12V output with mosfet

[Tuppe]

Hello,

I have a problem with 12V output from MOSFET to external device.

I need to have 12V output from my MCU, that is used as a signal to switch on a external device. The signal will be grounded within the external device(mutual GND)

The problem is, that I cannot find any circuit online that is used for this. Every circuit grounds the MOSFET and supplies 12V through it.

I cannot ground the MOSFET, because the voltage is going through the external device to GND. I got 12V on both Drain and Source.

My circuit works fully if I bypass the mosfet with a jumper, but I cannot electrically switch it ON, possibly due that I cannot ground it without creating 12V short to GND.

How can I make MOSFET to apply 12V to open lead? IS it possible? Do I really need a relay? I might have a brainfart here, but I seem unable to find solution.

Thanks!

 

[Audioguru]

You make no sense whatsoever.
1) You need a 12V signal from a Mosfet. Is it 0V most of the time then it goes to +12V when it signals?
2) Do you want the Mosfet to make the 0V part or the +12V part of the signal?
3) Is the +12V signal "forced" to ground in the external device? Why and how?
4) A voltage cannot go through a device to ground because then it has no voltage. Maybe a current goes through the external device to ground?? How much current?

Do you want to simply power an external device sometimes with +12V and want to use a Mosfet to do it?

 

[Tuppe]

You make no sense whatsoever.
1) You need a 12V signal from a Mosfet. Is it 0V most of the time then it goes to +12V when it signals?
2) Do you want the Mosfet to make the 0V part or the +12V part of the signal?
3) Is the +12V signal "forced" to ground in the external device? Why and how?
4) A voltage cannot go through a device to ground because then it has no voltage. Maybe a current goes through the external device to ground?? How much current?

Do you want to simply power an external device sometimes with +12V and want to use a Mosfet to do it?

Hello Audioguru, thanks for responding.
Sorry for not making sense.

1. Yes. 0V normal, 12V when activated. I can switch this around from the MCU code, so that's not a problem.
2. Mosfet should connect 12V to one activation pin. With N-Channel Mosfet I'm able to ground the pin, but I cannot "send" voltage to it.
3. I'm not sure. I need to feed 12V to a specific pin to activate the external device.
4. Yes, current goes in. It's not shorted. Only 20-50mA.

5. No. The device is powered all the time, the activation pin is only 1 pin that need +12V signal.
If I had 2 wires available, this would be a piece of cake. I only have activation pin that needs 12V signal. I'm not sure what it does inside, but it probably switches a transistor and signal goes to IC.

I forgot to mention that I have N-Channel Mosfet. Could I achieve this with P-Channel Mosfet only?
I could do this with a relay very easily, but for 20-50mA that would be crazy...

Problem said in other words:
I need to pass 12V signal to one activation pin. Just like with relay.

 

[Audioguru]

An N-channel Mosfet pulls to 0V when it is turned on. A P-channel Mosfet pulls to +12V when it turns on which is what you want.
But a P-channel Mosfet has its source pin connected to +12V then its gate must be +12V to turn it off and must go to +7V or to 0V to turn it on. Then you need a level-shifting transistor to convert the 0V to +5V from the MCU to +12V and 0V.

 

[Tuppe]

Thanks for the clarification.

P-channel Mosfet pulls to +12V when it turns on which is what you want.

Exactly! If you have any other ideas for this, I'd like to hear more. I can only come up with transistors, mosfets or relay.
Would PNP-transistor work the same as P-channel mosfet, as long as current rating is sufficient?

But a P-channel Mosfet has its source pin connected to +12V then its gate must be +12V to turn it off and must go to +7V or to 0V to turn it on. Then you need a level-shifting transistor to convert the 0V to +5V from the MCU to +12V and 0V.

Really? There is no 5V controlled P-Channel Mosfets?
This says -2 to -4v gate voltage:
https://www.adafruit.com/datasheets/1794datasheet.pdf

But that wouldn't be a problem. I can whack in transistor controlled mosfet easily.
I just have no experience with this kind of application with P channel components.

 

[FvM]

Controlling a 12V PMOSFET switch from 5V logic needs a level converter, independent of gate threshold voltage. As explained, this can be achieved by a single transistor (and a few resistors), actually not a big thing.

An alternative solution can be an integrated high-side switch with built-in level converter. They also offer features like short circuit or overload/overtemperature protection.

 

[Audioguru]

Would PNP-transistor work the same as P-channel mosfet, as long as current rating is sufficient?

A PNP transistor has the same level-shifting problem because its emitter is at +12V and its base must also be +12V to turn it off. But your MCU output only goes as high as +5V.

Really? There is no 5V controlled P-Channel Mosfets?

Of course you can connect the source pin to +5V but then when it is turned on your 12V load gets only 5V, not 12V by having its gate driven to 0V.

This says -2 to -4v gate voltage: FQP27P06

That is its "threshold" Vgs voltage when it is barely turned on conducting only 0.25mA. It needs a Vgs of -10V to be completely turned on. With its source pin at +12V its gate must also be +12V for it to be turned off and must be 0V to +2V for it to be turned on.

 

[D.A.(Tony)Stewart]

P type are used for High side switches with ground input ...
.. unless a boost gate voltage is available (>>Vdd) as N type tend to be lower RdsOn. ) just as NPN's are lower Rce than PNP
...Otherwise N type are used for Low side switch.

A variety of Vgs gate thresholds exist as well as RdsOn and Vds_max giving rise to >10k variety of part numbers.

Automotive grade Smart High Side switches are often best choice with logic level drive for 12V High Side Switches.

The other factor with FETs is gate capacitance rises as body diode has to handle same current as lower RdsOn so gate current becomes significant in PWM modes of switching just like Bipolar base current except FETS tend to have lower RdsOn than Bipolar Rce_sat. for similar cost.

 

[Tuppe]

Thanks for all the info!
I need really to play around with those components and study more to understand this.
I didn't know the term "High side switch", but now I can find info with that term.

I solved this problem with a relay due to resources atm, but I will get familiar with those switches too.

Thanks again for all the explanations and help!