# irf5210 mosfet always on

Hello,i want to use a p channel mosfet irf5210 as the attached circuit,but when I power the circuit the relay turns on and even if turn on/off the switch the mosfet keeps conducting.The Vgs(th)of irf5210 say -2 to -4 volts it means that to turn on the mosfet gate voltage must be below 10 volts right?

That is correct.
The output of an Arduino digital pin can be 0V, or 5V. Both are below 10V.

JohnLincoln:
That could be correct, but I can't see an attached circuit.
The output of an Arduino digital pin can be 0V, or 5V. Both are less than 10V.

hi,
I attached it now because it returned an error before.i am trying the circuit on a breadboard first to see how it works and then test it with Arduino.Why the mosfet turns on when I power the circuit with the switch open?

Southpark:
Relative to the terminal that they call 'the source terminal'.

yes but since my source voltage is 12volts the gate voltage must be below 10 volts.Is that correct?

nixtas:
yes but since my source voltage is 12volts the gate voltage must be below 10 volts.Is that correct?

You're right. To turn this device on hard, making the gate voltage 8 Volt or more LOWER than 12 Volt should do the trick. Eg.... when Vg (relative to GND, 0V) is 4 V or less. Maybe if Vg is 6 V or less, then could be ok too (ie. making V_sg (source to gate voltage) 6 V to 8 V should be ok for turning on.

Southpark:
You're right. To turn this device on hard, making the gate voltage 8 Volt or more LOWER than 12 Volt should do the trick. Eg.... when Vg (relative to GND, 0V) is 4 V or less.

ok,then why the mosfet turns on in my circuit.i have the circuit exactly as the diagram.i have five new mosfets and all behave the same.it should be of right?

nixtas:
ok,then why the mosfet turns on in my circuit.i have the circuit exactly as the diagram.i have five new mosfets and all behave the same.it should be of right?

Is it possible that you wired something incorrectly? Best bet is to have spare IRF5210, and wire that up on a breadboard with basic resistors - one for test load, and one for the current limiting of the switch current. And see if things work as expected. Test with a bench power supply and a switch, without arduino.

Another thing is.... did you order from a reputable supplier? Depends on where you got the devices from. If you ordered through a reputable supplier, then ok. But if from ebay, just never know if the item is fake or not..... until after enough testing is done.

Southpark:
Is it possible that you wired something incorrectly? Best bet is to have spare IRF5210, and wire that up on a breadboard with basic resistors - one for test load, and one for the current limiting of the switch current. And see if things work as expected. Test with a bench power supply and a switch, without arduino.

Another thing is… did you order from a reputable supplier? Depends on where you got the devices from. If you ordered through a reputable supplier, then ok. But if from ebay, just never know if the item is fake or not… until after enough testing is done.

thanks,all the test i made show that are n channel mosfets.I can’t remember where i bought them from its been a long time but they are fake.i have one more question,at maximum Vgs the specs write ±20 volts,so lets say i have 25 volts at source,i have to feed the gate maximum with 5 volts no less?if i ground the gate it will be over the limit?

It means that the gate should stay within 20volt of the source.

If the source of a P-channel fet is at 25volt (supply), you can't just ground the gate.
There are solutions though.

https://www.gammon.com.au/motors
Leo..

nixtas:
thanks,all the test i made show that are n channel mosfets.I can't remember where i bought them from its been a long time but they are fake.i have one more question,at maximum Vgs the specs write +-20 volts,so lets say i have 25 volts at source,i have to feed the gate maximum with 5 volts no less?if i ground the gate it will be over the limit?

Ok. Thanks. I assume you mean they are 'NOT' fake. In that case, just make sure your terminals (ie. pin out connections) are correct.

I reckon you may have inadvertently flipped this mosfet upside down, so that your drain and source are connected in reverse.

Southpark:
Ok. Thanks. I assume you mean they are 'NOT' fake. In that case, just make sure your terminals (ie. pin out connections) are correct.

I reckon you may have inadvertently flipped this mosfet upside down, so that your drain and source are connected in reverse.

No they are fake since they work as n channel mosfet.Thank's for your help

nixtas:
No they are fake since they work as n channel mosfet.Thank's for your help

Most welcome! Thanks for letting me know too.

Are you using an Arduino to drive that mosfet (in place of the switch on your diagram) or has this nothing to do with Arduino ?
If the mosfets really are N channel, then invert your circuit so the mosfets are on the low side, where one relay coil connection is to the +12 volt rail.
If you are driving a P channel mosfet from an Arduino pin and the mosfet is powered by a 12 volt supply, you need to make a level shifter (composed of a pull up resistor for the mosfet, and an npn transistor with base resistor).

6v6gt:
Are you using an Arduino to drive that mosfet (in place of the switch on your diagram) or has this nothing to do with Arduino ?
If the mosfets really are N channel, then invert your circuit so the mosfets are on the low side, where one relay coil connection is to the +12 volt rail.
If you are driving a P channel mosfet from an Arduino pin and the mosfet is powered by a 12 volt supply, you need to make a level shifter (composed of a pull up resistor for the mosfet, and an npn transistor with base resistor).

ok thank's

Hi,

I presume you are high side switching is because the relay is automotive (12V system) and connected to gnd.

Be aware that 12V auto systems can get to 14.2Vdc in charging conditions.

Tom...

Ok,thank you all for your replies.I upload two pictures,one is the original and one is the fake one I bought, in case someone else have same problems to not waste time searching what is wrong and maybe damaging further his circuit.

Wawa:
It means that the gate should stay within 20volt of the source.

If the source of a P-channel fet is at 25volt (supply), you can't just ground the gate.
There are solutions though.