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What amI doing wrong? MOSFET + RELAY + LED

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active_si

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I have the circuit as attached and the functionality is as follows:
- by default the relay is turned ON
- by default the LED doesn't light
when "CONTROL" is pulled down the LED lights up and the RELAY should turn off.

The problem that I have is that when I put some load on the place of relay the voltage drops quickly. For example if I put a LED instead of the relay the LED will light up but will graduately turn off very quickly.

What am I doing wrong?

thank you for any help resolving this
 

It looks like the voltage that drives the MOSFET's gate is to low ..
Digital MOSFETs (those designed to by driven by low-volt logic) like to be driven from roughly 4-5V, whereas standard MOSFET shoud have the U[GS] close to 10Vdc ..

Try to add 1kΩ in parallel with [LED+R13], see attached drawing ..

Rgds,
IanP
 

So if I'm reading the datasheet right for the MOSFET I'm using (MMBF170LT1G) the GATE voltage should be 20Vdc?

Should I use a NPN transistor instead of the MOSFET to work as I want to?

Thanks again!

Re, Uros
 

So if I'm reading the datasheet right for the MOSFET I'm using (MMBF170LT1G) the GATE voltage should be 20Vdc?
Click to expand...
If the data sheet says so - it's right ..

BJT should work ..

Here is the voltage budget:

3.3V - U[led] - U[R13] ≥ 0.7V --> that is U[BE]

In any case, connect a parallel resitor to help to drive, this time, the Base ..

Rgds,
IanP
 

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    active_si

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active_si said:
So if I'm reading the datasheet right for the MOSFET I'm using (MMBF170LT1G) the GATE voltage should be 20Vdc?

Should I use a NPN transistor instead of the MOSFET to work as I want to?

Thanks again!

Re, Uros
Click to expand...

No, I think you're reading the datasheet wrong. The 20v specified is the maximum you can apply. If your Vgs goes above 20v, your MOSFET will burn. This is usually for most MOSFETs. The voltage Vgs should be between 8v to 12v. There should be a graph in the datasheet showing how much Vgs you require for a specific amount of current. Check that.

I think, if your voltage is limited to 3.3v, you should use NPN transistor instead of MOSFET.

Hope this helps.
Tahmid.
 

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