A friend recently gave me an old Zenith TransOceanic (ZTO) H-500 and after re-aligning it to get it into proper working condition I decided that I wanted to build a battery pack for it - both for "completeness" and to allow the radio to be used outdoors, away from interference sources. While it might be said that the GoogleWeb is lousy with options to replace the obsolete "A/B" battery used to power the Zenith TransOceanic, that wasn't a deterrent for me to design and build yet another one. Even though it is easy to use a lot of 1.5 volt cells (e.g. 6 "D" cells and 60 "AA" or "AAA" cells or ten 9-volt batteries) I decided to make do something different.
I threw a computer at it.
While it might seem odd to wield a microcontroller to solve a relatively simple problem on an antique, tube-type radio, it does make sense in a few ways as I'll outline below.
Design goals:
There are several things that I decided that this voltage converter should do:
Generating the "B+" voltage:
The "B Battery"(high voltage) needs of the ZTO are rather modest - approximately 90 volts at 5-20 milliamps. Aside from using a battery of sixty 1.5 volt cells or ten 9 volt batteries in series there are two common ways to generate this sort of voltage electronically:
The second method - and the one that I chose - uses the boost-type converter, typically with a single inductor as depicted in Figure 2. The switching frequency must be much higher than one would use with an ordinary mains transformer - typically in the 5-30 kHz range - if one wishes to keep the inductance and physical size of that inductor reasonably small. With these higher frequencies and typically "square" drive signals, rich in harmonic content, there is a much greater potential to interfere with the reception on radio itself. While a bit of a nuisance, the interference potential of this approach may be easily mitigated by putting the entire circuit in a metal box and appropriately bypassing and filtering the leads in and out.
Raiding my inductor drawer I picked a few "power" inductors (those capable of handling at least half an amp) in the range of 100μH and 1 mH and threw together the circuit in Figure 2consisting of a high-voltage FET (Q), the inductor under test (L), a high voltage, high speed diode (D), a 22μF, 160 volt capacitor (C) and a 5.6k, 2 watt load resistor (R). Connecting the FET's gate to the square wave (50% duty cycle) output of the signal generator I measured each one in terms of output voltage, total output power and overall power conversion efficiency with respect to frequency.
As would be dictated by the plethora of articles on the subject - not to mention data sheets of switching regulator chips - I noted that neither the value of the inductance or switching frequency was particularly critical to achieve the desired results. In general, higher inductances produce a bit more output at the lower frequencies (a few kHz) while the lower inductances worked a bit better in the 10-30 kHz range but all of the inductors did work over the entire range to a greater or lesser degree. Settling on a decent-sized 330μH inductor - a value that is not particularly critical - I proceeded with the circuit design.
The circuit:
Rather than go through a lot of theory I'll just describe the circuit that I designed and built - See Figure 3, above.
When the radio's power switch is turned on its filament circuit is connected and a voltage appears in the negative lead across "Batt-" and "A-" and R7, a 10k resistor connected across the switched-off FET Q4. When this happens transistor Q3 is turned on pulling the base of Q1, a PNP transistor in the high side of the BATT+ line, toward ground and turning it on applying power to U3, a 78L05 voltage regulator, and microcontroller U1, a PIC12F683. After a short initialization delay the microcontroller activates the "PWR_SW" line which turns on Q2 which also assures that Q1 is always turned on even if the filament switch is turned off abruptly and Q3 turns off or, as we shall see, when the battery voltage is at or below the filament regulator's set point.
At this point the microcontroller executes the code to produce the high voltage (B+) output by monitoring the B+ output via resistor divider R18/R19/R20: If the voltage is below the threshold the duty cycle of the PWM signal output on the "SW_DRIVE" line is increased to force more energy storage in the inductor (L1) up to a maximum limit of around 80% set in software. If the voltage is above the threshold the duty cycle is decreased - down to zero and even into "discontinuous" mode if necessary as would be the case if there were no load on the output. In this way the output voltage is appropriately regulated, typically to 90 volts, as set by R19. In this circuit when the PWM signal turns off Q5, the high voltage FET, the magnetic field in L1 collapses and induces a voltage across it. This voltage is rectified by high-speed, high-voltage diode D2 and filtered by C8 and additionally filtered and smoothed by R21 and C9.
Because the battery voltage could be as high as 16 volts if ten fresh "1.5" volt cells were used it is necessary to regulate the filament voltage to something around 9 volts. Op amp section U2b is a "difference amplifier"(a.k.a. subractor) that measures the voltage difference between the "A-" and the "A+" lines (the filament supply to the radio) and this calculated voltage difference is applied to the inverting input of U2a via potentiometer R14. The voltage at the inverting input of U2a as set by R14 is compared to the the "reference" voltage applied to its non-inverting input and if the voltage is low, its output voltage is increased so that FET Q4, which is placed between the A- and BATT- connections, conducts more to increase the filament voltage. Conversely, if the voltage is too high, the output voltage of U2a to Q14's gate is reduced, decreasing its conductivity.
The use of the circuitry of U2b is necessary because neither the A- or A+ (filament) leads are referenced to the circuit ground (e.g. they are sort of "floating") which makes it necessary to measure the difference between those two leads to ascertain the actual filament voltage. If the battery voltage does get low enough that Q4 is completely "on", the voltage across R7 will disappear and Q3 will turn off: It is because this can happen that we must have activated Q2 to keep the microcontroller's power turned on and this is also why we cannot use this voltage drop to detect if the filament current has ceased to flow when the radio is turned off.
Note: It would have been possible to have used the microcontroller to regulate the filament voltage in a manner similar to that in which the high voltage is produced, but a programming bug or crash could cause the fragile, expensive tubes to be exposed to the full battery voltage whereas a malfunction of the high voltage generator is unlikely to cause damage to the radio.
A short time after the high voltage converter is enabled the "FIL_SW" line is set high. Because the microcontroller has low-impedance FET output drivers, this pin's voltage is essentially that the 5 volt regulator and it is used as the filament voltage reference. Similarly, if the microcontroller sets the "FIL_SW" line low (zero volts) this will shut off the filament supply.
With the use of a MOSFET (e.g. Q4) as the filament control device, the series regulation of the filament has a very low drop-out voltage - that of the voltage drop across the FET, limited by its own "on" resistance - so this drop can be as low a few 10s of millivolts. What this means is that if the filament voltage is set to 9.0 volts by R14, as long as the "A" battery voltage exceeds that by a few 10s of millivolts, the filament will always be maintained exactly 9.0 volts. If the "A" supply (battery voltage) drops below 9.0 volts, Q4 will be turned fully on and the filament voltage will be within 10-20 millivolts of that battery voltage. Comparing this circuit to a typical "low dropout" regulator IC that has around 0.15-0.3 volts drop, this circuit offers lower voltage drop and better radio performance in those situations, particularly when even a few tenths of a volt can make a lot of difference!
A second or so after the application of the filament voltage the microcontroller starts to "look at" the current drawn on the B+ lead, detected by U4, an opto-isolator in series with this supply. Once the tubes warm up and drawing current U4's internal LED turns on, activating its internal transistor which then pulls the "HV_IMON"(high voltage current monitor) line low, indicating to the microcontroller that the radio is now operating.
When the radio is turned off the current on the B+ line will disappear due the loss of the tubes' emission caused by the filaments being turned off and, possibly, the B+ line being disconnected. When this happens the LED in optoisolator U4 will turn off, its transistor will stop conducting and the "HV_IMON" line will be pulled high indicating to the microcontroller that the radio has been turned off. After a short "debounce" period to verify that this loss of current wasn't due erroneously detected, the microcontroller will shut off the high voltage generator and set the "FIL_SW" line low, powering down the filament regulator and then setting the "PWR_SW" line low which then disconnects the microcontroller's power source from the BATT+ line.
Why use eight 1.5 volt cells rather than just six to get the filament voltage?
Why not just use six 1.5 volt cells to get 9 volts for the filament string? As it turns out only a set of six fresh 1.5 volt cells will actually produce 9 volts - and the voltage drops from there. If one consults the manufacturers' specifications for alkaline cells it will be noted that the majority of the useful life of typical "1.5 volt" cells occurs with their voltage actually being in the range of 1.25-1.3 volts and it isn't until a cell gets all of the way down to 1 volt (for a total of 6 volts to the filaments from our example six cell battery) that 80% of the cell's capacity has been exhausted.
In this radio I noted that below an "A" battery of 8 volts (e.g. 1.33 volts/cell for 6 cells) the sensitivity started to drop and by the time it has dropped to around 7.5 volts (1.25 volts/cell for 6 cells) the radio was practically deaf with the oscillator abruptly stopping just below this. Poking around inside the radio I noticed that at 9 volts, the series voltage drop across each of the tubes' filaments was very close to that shown on the schematic diagram in the service manual, but by the time it dropped to 7.5 volts it had become unequal, with the 1L6 converter tube being disproportionately affected and its filament voltage at or below 1 volt. Interestingly this drop-off in sensitivity did not appear to be related to frequency: The radio still worked at all frequencies with a filament voltage just above where it cut off, but it was just as deaf on the low bands as it was on the high.
For this reason I decided to use a battery voltage higher than the "9 volts" obtained from six cells and use eight 1.5 volt cells for two important reasons:
Note that this circuit can be powered directly from a 12 volt supply or battery - just heed the warnings below about NEVER allowing the "Batt-" line to come in contact with the "A-" lead - or any part of the radio's chassis.
Additional comments about the circuit:
It should be noted that the "BATT-" and "A-" lines are isolated from each other. These two lines should never be connected to each other as that would prevent the closure of the filament switch from being detected when the radio is turned on and it would bypass the filament regulator, exposing the tubes' filaments to the full battery voltage, likely destroying one or more of them! The reason for putting the filament regulation in the negative lead is to avoid the use of a P-channel FET in the "high" side and the complications required in keeping its circuit stable (e.g. avoiding spurious turn-on events and even momentary loss of voltage regulation) when the unit is powering up or down.
A few more circuit comments:
How well does it work?
As can be seen in Figure 1the circuit board and the eight "D" cell battery is concealed in a replica battery box that is situated exactly where an original "AB" battery would be placed. Then the power switch is turned on it takes a bit over a second for the computer to power up, do its checks and the tubes to warm up and the radio begins playing while the power-off is detected within two seconds of the radio being turned off.
With the shielding of the circuity and bypassing of its leads there is no detectable interference caused by switching voltage converter. With the filament and B+ voltage being regulated to the same as a "fresh battery" or AC mains voltage, the sensitivity and audio output capability are maintained until the battery is more than 80% depleted.
In other words, it works just as it should!
If you are interested in the code for this (written in "C" using the PICC compiler) or just a .HEX file so that you can program a PIC12F683 yourself, or if you are interested in getting an already-programmed PIC12F683, let me know via a comment.
And before you ask: Sorry, but I can't build you one at this time...
[End]
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| Figure 1: The faux A400 "AB" battery, installed and working in the Zenith Trans Oceanic H-500. Contained therein are eight "D" type cells and circuitry to produce the 90 volt "B" voltage and a regulated 9 volts for the filament supply. Click on the image for a larger version. |
While it might seem odd to wield a microcontroller to solve a relatively simple problem on an antique, tube-type radio, it does make sense in a few ways as I'll outline below.
Design goals:
There are several things that I decided that this voltage converter should do:
- Automatically power up and shut down when the radio is turned on and then off.
- Cause no interference to radio reception.
- Consume minimal current when the radio is turned off.
- Produce a regulated B+ voltage.
- Regulate the filament voltage so that the radio functions properly even when the battery is mostly discharged so that maximum use can be made of its total capacity.
- If the radio is on for a very long time (e.g. more than about 2 hours) do a "power save" shut down to (hopefully) prevent the batteries from being completely flattened.
- "Lock out" the operation of the radio if the batteries are already extremely low. Avoidance of completely killing the battery may reduce the possibility of their leaking.
Generating the "B+" voltage:
The "B Battery"(high voltage) needs of the ZTO are rather modest - approximately 90 volts at 5-20 milliamps. Aside from using a battery of sixty 1.5 volt cells or ten 9 volt batteries in series there are two common ways to generate this sort of voltage electronically:
- Use a step-up transformer to take the low battery voltage to the appropriate B+ potential, typically using a low-voltage mains transformer in "reverse"(e.g. applying drive to the secondary, rectifying high voltage from the primary.)
- The use of a simple boost-type converter using a single inductor.
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| Figure 2: Test circuit to determine the suitability of various inductors and transistors and to determine reasonable drive frequencies. Diode "D" is a high-speed, high-voltage diode, "R" can be two 10k 1 watt resistors in parallel and "Q" is a power FET with suitably high voltage ratings (>=200 Volts) and a gate turn-on threshold in the 2-3 volt range so that it is suitable to be driven by 5 volt logic. V+ is from a DC power supply that is variable from at least 5 volts to 10 volts. The square wave drive, from a function generator, was set to output a 0-5 volt waveform to make certain that the chosen FET could be properly driven by a 5 volt logic-level signal from the PIC as evidenced by it not getting perceptibly warm during operation. |
Raiding my inductor drawer I picked a few "power" inductors (those capable of handling at least half an amp) in the range of 100μH and 1 mH and threw together the circuit in Figure 2consisting of a high-voltage FET (Q), the inductor under test (L), a high voltage, high speed diode (D), a 22μF, 160 volt capacitor (C) and a 5.6k, 2 watt load resistor (R). Connecting the FET's gate to the square wave (50% duty cycle) output of the signal generator I measured each one in terms of output voltage, total output power and overall power conversion efficiency with respect to frequency.
As would be dictated by the plethora of articles on the subject - not to mention data sheets of switching regulator chips - I noted that neither the value of the inductance or switching frequency was particularly critical to achieve the desired results. In general, higher inductances produce a bit more output at the lower frequencies (a few kHz) while the lower inductances worked a bit better in the 10-30 kHz range but all of the inductors did work over the entire range to a greater or lesser degree. Settling on a decent-sized 330μH inductor - a value that is not particularly critical - I proceeded with the circuit design.
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| Figure 3: Schematic diagram of the voltage converter. See text for details. Click on the image for a larger version. |
The circuit:
Rather than go through a lot of theory I'll just describe the circuit that I designed and built - See Figure 3, above.
When the radio's power switch is turned on its filament circuit is connected and a voltage appears in the negative lead across "Batt-" and "A-" and R7, a 10k resistor connected across the switched-off FET Q4. When this happens transistor Q3 is turned on pulling the base of Q1, a PNP transistor in the high side of the BATT+ line, toward ground and turning it on applying power to U3, a 78L05 voltage regulator, and microcontroller U1, a PIC12F683. After a short initialization delay the microcontroller activates the "PWR_SW" line which turns on Q2 which also assures that Q1 is always turned on even if the filament switch is turned off abruptly and Q3 turns off or, as we shall see, when the battery voltage is at or below the filament regulator's set point.
At this point the microcontroller executes the code to produce the high voltage (B+) output by monitoring the B+ output via resistor divider R18/R19/R20: If the voltage is below the threshold the duty cycle of the PWM signal output on the "SW_DRIVE" line is increased to force more energy storage in the inductor (L1) up to a maximum limit of around 80% set in software. If the voltage is above the threshold the duty cycle is decreased - down to zero and even into "discontinuous" mode if necessary as would be the case if there were no load on the output. In this way the output voltage is appropriately regulated, typically to 90 volts, as set by R19. In this circuit when the PWM signal turns off Q5, the high voltage FET, the magnetic field in L1 collapses and induces a voltage across it. This voltage is rectified by high-speed, high-voltage diode D2 and filtered by C8 and additionally filtered and smoothed by R21 and C9.
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| Figure 3: The (mostly complete) converter board. The high-voltage FET (Q5) is in the lower left corner while the filament regulator FET is in the lower- right corner. In the upper right corner is U2, the rail-to-rail dual op-amp that is part of the filament regulator. Because of the very small amount of heat being dissipated by any component, no heat sinks were required. The high voltage filtering components and the optoisolator are in the upper left corner. No circuit board is available - but if you design one, I'd be happy to post information about it and give you credit! Click on the image for a larger version. |
Because the battery voltage could be as high as 16 volts if ten fresh "1.5" volt cells were used it is necessary to regulate the filament voltage to something around 9 volts. Op amp section U2b is a "difference amplifier"(a.k.a. subractor) that measures the voltage difference between the "A-" and the "A+" lines (the filament supply to the radio) and this calculated voltage difference is applied to the inverting input of U2a via potentiometer R14. The voltage at the inverting input of U2a as set by R14 is compared to the the "reference" voltage applied to its non-inverting input and if the voltage is low, its output voltage is increased so that FET Q4, which is placed between the A- and BATT- connections, conducts more to increase the filament voltage. Conversely, if the voltage is too high, the output voltage of U2a to Q14's gate is reduced, decreasing its conductivity.
The use of the circuitry of U2b is necessary because neither the A- or A+ (filament) leads are referenced to the circuit ground (e.g. they are sort of "floating") which makes it necessary to measure the difference between those two leads to ascertain the actual filament voltage. If the battery voltage does get low enough that Q4 is completely "on", the voltage across R7 will disappear and Q3 will turn off: It is because this can happen that we must have activated Q2 to keep the microcontroller's power turned on and this is also why we cannot use this voltage drop to detect if the filament current has ceased to flow when the radio is turned off.
Note: It would have been possible to have used the microcontroller to regulate the filament voltage in a manner similar to that in which the high voltage is produced, but a programming bug or crash could cause the fragile, expensive tubes to be exposed to the full battery voltage whereas a malfunction of the high voltage generator is unlikely to cause damage to the radio.
A short time after the high voltage converter is enabled the "FIL_SW" line is set high. Because the microcontroller has low-impedance FET output drivers, this pin's voltage is essentially that the 5 volt regulator and it is used as the filament voltage reference. Similarly, if the microcontroller sets the "FIL_SW" line low (zero volts) this will shut off the filament supply.
With the use of a MOSFET (e.g. Q4) as the filament control device, the series regulation of the filament has a very low drop-out voltage - that of the voltage drop across the FET, limited by its own "on" resistance - so this drop can be as low a few 10s of millivolts. What this means is that if the filament voltage is set to 9.0 volts by R14, as long as the "A" battery voltage exceeds that by a few 10s of millivolts, the filament will always be maintained exactly 9.0 volts. If the "A" supply (battery voltage) drops below 9.0 volts, Q4 will be turned fully on and the filament voltage will be within 10-20 millivolts of that battery voltage. Comparing this circuit to a typical "low dropout" regulator IC that has around 0.15-0.3 volts drop, this circuit offers lower voltage drop and better radio performance in those situations, particularly when even a few tenths of a volt can make a lot of difference!
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| Figure 4: Inside the completed voltage converter. All leads going in and out are bypassed with low-ESR electrolytic capacitors and further filtered with series chokes as shown in Figure 3. The use of a completely shielded enclosure (top not shown) is necessary as direct E-field radiation from the circuit will otherwise be heard on the radio. This box is made from cut pieces of circuit board material, soldered at the seams inside and out, with cut-in-half nickel-plated brass standoffs soldered to the board being used to support the circuit and at the corners to attach the lid. Click on the image for a larger version. |
A second or so after the application of the filament voltage the microcontroller starts to "look at" the current drawn on the B+ lead, detected by U4, an opto-isolator in series with this supply. Once the tubes warm up and drawing current U4's internal LED turns on, activating its internal transistor which then pulls the "HV_IMON"(high voltage current monitor) line low, indicating to the microcontroller that the radio is now operating.
When the radio is turned off the current on the B+ line will disappear due the loss of the tubes' emission caused by the filaments being turned off and, possibly, the B+ line being disconnected. When this happens the LED in optoisolator U4 will turn off, its transistor will stop conducting and the "HV_IMON" line will be pulled high indicating to the microcontroller that the radio has been turned off. After a short "debounce" period to verify that this loss of current wasn't due erroneously detected, the microcontroller will shut off the high voltage generator and set the "FIL_SW" line low, powering down the filament regulator and then setting the "PWR_SW" line low which then disconnects the microcontroller's power source from the BATT+ line.
Why use eight 1.5 volt cells rather than just six to get the filament voltage?
Why not just use six 1.5 volt cells to get 9 volts for the filament string? As it turns out only a set of six fresh 1.5 volt cells will actually produce 9 volts - and the voltage drops from there. If one consults the manufacturers' specifications for alkaline cells it will be noted that the majority of the useful life of typical "1.5 volt" cells occurs with their voltage actually being in the range of 1.25-1.3 volts and it isn't until a cell gets all of the way down to 1 volt (for a total of 6 volts to the filaments from our example six cell battery) that 80% of the cell's capacity has been exhausted.
In this radio I noted that below an "A" battery of 8 volts (e.g. 1.33 volts/cell for 6 cells) the sensitivity started to drop and by the time it has dropped to around 7.5 volts (1.25 volts/cell for 6 cells) the radio was practically deaf with the oscillator abruptly stopping just below this. Poking around inside the radio I noticed that at 9 volts, the series voltage drop across each of the tubes' filaments was very close to that shown on the schematic diagram in the service manual, but by the time it dropped to 7.5 volts it had become unequal, with the 1L6 converter tube being disproportionately affected and its filament voltage at or below 1 volt. Interestingly this drop-off in sensitivity did not appear to be related to frequency: The radio still worked at all frequencies with a filament voltage just above where it cut off, but it was just as deaf on the low bands as it was on the high.
For this reason I decided to use a battery voltage higher than the "9 volts" obtained from six cells and use eight 1.5 volt cells for two important reasons:
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| Figure 5: Inside the faux "AB" battery box for the Zenith TransOceanic. Eight "D" cells are used in four holders (one 4 cell, one 2 cell and two 1-cell) which, along with the converter box, are screwed down to some plywood (3 layers of 3.2 mm "luon") which itself is glued to the bottom of the box. The cover, made from the same circuit board material as the box containing the circuits, has both of its surfaces electrically connected using thin, copper foil soldered to each side to assure that an electrical connection is made to the box itself when the cover screws are tightened. The authentic-looking replica battery box and radio connector were obtained from "Edsantiqueradios.com". Click on the image for a larger version. |
- The higher voltage of eight 1.5 volt cells (12+ volts when fresh) would allow the total filament potential ("A" voltage) to be regulated down to 9 volts.
- The use of an extra two cells will allow the use of more of the battery capacity. For example, with 8 cells discharged to 1 volt, each, around 80% of the cell's useful life has been utilized with the ending voltage still being 8 volts. Contrasting this to the use of just six cells, at a total "A" voltage of just 7.75 volts (approx. 1.3 volts/cell for 6 cells) 40-60% of the life of the cells will remain, but the radio will likely not be usefully operational!
- In theory, ten 1.5 volt cells could be used. Because the voltage of a "fresh" 1.5 volt alkaline is around 1.6 volts, this could expose some of the devices - particularly the electrolytic capacitors and U2 - to voltage at or above the official maximum rating. Practically speaking these devices will likely survive this, particularly since the voltage will very quickly drop under the load presented by the radio into the "safe" range. The use of one or two additional 1.5 volt cells (e.g. 9 or 10) won't add more than 10-15% of "run time" to the radio so it is not likely to be worth using more than eight 1.5 volt cells.
- The typical filament current of this radio is on the order of 50 milliamps. At a battery voltage of 12 volts where 3 volts is dropped by the series regulator, approximately 150 milliwatts is dissipated as heat - about 25% of the total filament power. Were a switching regulator used for the filament its efficiency would likely be in the 85-90% range and increase of efficiency over the linear regulator would likely not be worth the added complexity. Considering that the average voltage of the battery over its life will be closer to 10.4 volts (approx. 1.3 volts/cell) with a dissipation of only 70 milliwatts the difference in loss will be even lower.
- For "AA" size: 15-20 hours with reduced performance for an additional 1-2 hours.
- For "C" size: 30-40 hours with reduced performance for an additional 3-5 hours.
- For "D" size: 70-90 hours with reduced performance for an additional 6-10 hours.
Note that this circuit can be powered directly from a 12 volt supply or battery - just heed the warnings below about NEVER allowing the "Batt-" line to come in contact with the "A-" lead - or any part of the radio's chassis.
Additional comments about the circuit:
It should be noted that the "BATT-" and "A-" lines are isolated from each other. These two lines should never be connected to each other as that would prevent the closure of the filament switch from being detected when the radio is turned on and it would bypass the filament regulator, exposing the tubes' filaments to the full battery voltage, likely destroying one or more of them! The reason for putting the filament regulation in the negative lead is to avoid the use of a P-channel FET in the "high" side and the complications required in keeping its circuit stable (e.g. avoiding spurious turn-on events and even momentary loss of voltage regulation) when the unit is powering up or down.
A few more circuit comments:
- Resistors R8 and R17 are used to bias their respected FETs off by default. This is necessary as the outputs of the microcontroller are high-Z unless/until it is operating and these FETs could randomly turn on due to leakage currents without them.
- Similarly R15, on the "reference" voltage for U2's filament regulator circuit from the microcontroller, pulls that output down before the processor initializes its outputs, eliminating a possible "glitch" of the filament voltage during circuit start-up and shut-down.
- U2, the filament voltage regulator, MUST be a rail-to-rail input and output op amp: An "ordinary" op amp such as the '1458 or '358 WILL NOT WORK PROPERLY under all conditions. Some parts suggestions for suitable op amps are included in the schematic diagram of Figure 3.
- Resistor R9, a 470 ohm resistor in series with the output of U2a and FET Q4, isolates Q4's gate capacitance, preventing instability of the op-amp.
- When powered down the quiescent current of this circuit is approximately 7μA caused by the battery voltage (minus the drop of D1) always being applied across the B+ voltage divider string R18, R19 and R20. This amount of current is comparable to the self-discharge rate of modern alkaline cells and can generally be ignored. If this amount of current were to really bother you, the voltage converter circuit could be powered from the "V+_SW" line and transistor Q1 could be replaced with a P-channel power FET as noted on the diagram.
- LED1 is optional. It will glow when the microcontroller activates the "PWR_SW" line and can be used for troubleshooting. For example, if no current is being drawn from the B+ line - or the converter is not working - the software will continually cycle: It will turn on the high voltage, wait for current to flow and when not seeing it, it will turn off the high voltage again and retry after a few seconds causing the LED to turn on and off.
- Transistor Q5, used in the high voltage "boost" converter, must be rated for at least 200 volts and it should have a "logic level" gate capable of turning the device (more or less) fully on at just 5 volts: Some suggested device types are noted on the diagram (Figure 3). An additional device worth considering is the ON Semiconductor NDD02N40-1G, a 400 volt, 1.1 amp FET that has a suitably low turn-on threshold - and it's pretty cheap.
- Components TH1, a 1 amp self-resetting fuse and diode D1 protect the circuit against shorts or accidental reverse polarity by limiting the current to a reasonable value should this occur. TH1 may be replaced with a 0.75-1 amp fast-acting fuse if so desired.
- The PWM (switching) frequency is approximately 15.625 kHz based on the microcontroller's internal 8 MHz clock. Both 7.8125 and 31.25 kHz were tried and the conversion efficiency was slightly lower (e.g. approx. 1-5%) with the 330 μH inductor value chosen - an indication that the actual value of L1 isn't particularly critical.
- The value of L1 may be anything from 220μH to 470μH - and even a bit beyond this. Make sure that the inductor used has a current rating of at least a half an amp or else internal resistive losses will significantly impact conversion efficiency. If available, a toroidal inductor is preferred as it better-contains its magnetic field than solenoid types.
- The measured efficiency of the boost converter is greater than 80%, including the power lost in R21, the "filter" resistor in series with the B+ output.
- The 15 volt limit is set by the voltage rating of op amp U2 and the ratings of the electrolytic capacitors.
- If one chose to use just six 1.5 volt cells instead of eight, the "FIL_SW" line would be connected directly to the gate of Q4 and the circuitry related to U2 would be omitted.
- The diagram and pictures show the use of feedthrough capacitors (4000pF) to pass the voltages through the shielded box. Feedthrough capacitors are somewhat difficult to get, but good results may be obtained by using good-quality monolithic ceramic(NOT disk ceramic) capacitors instead. These capacitors are typically square in shape and rather compact and available in both leaded and surface-mount form. Remember that for the B+ output a capacitor with a rating of at least 100 volts must be used. Any value from 0.0022μF to 0.1μF may be used.
- If you build this sort of circuit make absolutely certain that you simulate the filament string with a 150-200 ohm 1/2-1 watt resistor and the B+ load with a 10k, 1-2 watt resistor and verify that the circuits are working properly BEFORE connecting it to a radio. While a brief bit of over-voltage on the B+ line (to perhaps 130-150 volts) will likely not harm the radio, more than 9 volts on the filament line will probably ruin one or more of the fragile and expensive tubes!
- About that "auto power save" feature? After two hours of uninterrupted operation the microcontroller will modulate the filament line with an intermittent tone and drop the B+ voltage to about 50%causing the radio to partially mute with the alarm tone sounding in the speaker. This will continue for about a minute before the microcontroller shuts down the radio, dropping the current consumption from around 150 milliamps to about 6-12 milliamps. Turning the radio off for 5-10 seconds and then back on will reset it at any time. The down-side of this is that one may forget that the radio is even on, except for the fact that the front lid will be in its upright position. If the battery voltage is less than around 7.5 volts (0.9375 volts/cell) the radio will not even turn on, but at this voltage the batteries are not only quite discharged, but their internal resistance will be rapidly increasing as well.
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| Figure 6: A handy "map" showing where the various RF adjustments may be found. This doesn't really have too much to do with the article, but since I made it when I was aligning the radio I thought that I might as well post it here! Note that locations of some of the trimmer capacitors - particularly those in the lower-left corner - will vary with different production runs. Some of the alignment points shown in this picture are also omitted in the "official" H500 service manual and thus have no parts designations: These adjustments are peaked at the frequencies indicated on the drawing. Click on the image for a larger version. |
As can be seen in Figure 1the circuit board and the eight "D" cell battery is concealed in a replica battery box that is situated exactly where an original "AB" battery would be placed. Then the power switch is turned on it takes a bit over a second for the computer to power up, do its checks and the tubes to warm up and the radio begins playing while the power-off is detected within two seconds of the radio being turned off.
With the shielding of the circuity and bypassing of its leads there is no detectable interference caused by switching voltage converter. With the filament and B+ voltage being regulated to the same as a "fresh battery" or AC mains voltage, the sensitivity and audio output capability are maintained until the battery is more than 80% depleted.
In other words, it works just as it should!
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If you are interested in the code for this (written in "C" using the PICC compiler) or just a .HEX file so that you can program a PIC12F683 yourself, or if you are interested in getting an already-programmed PIC12F683, let me know via a comment.
And before you ask: Sorry, but I can't build you one at this time...
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