I have an N-channel logic level FET, IRLZ44, that I need to turn on a DC motor. However the triggering signal is only about 2.3 Volts.
This is not enough to drive the gate directly so I need a circuit to do this. I found this transitor drive circuit (attached). However it
warns you that this will turn the FET off when voltage goes up and ON when voltage goes to zero. How can I invert this action so
that the FET turns on when input voltage goes to 2.3V? Use a PNP transistor? If so how would that circuit look?
The second question: Is it OK if the 12V used for the gate transistor is the same 12V source that will power the motor?

 

You could use a P-channel MOSFET and swap the positions of the load and the MOSFET.
You could add an extra NPN transistor duplicating the circuit around T1 on the input so you have a double inversion - two negatives equal a positive.

 

You could use a P-channel MOSFET and swap the positions of the load and the MOSFET.
You could add an extra NPN transistor duplicating the circuit around T1 on the input so you have a double inversion - two negatives equal a positive.

I don't believe I have any P channel FETS. But I could do two NPN transistors. Does the collector of the first connect to the base of the 2nd?

 

Does the collector of the first connect to the base of the 2nd?

Yep.

 

I don't believe I have any P channel FETS. But I could do two NPN transistors. Does the collector of the first connect to the base of the 2nd?

 

BTW, the resistor on the gate of the MOSFET should not be connected to +12V - the maximum rating of the MOSFET is 10V. Connect that resistor to +5V instead.

 

If you want to run everything from +12V in GopherT's schematic, change the 1kΩ resistors to 2kΩ, change the 330Ω resistor to 500Ω, and add a 500Ω resistor in series with the collector of the PNP transistor.

 

Two NPN inverters, NPN/PNP, NPN/P MOSFET:

EDIT: label 12V supply
EDIT 2: To avoid more superfluous comments pointing out excessive gate voltage, refer to updated schematics below.

 

Hey, thank you all for the suggested circuits. I will study them tomorrow. Late now. BTW, since the gate of a FET is really just a capacitor does it really matter if you have 10V or 12V on it? I suppose there is a capacitor breakdown voltage limit on the gate but 12V seems low..

 

Hey, thank you all for the suggested circuits. I will study them tomorrow. Late now. BTW, since the gate of a FET is really just a capacitor does it really matter if you have 10V or 12V on it? I suppose there is a capacitor breakdown voltage limit on the gate but 12V seems low..

Check the DATASHEET of the specific MOSFET you plan to use. There is an Absolute limit listed in most Datasheets.
Most MOSFETS are specified at 10v or 5v gate voltage and list a 15 to 20v absolut max. Some may be lower.

Edit : so you did say which MOSFET. DATASHEET fir irfz44 is a max of 10v.

Per your question, isn't the gate like a cap so does voltage matter? Well, voltage matters for caps so it matters for gates.

 

BTW, since the gate of a FET is really just a capacitor does it really matter if you have 10V or 12V on it? I suppose there is a capacitor breakdown voltage limit on the gate but 12V seems low..

Absolute maximum gate-source voltage on the MOSFET you're using is 10V.

With the circuits I posted, you don't need a logic level MOSFET. If you don't have one, the gate drive will need to be changed to limit maximum gate voltage.

 

Here, 12v split with a resistor.

 

If you have a zener diode in the range of 2.7-5.6V, you can do this:


Resistor values aren't critical. The resistor on the gate will affect switching time.

 

Two NPN inverters, NPN/PNP, NPN/P MOSFET:
View attachment 122634
EDIT: label 12V supply

All three of these circuits put 12V across the gate-source of the MOSFET. Its absolute maximum rating is 10V so there is a serious risk of damaging the MOSFET.

 

All three of these circuits put 12V across the gate-source of the MOSFET. Its absolute maximum rating is 10V so there is a serious risk of damaging the MOSFET.

That was before I read the datasheet. The circuits were updated in post #13; just before above your post; and hours before...

 

Thanks guys. I will build the circuit using 2 NPNs from dl324 and see if I have a zener somewhere.

 

I will build the circuit using 2 NPNs from dl324 and see if I have a zener somewhere.

If you don't have an appropriate zener, you could use a voltage divider with a ratio of 1:2; that would give you a maximum Vgs of 8V.

EDIT: Add schematic for clarity.
EDIT 2: Corrected placement and values for R3 and R4.

Notes:

1. Snubber diode needs to be able to handle the motor current.
2. Resistor values are not critical.
3. R1 needs to provide Ib1 >= 0.1 Ic1.
4. R2 needs to provide Ib2 >= 0.1 Ic2.

 

I had already built the NPN circuit by dl324 before seeing the one above. I checked the Vgs breakdown for the IRLZ44 and it is not 10 Volts. It is +/-16 Volts so I eliminated the zener and the original schematic did not have an R4 in so that is not in either. Nevertheless...
The circuit works perfectly to drive my 12V motor from a 2.3V signal. Success!
Thanks to all for the suggestions and interesting discussions.

 

I checked the Vgs breakdown for the IRLZ44 and it is not 10 Volts. It is +/-16 Volts

I just checked both Vishay and IR data sheets for the IRLZ44. They both say ±10V absolute maximum voltage Vgs.
Where did you find the ±16V figure?