Issues w/ 12v power supply
The Lazy Destroyer
Posts: 21
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:
http://www.mitedu.freeserve.co.uk/Circuits/Power/boosti.htm
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.
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...
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:
http://www.mitedu.freeserve.co.uk/Circuits/Power/boosti.htm
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
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 - 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...
Comments
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.