# How to Build a Current Limiting Circuit

#### laserau77

Joined Feb 11, 2018
12
Hi Guys,

I have an application where I need to build a current limiting circuit to protect a circuit board.
It is a solar application. The existing circuit board can only handle max 1.6amps input but the solar panels used are delivering up to 4 amps.
I understand the easiest option is replace the panel with a smaller one. Unfortunately its a messy situation and this is not possible to do this so looking for a simple circuit that will do this.
I have found lots of circuits with MOSFETs online but not really sure how well they work.
The input voltage ranges from 20V to 28V from the panel and that is no problem. I just need to limit the current to max 1.6A
So lets say ideally it will be 24V 1.6A
Any ideas?

Something along these lines would be ideal. http://320volt.com/en/irf4905-ile-5-amper-akim-regulatoru/
The original article on this is in german, I tried translating but its not clear if I can change components to reduce the output current.

#### crutschow

Joined Mar 14, 2008
24,426
Here's the LTspice simulation of a simple circuit that limits the current to about 1.55A with the circuit values shown.
It has a voltage drop when not limiting, equal to the current times (0.45Ω + the MOSFET on resistance).

You can alter the limit by changing the value of R1. The limit is ≈ 0.7V/R1.
The value of R2 will also cause a small changes in the limit (lower value causes a higher limit).

The P-MOSFET can be just about any with the a voltage rating of 40V or greater and an on-resistance ≤0.1Ω.
It should be in a TO-220 case so you can mount in on a heatsink, since it will dissipate power equal to (Vin-Vout) x 1.6A when limiting (blue trace).

#### wayneh

Joined Sep 9, 2010
16,292
What is your load and why can't it control its own current draw? I mean, the capacity of the panel might be 4A but it's not going to provide that much current into a load that does not demand it.

#### laserau77

Joined Feb 11, 2018
12
Thanks Crutslow, that is perfect. Where did you find that calculator tool and software?
What is the wattage on the resistors? 1/4 or 1/2W?
Will the output voltage track the same as the input voltage minus the voltage drop. I am sure it does, just checking.

Wayneh, thanks for your response. You are correct, the board will only use the full amount of current when required.
It is only designed to handle a max current of 3.6A and quite often needs to delivery this current to charge a battery.

We have a new board in the design stage at the moment and is a month or 2 away from being ready.
So I need a way to limit the current in the mean time to stop any unnecessary failures.

#### crutschow

Joined Mar 14, 2008
24,426
Where did you find that calculator tool and software?
It's the free LTspice program from Linear Technology/Analog Devices.
What is the wattage on the resistors? 1/4 or 1/2W?
R1 should be 2W.
R2 can be 1/4W.
Will the output voltage track the same as the input voltage minus the voltage drop.
Yes.
Look at the red and yellow traces before it limits.

#### sghioto

Joined Dec 31, 2017
1,038
Another simple circuit is to use a voltage regulator configured as a current limiter. The LM338 would be ideal for this. So for 1.6 amps you need a .75 ohm resistor rated at least 5 watts.
SG
View attachment 148002

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#### crutschow

Joined Mar 14, 2008
24,426
Another simple circuit is to use a voltage regulator configured as a current limiter. The LM338 would be ideal for this.
But note that the drop across the LM338 circuit is about 2V plus the output current caused voltage drop across the 0.75Ω resistor.

#### sghioto

Joined Dec 31, 2017
1,038
But note that the drop across the LM338 circuit is about 2V plus the output current caused voltage drop across the 0.75Ω resistor.
I don't see that as an issue. He asked for a current regulator not a voltage regulator.
SG

#### crutschow

Joined Mar 14, 2008
24,426
I don't see that as an issue. He asked for a current regulator not a voltage regulator.
SG
True.
But at low solar panel output voltages that extra drop could be a problem.
Notice this question in post #4.
Will the output voltage track the same as the input voltage minus the voltage drop. I am sure it does, just checking.
That implies to me that he's concerned about voltage drop.

#### AnalogKid

Joined Aug 1, 2013
8,330
Without more information about the board that can "handle" only 1.6 A, the question in post #1 can't really be answered. part of it says current regulator, and part of it says buck-boost converter the t turns a variable 20 V-to-28 V input into a 24 V output.

???

ak

#### laserau77

Joined Feb 11, 2018
12
Thanks for the responses guys.
So the control board I am using at the moment can accept an input voltage between 20-28V
I am not too worried about a voltage drop, but the less drop the better.
The Solar Panels I am using are 24V but have Voc of around 30V.
I will need to check exactly what the voltage is under load.
The Battery voltage that it is charging is 16.8 Volts.

So yes, I am looking for a solution that will limit the current to a max of 1.5amps.

Your first comment AnologKid about the circuit only limiting inrush current etc. Are you talking about the link I shared or the information crutschow shared?
Will Crutschow circuit only stop inrush current as well?
From the screenshot he shared with the Vin, Vout, Max current etc it looks to continuously limit the current.

#### laserau77

Joined Feb 11, 2018
12
Crutschow you said:

The P-MOSFET can be just about any with the a voltage rating of 40V or greater and an on-resistance ≤0.1Ω.
I found this P- MOSFET, would it be suitable? The R On is 0.14 not 0.1 as you suggested.

Element 14 have a lot of these in stock so I would be able to get them the next day, other options would take over a week and I am hoping for something made by the end of this week.

#### crutschow

Joined Mar 14, 2008
24,426
Will Crutschow circuit only stop inrush current as well?
No, mine will give a steady limit.
AK was referring to your first reference about a current limit circuit you found.
Crutschow, would this P MOSFET be suitable? Only thing is the R on resistance is 0.14ohm.
You said it must be less than or equal to 0.1
http://au.element14.com/vishay-siliconix/irf9z34pbf/p-channel-mosfet-60v-18a-to-220/dp/1351211
That was not a hard limit and I was probably unnecessarily conservative.
0.14 will give a drop of 224mv @ 1.6A which would add to the 728mV drop from R1, for a total of 944mv @ 1.6A.
I assume that's acceptable?

#### laserau77

Joined Feb 11, 2018
12
No, mine will give a steady limit.
AK was referring to your first reference about a current limit circuit you found.

That was not a hard limit and I was probably unnecessarily conservative.
0.14 will give a drop of 224mv @ 1.6A which would add to the 728mV drop from R1, for a total of 944mv @ 1.6A.
I assume that's acceptable?
Yes that is fine.
Really appreciate your help. I thought yours looked like it would work fine.
I have ordered the parts so they should arrive in a few days.
Thanks again.

#### BobTPH

Joined Jun 5, 2013
2,266
There is something wrong with your description of the problem.

If the the board is a battery charger designed to operate on 20 to 28V, it should be controlling the current, abd it should not matter if your voltage source can supply more current than it needs.

If it is not controlling the current, then it a stretch to call it a battery charger.

Is this a problem that you have observed, or one that you have imagined and are trying to prevent?

Bob

#### Bordodynov

Joined May 20, 2015
2,504
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#### crutschow

Joined Mar 14, 2008
24,426
What determines the value of the current limit in that circuit?

#### AnalogKid

Joined Aug 1, 2013
8,330
R1. As the current in R1 increases, the voltage at the base of Q1 decreases, turning it more "on" and pulling up the M2 gate, turning it more off.

ak