[SOLVED} MOSFET continuously conducts

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 ?

Without schematics of your circuits there is little or nothing we can do to help you.

There are 3 possibilities -

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

  2. It's wired wrong.

  3. There is another current path.

Only 3 ?

What about "OP doesn't know enough about mosfets "

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...

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.

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)

terryking228:
You may need TVS transient suppressors right across your load.

Hello, is this the Department of Redundancy Department?

incomplete.
Vin ? show EXACTLY WHAT is connected here and how

raschemmel: 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.

"Incomplete" is putting it mildly, that's literally the most important part of the problem.

Incomplete is incomplete

I know, I'm saying that was an understatement.

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

Why the OP is converting the PWM to dc is unclear

raschemmel:
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.

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

How's your heat dissipation? You're going to have 2.5A max going though that MOSFET and it's going to be taking all that heat. If you don't have good heat sinking you're going to toast it.

Do you have a large enough heatsink with a good thermal connection to the MOSFET?

Can LM324 pull all the way down to 0V?

Only with a +/- gnd referenced supply such as a +12 -12 GND supply. It cannot go “rail to rail”.

An IRF520 is an enhancement mode device, and should be pretty well off with 0v g-s

Would need 8ish volts to turn it on hard.

Since it's being used in an open loop linear mode, it'll probably get pretty hot - but unlike a bipolar that should try and shut it down..

OP - there's something else going on - more details of the load please - or is the 2ohm the load and you're strapping the drain directly to +ve?

The LM324 will pull down to a few mv, but won't sink much current - a 10k from g-s may help here

regards

Allan

It is possible that that the MOSFET is being damaged during soldering. You really should be using a grounded iron. It is not real difficult to add a ground wire. The FET has a reasonably large input capacitance but they can be damaged. Even with a grounded tip, install the MOSFET last. If the voltage of the opamp output is zero than I'd say your looking at a leaking gate. Your inputs for the 324 are not well balanced on impedance. As has been noted, the 324 does not do well near the negative terminals level. Have you measured across the 1K resistor? There should be zero volts. Any voltage at all is an indication of a blown FET. Lastly, you should have a method of clamping the voltage into the analog input. This is especially true with your lack of filtering. Pulses could easily exceed the input spec for analog in. Dwight