Electronic – Simplest, cheapest, fast and minimum footprint current limiting circuit with low resistance in normal state


I have a digital output, driven by the high-side driver with nominal voltage of 24V DC. The load current normally is below 100 mA. The output is monitored, so I can switch it off quickly if I detect a short circuit at the load side.
The problem is that the driver itself is not protected and short-circuit makes it generating a lot of smoke. So what I need is a simple circuit at the output of the driver which:

  • has low resistance of below 10 Ω if output current is under 100 mA
  • rapidly increases its resistance to limit the driver current at 500 mA level or lower
  • withstand capability at short-circuit current shall be at least 20 ms for short circuit to be detected and driver switched off
  • has a working voltage of 50V or higher
  • has minimum components and cheap (0,20$ per channel max)
  • is not single-source supplier

I tried PTC resettable polyfuses, but they are too slow. Microchip's FP0100 should be good but it's expensive (I need at least 60 channels on my PCB). Bourns TBU series are also OK, but also expensive.

Any other options?

UPD1. My current output circuit is MIC2981/82 driven by 74HC594 shift register. On each output I have Littelfuse 1206L012 PTC.
On my board I need 64 channels like this, and this is small series board so total price per channel and footprint are important.

Best Answer

Your typical double transistor current limiter may be your best bet. Shown below is the top-side and bottom side versions.


simulate this circuit – Schematic created using CircuitLab

Note there is a penalty of about a volt drop with this circuit.

Buy dual transistors in a single 6 pin package.

The small resistor will cause the current to fold back when it reaches Vbe. The other resistor sets the base current and needs to be calculated to produce sufficient collector current taking into account Hfe.

HOWEVER: Be aware that transistor needs to handle a few watts for the duration of the short since it only limits the current to your threshold value.