# Minimal circuit for a current limiter for small dc voltages and low power

I remember to be able to build a current limiter for small dc voltages around ttl levels using a zenner diode, transistor and resistor and some math. I couldn't find one when browsing, how do I build one?

Suppose I have a 5v, 5W capable power supply, feeding a led lamp that takes 5V and 0.5A, and I want to limit the current as a small safety If I connect a different one that I have, that is 5V and 1.5A.

• Hope do you hope to use it? Please provide an example case to illustrate using a realistic situation. – jonk Mar 23 '19 at 23:39
• questiona about use of appliances are off topic, but why do you thing you need a current limit when using the 1.5A power supply if you don't when using the 1A power supply? – Jasen Mar 24 '19 at 2:04

It doesn't have your Zener, but I've always like this current limiter circuit:

simulate this circuit – Schematic created using CircuitLab

Very simple to use - just set $$\R_{sense} = \frac{0.6}{I_{limit}}\$$.

The tradeoff for that simplicity is (1) efficiency and (2) precision:

1. There's up to 0.6V of voltage drop across $$\R_{sense}\$$ in series with your load. In many low current limit circuits this is not an issue, but in power efficient circuits this will definitely be an issue.
2. There's a fairly vague transition region around the current limit, which is sensitive to the $$\\beta\$$ gain of the transistors.

But if efficiency and precision aren't high on your requirements list, it's a winner!

There are plenty more alternatives here: Current limiters using transistors and diodes

• It's a nice simple circuit, but when it's not limiting the current it still reduces the output voltage by one diode drop, so be aware of that. The limit current is also more complicated than just 0.6/ilimit, because Q1 and Q2 have finite β. – Hearth Mar 24 '19 at 0:04
• just a curiositym how do I measure the voltage in the Rload in circuitlab ? – eri0o Mar 24 '19 at 0:12
• Very good points @Hearth, will add them to the answer. – Heath Raftery Mar 24 '19 at 2:40