# help make a high current (20A 12v) current limiter?

Discussion in 'The Projects Forum' started by dacrazyazn, Apr 19, 2010.

1. ### dacrazyazn Thread Starter Member

Mar 30, 2009
22
0
Hi guys,

Im trying to build a current limiting circuit that can withstand up to 20A. I looked around and the max is up to 10A only.. Im an ee minor and know a little about circuits but not enough to tackle this on my own.. Is there a place that i can start?

My input is going to be a car battery at 12v. and my goal is to limit the load from pulling more than a specified amount of current. (variable current limiter)

I am willing to learn. But need a small push.. Thanks!

Jul 17, 2007
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3. ### Externet AAC Fanatic!

Nov 29, 2005
1,231
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Last edited: Apr 19, 2010
4. ### DickCappels Moderator

Aug 21, 2008
5,017
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And, how much voltage can you stand to loose across the current limiting circuit?

5. ### kubeek Expert

Sep 20, 2005
5,297
996
Have a look here, http://forum.allaboutcircuits.com/showthread.php?p=95034&postcount=11 , this is my approach to a current limiter. I never tried it in an actual circuit, but in simulation it works like a charm.
X2 with X4 make a differential amplifier, and X3 with D1 create the limiter.
The inductor L1 is used for simulating R5 as a wire-wound resistor and does not belong to the actual schematic.
The mosfet M1 is conducting until the voltage drop at R5 exceeds the preset current, then it starts to slowly close to maintain the current.

For your needs the R5 should have lower value, probably 0.01Ω, because 2V drop could be a little to much. Then you will have to change either the gain of the diff amp, or the range of the potentiometer.

6. ### dacrazyazn Thread Starter Member

Mar 30, 2009
22
0
My load will be Batteries, i'm trying to charge A123 batteries from 10-20A from a car battery through a zip cord. I know i can do a cheap limiter by using something like a lamp cord, but i want it to be a variable limiter.

the voltage drop should not be more than about 1 volt from the source. i need it to be above my battery nominal voltage of 9.9, and full charge of about 11.1v.

I will definitely look into the workings of kubeek's schematic..

But instead of taking something someone already made. I would love to understand how i can start to design something like this from scratch. Thanks for your replies guys!

7. ### ifixit AAC Fanatic!

Nov 20, 2008
650
118
Hi,

Will the car battery be under charge itself at 14V or so? If not, you don't have much headroom to work with from a 12V car battery to 11.1V A123 is 0.9V.

Design procedure to get you started

1. Write down the specifications in detail, then design/build to that.
2. Calculate the worst power dissipation scenario. E.g. Car battery fully charged to A123 at 9.9V = 12.6  9.9 = 2.7V at 20A = 54W.
3. As the battery nears full charge the current will likely have to be less than 20A. Is this okay?
4. Select a MOSFET that could bridge the gap between the 2 batteries and control the current without failing. Id min =>20A, Rds(on)=<0.01Ω, Pchan. A power transistor could do the job better if there is enough headroom.
5. Decide how to sense the current with minimal voltage drop. A 0.01ohm resistor? 0.01Ω x 20A = 0.2V.
6. Design an electronic circuit that takes the voltage from the current sensor, your setting requirements, the A123 voltage, and then drive the MOSFET to regulate the current until the termination voltage is reached (11.1V).
7. The control circuits will be powered from the car battery?

Get back to us when you have questions, or have a design to be inpected.

Regards,
Ifixit