Modifying an "O2-Cool" battery fan to (also) run from 12 volts

A blog posting about a fan?  Really?

Why not!

Figure 1: The modified fan on my cluttered workbench, running from 13 volts. The external DC input plug is visible on the lower left. Click on the image for a larger version.

This blog post is less about a fan, but is more of example of the use of a low-cost buck-type converter to efficiently power a devices intended for a lower voltage than might be available - in this case, a device (the fan) that expects 3 volts.  In many cases, "12" volts (which may be anything from 10 to 15 volts) will be available from an existing power source (battery, vehicle, power supply) and it would be nice to be able to run everything from that.

Background

Several years ago I picked up a 5" battery-operated DC fan branded "O2 Cool" that has come in handy occasionally when I needed a bit of airflow on a hot day.  While self-contained, using two "D" cells - it can't run from a common external power source such as 12 volts.

Getting 3 volts

Since this fan uses 3 volts, an obvious means of powering it from 12 voults would be to simply add a dropping resistor - but I wasn't really a fan of this idea (pun intended!) as it would be very wasteful in power and since doing this would effectively defeat the "high/low" speed switch - which, itself is a 2.2 ohm resistor.

The problem is that the fan itself pulls 300-400 mA on high speed.  If I were to drop the voltage resistively from 12 volts (e.g. a 9 volt drop) and if we assume a 300mA current, we would need to add (9/0.3 = ) 30 ohms of resistance.  The "low" switch inserts a 2.2 ohm resistor and adding this amount to 30 ohms would result in a barely noticeable difference in speed, effectively turning it into a single-speed fan.

Fortunately, there's an answer:  An inexpensive buck converter board.  The board that I picked - based on the MP1584 chip - is plentiful on both EvilBay and Amazon, typically for less than US$2 each.  These operate at a switching frequency of about 1 MHz and aren't terribly prone to cause radio interference, having also been used to power 5 volt radios from 12 volts without issues.

These buck converters can handle as much as 24 volts on the input and up to 3 amps - more than enough for our purpose and can also be adjusted to output about any voltage that is at least 4 volts lower than the input voltage - including the nominal 3 volts that we need for the fan.

An additional advantage is the efficiency of this voltage conversion.  These devices are typically 80% efficient or better meaning that our 300 mA at 3 volts (about 0.9 watts of power) would translate to less than 100mA at 12 volts (a bit more than a watt).  Contrasting this to our hypothetical resistive divider, we would be burning up nearly 3 watts in the 30 ohm resistor by itself!

Implementation

One of my goals was to retain the ability of this fan to run at 3 volts as it can still be convenient to have this thing run stand-alone from internal power.  Perhaps overkill, but to do this I implemented a simple circuit using a small relay to switch to the buck converter when external power was present and internal power when it was not, rather than parallel the buck converter across the battery.

If I never intended to use the internal "D" cells ever again I would have dispensed with the relay entirely and not needed to make the slight modifications to the switch board mentioned below.  In this case I would have had plenty of room in the case and freedom to place the components wherever I wished.  In lieu of the ballast of the battery to hold the fan down and stable, I would have placed some weight in the case (some bolts, nuts, random hardware) to prevent it from tipping over.

The diagram of this circuitry is shown below:

Figure 2: Diagram of the finished/modified fan. On the left, J1 is the center-positive coaxial power connector with diode D1 and self-resetting resetting thermal fuse F1 to protect against reverse polarity.  The relay selects the source of power. Click on the image for a larger version.

The original parts of are the High/Low switch, the battery and the fan itself on the right side of the schematic with the added circuits being the jack (J1), the self-resetting fuse (F1), D1, R1, the buck converter and the relay (RLY).

How it works:

When no external power is applied, the relay (RLY) is de-energized and via the "NC"(Normally-Closed) contacts, the battery is connected to the High/Low switch and everything operates as it originally did.

External power is applied via "J1" which is a coaxial power jack, wiring the center pin as positive:  The connector that I used happens to have a 2.5mm diameter center pin and expects an outer shell diameter of 5.5mm.  There's nothing special about this jack except that I happen to have it on-hand.

When power is applied, the relay is energized and the switch is disconnected from the battery but is now connected, via the "NO"(Normally Open) contacts, to the OUT+ terminal of the buck converter.  

Ideally, a small 12 volt relay would be used, but the smallest relay that I found in my junk box was a 5 volt unit, requiring that the coil voltage be dropped.  Measuring the relay coil's resistance as 160 ohms, I knew that it required about 30 mA (5/160 = 0.03) and if we were to use 12 volts, we'd need to drop (12 - 5 =) 7 volts.  The resistance needed to drop 7 volts is therefore (7/0.03 = ) 233 ohms - but since I was more likely to operate it from closer to 13 volts much of the time I chose the next higher standard value of resistance, 270 ohms to put in series for R1.

Figure 3: Modification of the switch board.  The button is the positive battery terminal and traces are cut to isolate it to allow relay switching. Click on the image for a larger version.

The diode D1 is a standard 1 amp diode - I used a 1N4003 as it was the first thing that I found in my parts bin, but about any diode rated for 1 amp or greater could be used, instead.  Placing it in reverse-bias across the input of the buck converter means that if the voltage was reversed accidentally, it would conduct, causing the self-resetting thermal fuse F1 to "blow" and protect the converter.  I chose a thermal fuse that has several times the expected operating current so I selected a device that would handle 500-800 mA before it would open.

Modification to the switch board

The High/Low switch board also houses the positive battery contact, but since it is required that we disconnect the battery when running from external power, a slight modification is required, so a few traces were cut and a jumper wire added to isolate the tab that connects to the positive end of the battery as seen in Figure 3.

Figure 4: The top of the board battery board. The connection to the Batt+ is made by soldering to the tab. Click on the image for a larger version.

Near the top of the photo in Figure 3 we see the the end of the 2.2 ohm resistor has been separated from the battery "+" connector (the round portion) and also along the bottom edge where it connects to the switch.  Our added jumper then connects the resistor to the far end of the switch where the trace used to go and we see the yellow wire go off to the "common" contact of the relay.

In Figure 4 we can see the top of the board with the 2.2 ohm resistor - but we also see the wire (white and green) that connects to one of the tabs for the Battery + button on the bottom of the board:  The wire was connected on this side to keep it out of the way round battery tab and the "battery +" connection.

The mechanical parts

For a modification like this, there's no need to make a circuit board - or even use prototyping boards.  Because we are cramming extra components in an existing box, we have to be a bit clever as to where we put things in that we have only limited choices.

Figure 5: Getting ready to install the connector after a session of drilling and filing. Click on the image for a larger version.

In the case of the coaxial power connector, there was only one real choice for its location:  On the side opposite the power switch, near the front, because if it wereplaced anywhere else it would interfere with the battery or with the fan itself as the case was opened.

Figure 5 shows the location of this connector.  Inside the box. this is located between two bosses and there is just enough room to mount it.  To mount it, small holes were drilled into the case at the corners of the connector and a sharp pair of flush-cut diagonal nippers were used to open a hole.  From here it was a matter of filing and checking until the dimensions of the hole afforded a snug fit of the connector.

Figure 6: A close-up of the buck converter board with the attached wires and BATT- spring terminal. The tiny voltage adjustment potentiomenter is visible near the upper-left corner of the board. Click on the image for a larger version.

Wires were soldered to the connector before it was pressed into the hole and to hold it in place I used "Shoe Goo" - a rubber adhesive - as I have had good luck with this in terms of adhesion:  I could have used cyanoacrylate ("Super" glue) or epoxy, but I have found that these bonds tend to be a bit more brittle with rapid changes of temperature, shock or - most applicable here - flexing - something that the Shoe Goo is meant to do.

Because this jack is next to the battery minus (-) connector, a short wire was connected directly to it, and another wire was run to the location - in the adjacent portion of the case - where the buck converter board would be placed.

Figure 6 shows the buck converter board itself in front of the cavity in which it will be placed, next to the negative battery "spring" connector.  Diode D1 is soldered on the back side of this board and along the right edge, the yellow self-resetting fuse is visible.  Like everything else the relay was wired with flying leads as well, with resistor R1 being placed at the relay for convenience.

Figure 7: The relay, wired up with the flying leads. Click on the image for a larger version.

Figure 7 shows the wiring of the relay.  Again, this was chosen for its size - but any SPDT relay that will fit in the gap and not interfere mechanically with the battery should do the job.

The red wire - connected to the resistor - comes from the positive connector on the jack and the "IN+" of the buck converter board - the orange wire is the common connection of the High/Low switch, the white/violet comes from the "OUT+" of the buck converter and goes to the N.O. (Normally Open) contact on the relay, the white/green goes to the N.C. (Normally Closed) relay contact and the black is the negative lead attached to the coil.

Everything in its place

Figure 8 shows the internals of the fan with the added circuitry.  Shoe Goo was again employed to hold the buck converter board and the relay in place while the wires were carefully tucked into rails that look as though they were intended for this!

Now it was time to test it out:  I connected a bench power supply to the coaxial connector and set the voltage at 10 volts - enough to reliably pull in the relay - and set the fan to low speed.  At this point I adjusted the (tiny!)potentiometer on the buck converter board for an output of 3.2 volts - about that which could be expected from a very fresh pair of "D" cells.

Figure 8: Everything wired and in its final locations.  On the far left is the switch board.  To the left of the hinge is the relay with the buck converter on the right side of the hinge.  The jack and negative battery terminal is on the far right of the case. Click on the image for a larger version.

The result was a constant fan speed as I varied the bench supply from 9 to 18 volts indicating that the buck converter was doing its job.

The only thing left to do was to make a power cord to keep with the fan.  As is my wont, I tend to use Anderson Power Pole connectors for my 12 volt connections and I did so here.

As I also tend to do, I always attach two sets of connectors to the end of the power cord - the idea being that I would not "hog" DC power connections and leave somewhere to plug something else in.  While the power cord for the fan was just 22 gauge wire, I used heavier wire (#14 AWG) between the two Anderson connectors to carry heavier current than so that I could still run high-current devices.

* * *

Does it work?

Of course it does - it's a fan!

The relay switches over at about 8.5 volts making the useful voltage range via the external connector between 9 and 16 volts - perfect for use with an ostensibly "12 volt" system where the actual voltage can vary between 10 and 14 volts, depending on the battery chemistry and type.

Figure 9: The fan, folded up with power cord. The two connectors and short section of heavy conductor can be just seen. Click on the image for a larger version.

Without the weight of the two "D" batteries, the balance of the fan is slightly precarious and prone to tip forward slightly, but this could be fixed by leaving batteries in the unit - but this is not desirable for long-term storage as leakage is the likely result.  Alternatively, one may place some ballast in the battery compartment (large bolt wrapped in insulation, a rag, paper towel, etc.) or simply by placing something (perhaps a rock) on the top.  Alternatively, since the fan is typically placed on a desktop, it is often tilted slightly upwards and that offsets the center of gravity in our favor and this - plus the thrust from the airflow - prevents tipping.

This page stolen from ka7oei.blogspot.com

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