Comments on: Current Limited PC Power

By: dfowler

dfowler — Mon, 18 Jun 2007 00:26:42 +0000

1000ma or 1A would require that you change the pass components to high power parts. A power transistor for example. R1 will need to be set to drop about .7V at 1A and would be disapating .7W I would go for a 1W resistor and I would assume the transistor would need to disapate about 1W as well. A T0-220 package for the transistor would likely be OK. You should mount it to something to disapate the heat.

By: Granz

Granz — Sat, 16 Jun 2007 08:45:21 +0000

DFowler,

I would like to be able to provide about 1000 mA for my proto system. In addition to the change in R1, what should I change in the rest of the components (especially in the transistors)?

Thanks

By: dfowler

dfowler — Sat, 21 Apr 2007 20:58:23 +0000

Kevin,

Remember that there is some voltage drop on this current regulator. If you start with 5V, like the USB port, you will end up with less at your hardware. The 5V from the main PC supply tends to be higher then 5V. The USB maybe as well as it probably comes from the same point.

By: dfowler

dfowler — Sat, 21 Apr 2007 20:57:58 +0000

Juan,

Probably the best way to make it variable is to add an adjustable voltage regulator. Maybe a LM317 which also has built in overload protection but I would still use a current limiter on the PC side. If you were to short the regulated supply output it would likely surge the current from the PC and cause it to reset.

By: Juan Cubillo

Juan Cubillo — Sat, 21 Apr 2007 20:04:57 +0000

Is there any way to make this a Variable power supply?

By: Kevin

Kevin — Sat, 21 Apr 2007 10:17:06 +0000

Thanks a lot, exactly what i was looking for to not damage my usb port while playing with my atmegas

By: dfowler

dfowler — Fri, 29 Dec 2006 06:48:38 +0000

Jeff,

You are correct. 3.6 ohms is the value I used. I had an error on the schematic, good catch.

David

By: Jeff Epler

Jeff Epler — Thu, 28 Dec 2006 14:00:46 +0000

Isn’t the correct value for R1 3.6 ohms, not 36 ohms?