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My light bulb discharger build(s)

The following article is nothing more than a project report. I take no responsibility for how others may use this information.
My discharger build
I went a long time without a discharger. Basically I just used my rc heli as the discharger any time I needed one. That works great when you are just flying packs but when you start doing more with them, having a way to discharge packs on the bench can be handy. So I decided to finally put something together.

The light bulbs I chose
I started by doing a little research in an attempt to find the easiest bulb to deal with that also offered a good bang for the buck. What I turned up is referred to as a "G4 base JC type" bulb, as is very common both at places like WalMart and online. They seem to mostly be used for household 12V applications and come in wattage ratings from 5W to 20W.

Having picked out the bulbs, I did some quick math and found that 6 in parallel would create a 12V 10A load. That also meant that 3 bulbs in parallel would draw 5A and 12 bulbs in parallel would draw 20A. I ended up ordering 20, 12V 20W bulbs on eBay for about $.50 each.

The box
I work on computers for a living, so I have tons of old equipment laying around. Knowing that I was going to be using lots of 20W halogen bulbs, that get really hot, I wanted to enclose the bulbs in a case and actively cool them. I immediately decided to use an old power supply case. Not only is it a good size but it is also already designed to accept a circuit board and a fan.

The circuit board
The bulbs I chose have pins on them, so my first thought was to solder them to a circuit board. That made me think about trying to make a board that would mount in place of the original power supply board, using the stock hardware. Unfortunately the only circuit board I could find was a 2-sided board. So I cut off a piece and mounted it in the case. That part was easy and went well.

Next up was finding a way to etch the board. I have access to standard etching equipment but I have seen another method used that I wanted to try. Since the design of this circuit board is going to be very simple and does not need to be perfect, I planned to use a Dremel with a round tip carbide bit to remove copper where needed.

The first step was to draw out what needed "etched". I just used a black Sharpie to free-handed some light lines where I thought I needed. It was really not a precise exercise, but rather a "looks like I need a line here" kind of exercise. Then I got the Dremel out. It took a little practice to learn how to hold the Dremel right but after a few tries it turned out to be surprising easy. If I had to guess, I would say I had the whole board done in about 15min. I later added some other things to try, like a power point for the fan.

I originally made it with 5A, 10A and 20A 12V loads. Then later I changed the 12V 20A into a 24V 10A load. It required nothing more than rearranging a few wires. Here is the circuit as it currently stands.


Some may notice that if I would have done a little more thinking before I built it, I could have done all this, and more, with 12 bulbs. Well you know what they say, hindsight is 20-20, and so I might just have to redo it ;)

Wiring it up
I used the same banana plugs I have used in all my projects. I made sure to install them in a fixed portion of the case and not in the lid, as that makes it harder to work on. The wire I used was 16awg silicone noodle wire.

The fan
One thing I knew I would need is active cooling and that is one reason I used the case I did, it had a fan build into it. I had the option of powering the fan externally with some other source, like a power supply, but I chose to power it with the circuit. There always going to be a 12V max circuit present but where that circuit was would shift. This is another thing about the circuit I originally made compared to the simpler 12 bulb one. I ended up using a pin header to allow for 2 connections, one came off the 12V 10A circuit and the other came off half the 24V 10A circuit. The power supply case had a hole in it that allowed for easy access to the pin header, so I could change where the fan was plugged in from outside the case. I could have used a switch but didn't see the need.

The finished product
What I ended up with has worked great for over a year now. I use it mostly for testing things but do use it to discharge packs on occasion. The case part of the design is awesome but I believe I may redo the circuit board, using the simpler 12 bulb design and maybe adding a second set of 12 bulbs for up to 12V 40A load capability.

A few photos

A look at the back of the circuit board. Not pretty but it
works well.



I closer look at the bulbs and circuit board.


The board in the case.


The internal connections.


The external connections.


The finished product.


Another smaller discharger or two
When I wrote this how-to I swung by eBay to see what the little 12V 20W bulbs cost these days and found that I could buy 30 of the little buggers for $4 shipped from China. Well lets just say I decided to grab a set, but what was I to do with my new pile of 20W bulbs? Well why not get a little creative and see what I could come up with.

The round one
The first one I made was inspired by trying to keep the bulbs cool. So I grabbed a 60mm fan, a few standoffs and a perf board about the same size as the fan. I soldered 12 bulbs in a circle, 6 in each half circle are wired in parallel, and wired it up. That's about it.


The cute one
I spent 2 days watching and thinking about what I could use as a case for a discharger and it finally hit me, why not use one of my little metal Scorpion motor boxes? While I was at it, why not make use of one of the little nasty fans I pulled out of the HP server power supplies? I had a plan and with a quick stop by the hardware store, I picked up a few of the parts I needed.

My initial intent was to use 8 bulbs in 2 sets of 4, and I built that but it just made too much heat for such a small enclosure. I had them configured so the 4 corners were on one circuit and the 4 middles where on the other. To remedy the heat problem I removed 2 bulbs, the lower right corner bulb and upper middle bulb (as per the photo below). The result is not as pretty but works better and I so I will leave it be for now.


Here is a rundown of what I did to make this little guy. I started out by cutting the 3/4" holes in each end. I ended up using my pocket knife to cut the holes. That actually worked really well. The resultant holes were a little rough but the grommets cover it up easily. Next came the fan install. I used 4 computer motherboard standoffs and 4 screws to hold that it in place. Last up was to install the 3 grommets in the front of the case that would hold the 4mm bullets. For the circuit board I used one of the crappy but handy perf boards. I used stripped 22awg phone wire to connect all the bulbs together and some loose wire I had laying around to connect the bullets to the board.