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Basic Electronics » mosfet control during start up

May 22, 2017
by scootergarrett
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So I have a circuits question but its tied to a NerdKit. I need the MCU to control high side 12V switching. So I made this circuit; it works great, the only problem is then when I power up the circuit. It takes 68ms for the MCU to become active and in that time the 12V output has been high. Here is a scope capture line colors go with circuit. I need the output to be low during start up, any ideas how to change the circuit? I have already designed and populated a custom PCB and am now descovering this issue; wrong time in the development curve, oh well.

This is going to connect to a starter relay on my motorcycle, so it cant send power to the starter when I power up or it may lurk the motorcycle forward.

May 22, 2017
by esoderberg
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scooter,

Try putting a pull down resistor on your NFET gate so it is off by default until your MCU drives it high.

ES

May 23, 2017
by scootergarrett
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YEP pull down resistor. I just need to add a resistor as shown. Here is the new scope output. the 2N7000 mosfet is a surface mount; so fortunatly a 0603 resistor fits perfectly on between the legs, no new board needed! I'm thinking a 500Ohm resistor, which will only pull 13mA between the new resistor and the LED, below the 20mA a pin can source.

THANKS

May 24, 2017
by esoderberg
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Thanks for the update. It's nice to see it work out.

June 08, 2017
by BobaMosfet
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scooter,

Why don't you have a resistor on your gate on the 2N7000? Without a current-limiting resistor, the output pin will output max possible allowed current from the MCU (gate is in parallel with your pull-down, but nothing limits current to gate).

A pull-up/pull-down resistor usually only has to pull 1mA in the desired direction for a pin in the non-powered state, as this is sufficient to drain parasitic voltage, and is easily overcome by stronger resistor. Limiting to 1mA, allows more current for other MCU pins, and less overall power-drain on that specific pin.

I didn't know if this was by design, or something not realized. Thought I'd mention it for any future productions. It probably won't really affect your objective at this point, because you have an actual on/off switch which prevents a constant drain when the bike is 'off'.

Nice job, nonetheless. Application of electronics is the best way to learn, explore, and retain it-- causes one to solve 'real' problems they need a solution for. ;)

BM

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