Latching Current Limiter (resettable fuse if you will!)

Posted by on April 8, 2010

As my first electronics-related entry, I would like to share a modification of the “current limiter add-on” by Ron J. The original circuit works as advertised, however I needed to disconnect the load immediately after the maximum current was reached, what I needed was pretty much a resettable fuse.

The following schematic depicts this concept:

With only a few extra components you can make a trusty yet simple electronic fuse.

( This schematic is also available in PDF: LatchingCurrentLimiter )

Should you attempt to draw over ~500mA, the circuit will disconnect the load. Even if the load no longer attempts to draw half an amp, it will remain disconnected until the reset button is hit or the load is physically reconnected.

BC337 was chosen in this case but any general purpose or medium power transistor will suffice as long as it can handle at least ~150mW (worst case scenario someone glued the reset button and Q1 blows). All resistors 1/4W 5%, the shunt resistor should be at least 1/2W and 1%.

(If you can’t find 1 Ohm resistors, make your own by winding thin magnet wire onto a 1 meg resistor)

The latching behavior is possible thanks to Q2. It’s job is to pull the base of Q1 to ground, this effectively allows us to turn the transistor off. Now that it ceased to conduct, the “OUT-” lead is no longer tied to ground, therefore the LED is allowed to conduct. R5 will now provide enough base current for Q2 to remain active. Discharging the capacitor through the reset button unlatches the circuit. ie. Now that R5 is tied to ground, the base of Q2 no longer receives any current and therefore Q1 conducts again.

I must thank the regulars at ##electronics on for the invaluable help provided!.

If you find any mistakes or would like to enhance the circuit, feel free to do so! Keep in mind that I’ve successfully built and tested this; otherwise I wouldn’t be posting it 🙂