Battery Load Test, constant current circuit help.

Andytester

Aug 14, 2014
10
Hi All

This is my first post, hope it makes sense. I'm trying to test some 26-28v batteries for load capacity and I'm hoping to use this simple circuit to provide a 10A DC constant current load. I've used this circuit successfully for a while testing some 7v NiCd Battery's at 5A, but if I attach a 27v battery I blow the mosfet. I'm just a novice at electronics so can only use very basic maths but I'm wondering if I need to add protection the Gate of the Mosfet, or add an extra Mosfet as per dotted line?
As I understand it the Mosfet will take 30A and accept up to 20v on the gate so not sure what I need to do to make this survive a 27v load.

Thanks

successfully

Harald Kapp

Moderator
Moderator
Nov 17, 2011
13,067
Welcome to our forum.

At 10A, the "heat dump" resistors will develop a voltage of 0.7Ω*10A=7V. Therefore the rest of the voltage 21V (28V-7V) will drop across the transistor(s). This evaluates to a total power of 21V*10A=210W. The sihf22n60e is rated for max. 35W /datasheet) and is therefore heavily overloaded. You need to use multiple transistors (or a transistor rated for higher power dissipation) and a proper heatsink or, much simpler, use bigger load resistors e.g. 2.7Ohm - which then will will have to burn the excess power as heat.

Andytester

Aug 14, 2014
10
Thanks Harald

Your answer make sense, thanks for your help. I'll keep the original circuit for low voltage use and construct a new circuit using much bigger (1000w) FET's. I'll also add a bigger heatsink and some switchable load resistors. I should have a universal load for multiple jobs once its complete.

Cheers

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,508
Be sure to read the (not yet complete) resource dealing with heatsinks.

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