Battery Load Test, constant current circuit help.

[Andytester]

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]

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]

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*)]

Be sure to read the (not yet complete) resource dealing with heatsinks.