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Topic: [SOLVED} MOSFET continuously conducts (Read 8002 times) previous topic - next topic

SteveShaw

Sep 28, 2016, 05:24 pm Last Edit: Oct 02, 2016, 12:06 pm by SteveShaw Reason: Issue solved.
I seem to have a recurring issue with MOSFETS.

I'll make something up on a breadboard, it works, then build the project and solder everything together. Then at some point (not always immediately) my MOSFETs, rather than switching off with a gate voltage of 0, seem to continue to conduct current (just as if its a 50hm resistor). I'm using N channel devices with the Source to Ground, so the right way round.


I guess i'm some how killing them, but have no idea why any suggestions ?

raschemmel

Without schematics of your circuits there is little or nothing we can do to help you.
Arduino UNOs, Pro-Minis, ATMega328, ATtiny85, LCDs, MCP4162, keypads,<br />DS18B20s,74c922,nRF24L01, RS232, SD card, RC fixed wing, quadcopter

DVDdoug

There are 3 possibilities -

1. The MOSFET is defective (or burned-up).

2. It's wired wrong.

3. There is another current path.

raschemmel

Only 3 ?

What about "OP doesn't know enough about mosfets "
Arduino UNOs, Pro-Minis, ATMega328, ATtiny85, LCDs, MCP4162, keypads,<br />DS18B20s,74c922,nRF24L01, RS232, SD card, RC fixed wing, quadcopter

terryking228

Hi,
I've killed some NFETs lately and it appears to be very fast transients from a big 24VDC motor.

Subject is: Electromagnetic Interference and transient suppression.

What's your load??

You may need TVS transient suppressors right across your load.

Maybe there was enough inductance and resistance in your breadboard circuit to give some protection.

General info on transient problems  HERE:

Some other information  HERE:

Let us know what you find out...
Regards, Terry King terry@yourduino.com  - Check great prices, devices and Arduino-related boards at http://YourDuino.com
HOW-TO: http://ArduinoInfo.Info

raschemmel

I think it's a complete waste of time to continue to list all the possibilities under the sun when a simple schematic would clear everything up.
Arduino UNOs, Pro-Minis, ATMega328, ATtiny85, LCDs, MCP4162, keypads,<br />DS18B20s,74c922,nRF24L01, RS232, SD card, RC fixed wing, quadcopter

SteveShaw

#6
Sep 28, 2016, 06:16 pm Last Edit: Sep 28, 2016, 06:22 pm by SteveShaw
Here's the schematic - sorry its a drawing. Its the output side of a constant current load. I was getting oscillations on the output so took at the 1K to the gate of the MOSFET and replaced it with a direct connection Oscillations gone, the circuit works and provides constant current load .... but... now with a minimum of around 50mA and with the output off (i.e. PWM output of the ardunino at zero), then i still get a current of around 50mA through the MOSFET and the 2ohm load. Also disconnected the gate and connected it to ground - same result.

Could it be perhaps that in the soldering i've killed it ... (my soldering iron is not earthed - 2 pin connection)


Jiggy-Ninja

You may need TVS transient suppressors right across your load.
Hello, is this the Department of Redundancy Department?
Hackaday: https://hackaday.io/MarkRD
Advanced C++ Techniques: https://forum.arduino.cc/index.php?topic=493075.0

raschemmel

incomplete.
 Vin ? show EXACTLY WHAT is connected here and how
Arduino UNOs, Pro-Minis, ATMega328, ATtiny85, LCDs, MCP4162, keypads,<br />DS18B20s,74c922,nRF24L01, RS232, SD card, RC fixed wing, quadcopter

Jiggy-Ninja

incomplete.
 Vin ? show EXACTLY WHAT is connected here and how
"Incomplete" is putting it mildly, that's literally the most important part of the problem.
Hackaday: https://hackaday.io/MarkRD
Advanced C++ Techniques: https://forum.arduino.cc/index.php?topic=493075.0

raschemmel

Quote
"Incomplete" is putting it mildly, that's literally the most important part of the problem.
Incomplete is incomplete
Arduino UNOs, Pro-Minis, ATMega328, ATtiny85, LCDs, MCP4162, keypads,<br />DS18B20s,74c922,nRF24L01, RS232, SD card, RC fixed wing, quadcopter

Jiggy-Ninja

I know, I'm saying that was an understatement.
Hackaday: https://hackaday.io/MarkRD
Advanced C++ Techniques: https://forum.arduino.cc/index.php?topic=493075.0

raschemmel

#12
Sep 28, 2016, 06:47 pm Last Edit: Sep 28, 2016, 06:50 pm by raschemmel
The 2 ohm current sense resistor suggests a motor...

Why the OP is converting the PWM to dc is unclear
Arduino UNOs, Pro-Minis, ATMega328, ATtiny85, LCDs, MCP4162, keypads,<br />DS18B20s,74c922,nRF24L01, RS232, SD card, RC fixed wing, quadcopter

Jiggy-Ninja

The 2 ohm current sense resistor suggests a motor...

Why the OP is converting the PWM to dc is unclear
It's a constant current load, not a PWM driver. OP is using averaged PWM to set the current level instead of a DAC.
Hackaday: https://hackaday.io/MarkRD
Advanced C++ Techniques: https://forum.arduino.cc/index.php?topic=493075.0

SteveShaw

Sorry, Vin where i attach the device under test - to which i am applying the load to with the circuit - so far been testing with 5 and 10 v.

My question is more if there is something typical way i could be damaging mosfets so that they still function
 - but instead of high resistance when turned off they have a low resistance

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