# Inrush current limiter

Joined Jan 11, 2012
38
Hey, I'm trying to design an inrush current limiting circuit to limit a 24 volt dc source to 6-7 amps max. I've been working with the following attached circuit, where I have adjusted R2 to be .1 ohm using the equation I(max)=.7/R2.

For some reason though when I try to increase the load on the circuit higher than around 4 amps, it starts to limit the current already and reduces the voltage. Any ideas? Perhaps I need different diodes?

#### Attachments

• 32.9 KB Views: 185

#### jimkeith

Joined Oct 26, 2011
540
I have done this one before and found that the voltage drop across R2 is approx 0.5V or so. This depends upon the diode bias current, type of diode and size of power transistor--many variables. In your case, 0.4V sounds reasonable.

0.4V /0.1Ω = 4A.

Yes, it is limiting and you cannot expect more current unless you decrease R2 to perhaps 0.05Ω or so.

Reduce R1 to about 470Ω also to assure adequate base drive for the transistor while having enough extra to bias the diodes adequately--this alone will increase the output current slightly.

Note that when limiting the current, the power transistor dissipates significant power.

Joined Jan 11, 2012
38
Thank you I will try this and get back to you asap.

Joined Jan 11, 2012
38
Ok, well changing the 2 resistor values doesnt seem to help... It still starts limiting the current around 3.3 amps. I'm using an E-load to test this and I want the power supply this circuit is going on the end of to go into over current protection before the circuit starts to limit the current. The circuit is only to limit the current during the intial inrush spike of ~44 amps.

#### jimkeith

Joined Oct 26, 2011
540
What is an E-Load? Is it PWM?

Joined Jan 11, 2012
38
Its an electronic load I am using for testing. The OCP on the power supply kicks in around 5-6 amps and when I gradually increase the load up to that point, the circuit starts to attempt to limit the current around 3.3 amps as I can see the voltage dropping on the output and the transistor starts to get hot.

Joined Jan 11, 2012
38
Is there a way to do this with a relay and a resistor? It must go in line with the power supply and when the power supply is connected to the load, take the inrush current through say the resistor and then switch so that its not in the circuit anymore

#### jimkeith

Joined Oct 26, 2011
540
Thinking about it again--it appears that your bias resistor (R1) is far too high.

Assuming the hFE is 20, Ib = 4A/20 = 200mA.
Adding another 50mA to bias the diodes, the total current through R1 = 250mA.
Therefore, the bias resistor must be 100Ω.
P=EI = 23V * 0.23A = 5.3W. Use a 10W resistor.

So try it again with a 100Ω resistor.

If you do not want to dissipate this much power all the time, I can suggest a relatively simple solution.

Joined Jan 11, 2012
38
The simpliest solution the best lol I've tried a thermistor but the problem is the thermistor gets too hot after running for a while.

#### jimkeith

Joined Oct 26, 2011
540
You can use an Amatherm inrush limiter thermistor--talk to the application engineer there--they are very helpful.

I can also suggest an improvement to your circuit that will reduce standby power.

Joined Jan 11, 2012
38
I've tried an NTC thermistor in series with the output, but it gets too hot for our application.

also, put in a 100 ohm resistor, the transistor isn't getting quite as hot, but its applying some current limiting.

Last edited:

#### jimkeith

Joined Oct 26, 2011
540
It dissipates approx 3.2W at 4A because it has a voltage drop of approx 0.8V.

A more sophisticated circuit using an op amp and MOSFET would drop only 0.1V or so and run cool. Are you up to trying something like that?

Joined Jan 11, 2012
38
Sure, I would be willing to do it. Do you have a schematic that would work for this application? Thanks alot for your help man

#### jimkeith

Joined Oct 26, 2011
540
OK-- will get back wintin a few hours--must modify an existing circuit to general purpose applications like yours--was originally used to isolate two batteries that drew substantial current when tied together--what is your application?

Joined Jan 11, 2012
38
I have a 24 volt 2.2 A power supply that sparks when its plugged into the device on the DC output end. There is an inrush of about 54 amps when first plugged in which is causing the sparking. I want to have a solution that can be placed on the DC output cord essentially to initially reduce the current inrush.

#### jimkeith

Joined Oct 26, 2011
540
A better solution solution might be to replace the power supply with a modern switch mode power supply that has better current limiting. Is this possible? You can use one of the power supplies selected by Night-Eagle in his thread:

#### jimkeith

Joined Oct 26, 2011
540
See thumbnail--do not substitute op amp as this one has a common mode voltage range that extends to the positive rail.

If the output does not need to be grounded, everything can be inverted--it gets cheaper and more robust.

#### Attachments

• 27.6 KB Views: 153

Joined Jan 11, 2012
38
Awesome man I'm gonna give this a try. And the output doesn't need to be grounded, what do you mean by "inverted"? (Sorry been a long night)

Joined Jan 11, 2012
38
Also are these .1 and .01 uF caps?

#### jimkeith

Joined Oct 26, 2011
540
I'll see if I can draw an inverted schematic--yes these are 0.1 and 0.01uf caps.