Current limit to 500mA (maximum) at 12V

Question

I have a 12V DC output which can output 1A (maximum.)

I want a circuit that limits the current to maximum 500mA, so I don't accidentally draw more than 500mA as that would probably mean I have shorted something out.

If someone can help with a circuit that can limit the current to 500mA (max), so it doesn't output more than 500mA or opens the circuit if more than 500mA is drawn, please suggest which parts to use and the circuit.

Answer 1

You can use a circuit like this one:

Choose the transistors in such way that Q1 VCE will drop 0.7V when collector current is 500mA. This will trigger Q2 to shutoff Q1.To reset the circuit just remove the source and connect it again.

For a more precise current limit, independent of transistor gain, you can use the circuit below. Rsense is calculated as 0.4V / current. In your case 0.4V / 0.5A = 0.8ohm. However, you will need RB to be such that it draws 50mA, or more, as the circuit will be triggered when VCE is 0.3V and not 0.7V.

I have done some spice tests and here are the results when I vary the resistance of the load to show how the current varies and when Q1 shuts off.

Here is the chart of Q1 current:

You have to read the x-axis from 100ohm to 10ohm. And here is the spice code:

The way the circuit works is the following:

1. It uses the pass transistor (Q1) Collector to Emitter voltage (VCE) drop to trigger Q2.

2. When Q1 VCE is high enough (around 0.7 volts) it will make Q2 conduct.

3. When Q2 conducts because its base is 0.7V or higher, its VCE will lower Q1 Base to Emitter voltage below 0.7V, so Q1 base will not conduct anymore and it will shut off.

4. The trick here is that both transistors need to be driven in a such way that their collector to emitter voltage (VCE) is below the threshold of conduction of the base to emitter of the other transistor.

5. When Q1 is conducting, its collector to emitter (VCE) is below 0.7V so Q2 is off.

6. When Q1 VCE reaches 0.7V or higher, it triggers Q2 to conduct.

7. When Q2 conducts, it shuts off Q1. Detail, when Q2 is conducting, its VCE needs to be below 0.7V to keep Q1 off.

I have only done this circuit with BC557 which only draws a maximum of 100mA. So, I decided to do it with the BC327 and here are the results:

I have used an LED and 1K resistor to indicate when the circuit was ON. The 0.4A current was when ON, and the 0.05A when it was OFF. The 0.05A is the current on Q2 when the circuit if OFF.

Answer 2

The circuit by Victor is not stable, and will not provide the protection you want. It is based on a technique called "dangle-biasing". It relies on the gain of the transistor being absolutely constant as the transistor heats up, and this is not true.

Here is a much better 2-transistor current limit circuit. As with all simple limiter circuits, the output voltage will decrease slightly until the current limit is reached. In this case, the output voltage will decrease from 12 V at 0 mA to approximately 11.4 V when the the output current is at the limit defined by Rsense.

Help dimensioning resistors in a two NPN transistor current limiter circuit

Answer 3

If one can tolerate a little voltage drop ...

Here is a "circuit" that can do it ... In fact, it is a "classic" power supply. R1&R11 limit the current as usual. Internal impedance = ~ 0.6 Ohm. Can be used also to other "fix" voltages.

Some components may be "omitted". R10 & X2 ( 10V zener).

And if someone is interested in a "foldback" protection, one can use this schematic.

The internal impedance (because of the op-amp) is here 0.013 Ohm from 0mA to the "foldback point", here 300mA.

Answer 4

This circuit gives you an adjustable current limit. The current is sensed by R_POT. When the voltage across R_POT becomes high enough to turn on Q2, then Q2 will begin turning off Q1, which will restrict current flow.

^{simulate this circuit – Schematic created using CircuitLab}

COMPONENT SELECTION:

• Q2 may be just about any PNP transistor that has a VCE_sat < 0.5V.
• Q1 must be a PNP transistor that is rated to handle several watts of power. Something like a TO-220 screwed to the appropriate heat sink (which could just be some sheet metal if you had no other option).
• R2 biases Q1 base. Something around 500 ohms to 1K is a good choice depending on the gain of Q1. Due to the limited current gain of Q1, R2 imposes an additional limit on the output current regardless of R_POT. It's a good idea to select R2 as high a value as will work reliably.
• R_POT selects the output current limit. The current limit is roughly 0.7V / R_POT. R_POT should be rated to handle around 1W to be safe.

Answer 5

Using a fuse is the cheapest & easiest way, you can find resettable & non-resettable fuses on local stores. I was able to find the in Hyderabad. You can also get them delivered to India via RS components.