View Full Version : Issues w/ 12v power supply

The Lazy Destroyer
04-17-2007, 04:54 AM
So I'm running a bullet camera setup for my bike and decided to go a bit over-the-top and just build a power supply that can handle both my camcorder and two bullet cameras.

Built it and although powers up and spits out 12V, it does not power up even a lone bullet camera. With the bike running it powers 13.8-14.0V to the supply, and the supply spits out a pretty constant 12V. If I connect the supply to the camcorder while it is already on it will turn off and "blip" the power LED at a relatively quick(er) rate (than normal).

The camcorder has a power supply that is 12V and up to 1000mA. The one bullet camera I am testing with has a 12V supply rated at 300mA.

I used a schematic here:


to build a 78xx power supply that can handle more mA. It came up pretty frequently in my search for a schematic. I did things a little differently and want to double check that the design is right before going over everything yet again.

Input goes through a 4A fuse.

On +V-IN I put a 100uH RF choke. It has a max amp rating of 2A which is close to what I think I'd run w/ two cams (1.6A max) but okay.

The diagram shows 1R and 10R. I had 1K and 10K resistors laying around so I used those respectively.

The diagram lists 2955 as the power transistor. Couldn't find a 2955 so I went with a TIP42 available at Radio Shack.

www.ortodoxism.ro/datasheets/fairchild/TIP42.pdf (http://www.ortodoxism.ro/datasheets/fairchild/TIP42.pdf)
www.ortodoxism.ro/datasheets/unisonic/2N2955.pdf (http://www.ortodoxism.ro/datasheets/unisonic/2N2955.pdf)

Although they have similar properties, some of the values I am seeing are a little different. I wired this (left to right, when looking at the front of the transistor, pins pointing down): 1-base 2-collector 3-emittor, and I followed a PNP wiring diagram online here, second image:

www.geofex.com/fxdebug/trantest.htm (http://www.geofex.com/fxdebug/trantest.htm)

Instead of a 470u cap on the input side of the 7812, I used a 470u electrolytic and a .1u ceramic in parallel

Instead of a 470u cap on the output side, I used a 470u electrolytic and a 10u tantalum in parallel.

There is a 12v LED in parallel on the output of the power supply rated at 30mA. It does not dim when connecting either camera.

I double checked everything but with it spitting out 12v but still not seeming to power things up I'm stumped and wondering if maybe the TIP42 isn't working as a replacement or if maybe something else I did was wrong. The connectors for both cameras are wired right and and supplying proper + and - sides.

I'm not very experienced with power supplies and wanted to run this all by you all to see if maybe there is something obvious I am not seeing. Or if there is a good way to test something, that would also help me. I'm hoping to get things straightened out by this coming weekend. Thanks in advance...

Mike Green
04-17-2007, 05:23 AM
The substitutions are what's keeping it from working. You can't just willy-nilly substitute values without understanding what you're doing.
Basically, the "booster" transistor is not boosting at all and the 1K series resistor is limiting the current through the regulator to a few milliamps.
As soon as you hook up any kind of load, the output voltage drops to near zero.

I strongly suggest that you have two separate regulators, one for the camcorder and one for the bullet cameras, each using a low drop-out regulator like the LM2940-12.0. Make sure the regulators are adequately heatsinked since they will be dissipating a bit of heat (2V x 1A = 2Watts). You need at least a 0.47uF input capacitor for each regulator and at least a 22uF output capacitor. The output capacitor needs to have a voltage rating greater than the output voltage (I'd say at least 16V). The input capacitor should probably be rated at 25V.