# current limiter

#### Emesine

Jun 19, 2017
17
Hello!

I am filling a 12V, 5 farad capacitor array. The design of my power source requires that I limit charging current to approximately 80 mA.

I am looking for a simple solution for a current limiter. I'm considering the attached design based on the lm334.

I'm pretty sure this circuit will work, but I think it will oscillate. I'm interested in suggestions as to where to place a capacitor or resistor to limit oscillation. Thanks!

Andrew

#### Attachments

• limit.jpg
148.4 KB · Views: 6

#### crutschow

May 7, 2021
863
Below is the LTspice simulation of a simple, 2-transistor circuit that should not oscillate and should do what you want.
It uses the base-emitter voltage of Q1 as a reference for the current limit.
The limit is about 0.65V / R3.

The load current increases until the voltage drop across R3 starts to turn on Q1.
Q1's collector current then raises the voltage across R1 until M1 starts to turn off and limit the output current to maintain the drop across R3 equal to about 0.65V.

As the simulation shows, the source current (green trace) stays constant as the capacitor charges (yellow trace).

The PNP and P-MOSFET types are not critical and can be just about any devices you have.

Note that it takes over 13 minutes to charge the cap with an 80mA charging current.

Last edited:

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,799
The LM334 is a programmable current source and is imho not the right component for this purpose.

#### Emesine

Jun 19, 2017
17
Thanks Crutschow. I like your schematic and will incorporate it in my design.

My son is building a base station for launching model rockets. It needs to produce enough power to fire a 12V model rocket igniter using the 5V usb power supply from a laptop computer. I'm using a 1 watt boost converter to fill supercapacitors at 12V. Works beautifully, but will try smaller caps next time. At my first trial the capacitors lost ~15% of their voltage during a 1 second firing, and the igniter was fully aflame after ~140 ms. I'm also thinking about using a larger boost converter, though I am limited by the 500 mA available from my computer USB power supply.

Again, thanks for the schematic. Really helps.

Harald, lm334 officially scrapped!

Andrew

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