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Topic: FET selection - parallel use - overheating (Read 2400 times) previous topic - next topic

geoland

I moved the series 100R on the PWM pin to the camera side and the FET gate is going down without a pull down resistor - go figure?

Using only the laptop usb to power the board temperature drop was very slow and could not make 25 delta from ambient. Applying 12v from the wall adapter temperature shot through set point. Reducing to 6v maintained temperature accurately. I can work with this. Thanks for all the help.

dc42

Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

geoland

#17
Jan 17, 2012, 07:06 pm Last Edit: Jan 17, 2012, 07:35 pm by geoland Reason: 1
Thanks. I notice that powering the board with USB the FET will switch. If I then power the board using a 6v supply, and providing USB power is not removed the FET will continue to switch. Under the same condition an increase in power supply to 12v does not affect the FET and it will continue to switch.

However, if 12v is used first the FET will not switch, even if the supply voltage is subsequently reduced to 6v. That is, the reverse of the condition above.

If supply to the board is > 6v the FET will not switch. But the FET will switch at any voltage providing the first voltage does not exceed 6v.

I have reproduced this behaviour twice. I think pull down resistor is the only reliable option.

Also, unless 12v is applied to the board, the rate of cooling is quite slow and the temperature differential is reduced.

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