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# Current monitor/sensor

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Hello,

ia have one question regarding heatsink temperature, so i have two IRFP460 mosfet on pretty big heatsink en testing on 4A current and i measured temperature on heatsink after one hour arround 80 degrees, so high. I made calculation and says:

P_Dmax=5A*(12V-(5A*0.1Ω)-5V)=5A*6.5V=32.5W

So max power dissipation is one transistor is 32.5W, thermal resistance THj-c is 0.45C/W. why then so high temperature, i select pretty bigger heatsink that it sould be regarding my calculations??

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Simple arithmetic error.

5A x 0.1 ohms= 0.5V. 12V - 0.5V= 11.5V. 5A x 11.5V= 57.5W.The heatsink thermal resistance might be 1 degree C per Watt and the thermal grease might be 0.5 degrees C per Watt. The ambient might be 30 degrees C. Then the heatsink will be very hot and the chips inside the Mosfets might be hotter than allowed.

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Yes, but what with 5V voltage drop on 0.5E dischatge resistor? Is this nedd to take into account, or? Now i tasted with 4A, so 2A only through each mosfet and i measured 75degree per C on heatsink so from calculation is junstion temperature is allowed range.

Is that not truth?

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The datasheet for your Mosfet will tell you its maximum rated temperature and its thermal resistance.

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• 3 weeks later...

You didn't say if the current is AC or DC, and you didn't say how much current or voltage.

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• 3 months later...

Hi, you mentioned that you wanted to build a current monitor/sensor like the voltage monitor. I have some information for you and I really hope this will help you.

It is that Monitor-Current/Voltage product. You can check my link to see the detailed description of the product.

Hope you find the solution soon.

Lulu

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