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Looks like I killed a mosfet today, its an irf3205 http://www.irf.com/product-info/datasheets/data/irf3205.pdf

The dc motor always goes on as soon as +12v and GND for the motor are connected, no matter if the gate is connected or not.

This mosfet wasn't my first choice but the shop I was visiting was out of stock on the TTL mosfet I originally wanted. A quick google turned up the irf3205...but I missed that VGSon = 10v.

I was driving it via a bc547 from a avr a short test with a 35ma 5v motor worked even without the bc547 so I connected it directly.... there's a shottky diode in parrallel to the dc motor to kill spikes after shutdown.

A windshield washer pump is turned on by the mosfet, connected like in this circuit (the part to the right of r2): schematic

The pump should take about 2a(found different sources for that) when running and of course a lot more when it starts.

I'm missing a RDSon graphic in the datasheet for different VGS voltages - shouldnt there be one?

The on time for the pump was at most for 30s...the avr code still got some hickups. Could this be enough with the not so good VGSon value to kill the mosfet?

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    \$\begingroup\$ Use the circuit tool to create a schematic of your circuit. \$\endgroup\$
    – Passerby
    Feb 21, 2016 at 23:13
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    \$\begingroup\$ I don't see any form of inrush current limiting on that motor circuit. No inrush current limiting usually means your transistor gets hit with full VDD across D-S momentarily (across the MOSFET's very low Rd-s, thus ridiculously high current & wattage), which can sometimes be enough to start a (very small) silicon-based barbeque. \$\endgroup\$ Feb 22, 2016 at 0:21
  • \$\begingroup\$ Given what can happen with gate charge (and that this isn't your circuit to the left of R2), connect the gate to ground - if the motor runs, it's broken. If not, you have met gate charge with no path to ground. A MOSFET can quite happily run with no connection to the gate - it can even preserve its state through power cycles. Can be an amusing phenomenon, if you are aware of it; Confusing, if not. Please do enter your actual circuit. \$\endgroup\$
    – Ecnerwal
    Feb 22, 2016 at 3:30
  • \$\begingroup\$ Figures 1 & 2 tell you what you need to know about how DS current varies with VGS and VDS at two different temperatures, which will let you compute RDS for a particular VGS if you want to know it... \$\endgroup\$
    – Ecnerwal
    Feb 22, 2016 at 4:15
  • \$\begingroup\$ thank you guys, indeed grounding the gate does stop the turning on when no avr is connected. i will add resistors like in this circuit to the gate: i.stack.imgur.com/Ce6Ud.png cant test that today, but will report back \$\endgroup\$ Feb 22, 2016 at 8:28

2 Answers 2

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So, based on your comment, and your as-yet-unknown circuit that isn't the one you posted a picture of, even if it's similar on the right side, you discovered the joys of gate charge on MOSFETs; where not connecting the gate can leave you with an active, conducting MOSFET since there happens to be voltage on the gate, and being a MOSFET, that voltage isn't going anywhere in a hurry (leakage might pull it down over a few weeks, or sooner if you put your finger on it.)

I strongly suggest some active playing with a simple MOSFET - resistor - LED circuit if you don't have this one well-ingrained in your head.

schematic

simulate this circuit – Schematic created using CircuitLab

Connect the gate, briefly, to + or - and then disconnect. Observe behavior. Turn power supply off, go to lunch, come back, turn power supply on, observe behavior. Learn that you need to actively shut a MOSFET off, as well as turn it on. The above circuit is assuming the more-common enhancement mode MOSFET which is off when the Gate-Source voltage is 0V.

The good news would be that you evidently haven't killed your MOSFET, and perhaps you learned something, too.

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schematic

simulate this circuit – Schematic created using CircuitLab

R1 keeps the mosfet closed when no signal is attached, without R2 even after days/weeks it can still be "active" and allow current between Drain and Source even if there is no signal from the uC. disconnecting the uC doesnt help, can still go on. if R1 is to small the current trough it is high and the voltage output of the uC goes down to support the current.

D1 to save the mosfet when the inductance drives a surge in the other direction after the normal current flow stops.

the mosfet driver can provide a bigger current for the mosfet because even if its always prised as a voltage only surge, when it turns on it can draw up to 1A(lost the source for that) of current. also it has to be fast enough for the PWM frequency. can somebody recommend a "common" mosfet driver that can suppport ~20khz?

im not so sure about:

C1 - to smoothen out yerks from the motor

R2 - to reduce current draw from the mosfet, because the uC can only deliver <20ma all all ports combined.

can somebody confirm that?

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  • \$\begingroup\$ If R2 is to limit draw on the uC, is should be between the uC and the driver, if any driver is needed. If it's needed to limit the load on the driver, then you'd need one where it is. Mostly what you'll want in a driver is logic-level input and an output that's near 10-12V, if you are using a 12V supply (as your text indicates) rather than the 1V you have drawn, so that your MOSFET is turned fully on. \$\endgroup\$
    – Ecnerwal
    Feb 23, 2016 at 16:33
  • \$\begingroup\$ youre right, changed the voltage and the resistor \$\endgroup\$ Mar 9, 2016 at 22:31

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