PC Power 12 V Current Limiter

To safely power something from your PC, the attached loads have to be limited to a safe current in order to avoid the chance that a short circuit might cause inadvertent shutdown and damage to your data. This circuit can be used to safely bring out the 12V line from your computer to supply a limited amount of current to power external gadgets.

With normal loads, the power transistor is in saturation and so the regulated +12V line is directly available (minus only the Vcesat of the PNP transistor). If the output is accidentally shorted, the npn transistor turns off and removes the base drive of this transistor.

This circuit is intended for protection from short circuits. It might be possible to apply a load that brings the series transistor out of saturation without removing base drive to it: such a situation will cause heating and might cause it to fail short-circuit. Those who wish to draw significant current from this circuit will be well advised to check that the transistor is indeed in saturation with their intended load.

That said, this is a simple way to draw a few tens of milliamps at 12 V from your PC without danger to either load or the PC.

Step 1: The circuit

Any electronic project starts with a circuit. This one has a PNP power transistor in series with the +12V line from the PC. The collector supplies the external circuitry.

A small signal NPN transistor is fed with the output voltage through two resistors connected as a potential divider. Should the output voltage become less than a certain value, this transistor will turn off.

The collector of this transistor supplies the base current of the power PNP transistor. So the two transistors will always switch on or off together.

The power transistor (BD140) is supplied with a certain current via the green LED and 470 ohm resistor. It has a certain gain, around 50 or so, and its collector can supply only the gain times base current before it drops voltage (comes out of saturation).

So the maximum current draw can be changed by appropriately dimensioning the base resistor of the PNP power transistor.

The voltage divider at the output determines the voltage at which the current supplied by the transistor drops to zero.

The 470 ohm resistor in parallel with the PNP transistor supplies a certain amount of current so that the circuit will switch on when first powered on.

Comments

[drusilla]

Thanks! I needed this circuit. I put a 10k potentiometer in place of R5 which I think is useful.

I run this circuit with the 5V off my USB port. I didn't have to make any modifications except to make sure the transistors remain biased (changing value of the R5 pot).

[magma6]

By the way, I found a circuit of the same kindhere on wikipedia
Any comment from experts ? :-)

[neelandan]

It has a sensing resistor in series with the output. It can drop up to Vbe of the sensing transistor - around 600 mV - before current limiting takes place. It passes the full limiting current into a short circuit. This type of current limiting will be appropriate for an audio amplifier, for example. My circuit is intended to be a sort of electronic fuse which cuts off current when a short circuit is applied, and recover automatically when the short is removed.

[Jimmy Proton]

so its like a self-reseting circuit breaker?

[dfowler7437]

dfowler7437says:
I did an article about a current limiter to use with the PC supply at the following link. This limiter has a very sharp knee and would work with any input voltage.
http://www.uchobby.com/index.php/2006/11/12/current-limited-pc-power/

The circuit you show here would limit the current, based on the beta of the pass transistor, I dont think it would have a sharp cut-off.

[neelandan]

Your design continues to pass the full limit current into a short circuit. This one will pass only the current through the red LED into a short circuit. Your design has a variable voltage drop (resistive) upto 600 millivolts. This one has a constant and small voltage drop until the onset of limiting. Minor points, I am sure. But on these two points, I think my design is better.

[ReCreate]

so this is kinda like a self activating fuse type of thing ,right?

[neelandan]

A length of thin wire, or a small resistor (2.2 ohms, 1/4 Watt) will work as a self activating fuse.

This circuit works as a self-resetting fuse.

[ReCreate]

ah...yes self reseting fuse pretty neat...i might make this for some of my projects

[Erik Andersen]

Couldn't you just use a fuse? They're much simpler a circuit :) Or am I missing the whole point?

[neelandan]

Of course you can. But then I tend to get tired of changing fuses pretty quickly.

[Derin]

PTC self resetting fuse(thermal breaker)

[magma6]

I like the idea of a trimpot (on R1 ?) to select the max current but I have trouble to find which resistor values are needed. I understood that the maximum current depend of " the gain times base current before it drops voltage ". But it is not clear how to calculate it. Any clue ?
Can I find it empirically from the voltage at R1 ends ? (But then a gain of 50 in Q1 and the value you give will mean: Vr1= 470*.22/50 = 2V but that look a bit too few)
By the way what is the role of R5/(R4+R5) ? It look important to make the whole thing work but you did not say why :"That resistor at the output, originally 10K, changed to 36K, had to be changed to 22K." (in 8)

[neelandan]

10K - onset of current limiting too high 36K - circuit would not turn on with load 22K - current limiting around twice load current. I have modified step 8 accordingly.

[dfowler7437]

From my experience the PC supply is very noisy and often considerably over voltage. The 5V regulator would give a clean supply if powered from PC 12V. But it would not keep the PC from resetting if you shorted the output.

The regulator does have overtemp and short circuit protection but the input current would still spike.

I describe a different current limiting circuit on my site in case you are interested.
http://www.uchobby.com/index.php/2006/11/12/current-limited-pc-power/

[votecoffee]

The computer power supply pre-regulates the power. If you want 5 volts, pull off of a red wire instead of a yellow wire. Red to ground is 5 volts and yellow to ground is 12. The orange wire if available is even regulated at 3.3 for you. The point of his design is not as a voltage regulator but as a very low current overload protector. Fuses are normally rated higher than this and with no way to adjust and have to be replaced. Another advantage to this is that one could change the resistor with a trimpot to make a variable trip overload protector.

[wackyvorlon]

I have to admit to being more than a little puzzled here. Couldn't we drop about four components by using a 7805 regulator, with a series resistor on it's output? That should limit the current and give us a +5v supply. Probably wouldn't even need the bypass caps, I expect the output of a PC power supply is pretty stable.

[neelandan]

1. I need a 12 V supply. 2. Series resistors impose voltage drops. 3. It has to be protected from short circuits.

[blueroomelectronics]

The simple way is a PTC fuse. They are self resetting and simple devices.
Little fuse

[prodmod]

So this circuit prevents a short from causing a shut down and loss of data in your PC. But if I want more current I can just plug directly into the floppy power connector, right? Some power supplies offer 5v @ 30A for example. So if I want to draw 10 Amps or so, I just have to make sure I never short it otherwise my PC is damaged. Am i getting this right?

[neelandan]

Right. But I think if you need 10A on a regular basis you'll be better off investing in a separate power supply. A 12V, 7AH lead acid battery kept charged by a small (500 ma) wall wart is a good option, too, if you need it only intermittently.

[royalestel]

Is this to be plugged into the USB port?

[neelandan]

It has to be plugged into the power connector for the floppy drive.