MOSFET ISSUE i cant get it to turn on

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jcvince

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Hi I have a circuit (attached) with a 5v power supply operating a bjt allowing 12v to operate a MOSFET (TO-220 3N60P). The MOSFET completes the circuit for a motor running at 12v. The circuit may seem a little pointless, but its part of a larger circuit, and i thought this would simplify the issue. The reason im not running the mosfet at 5v is because at 12v the max drain source current is a LOT higher.
The Mosfet data sheet is at:
https://www.electro-tech-online.com/custompdfs/2011/05/99449.pdf
Using r1=470 ohms opens the bjt, however i cant find suitable values for r2 and r3 which open the mosfet. r3 is a pull down resistor allowing the mosfet to turn off again once power is cut to the gate pin.

Help would be appreciated, because I am quite stuck.
THANKYOU
 

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I don't see your attached circuit. How did you attach it?

How much current does the motor require?
 
Hi I have a circuit (attached) with a 5v power supply operating a bjt allowing 12v to operate a MOSFET (TO-220 3N60P).
Click to expand...

The only circuit I see is the MOSFET data sheet link?

Ron
 
sorry, now its attached.
the motor draws around 2A. Thats part of the reason why we need to run the mosfet at 12v, because it means higher drain source current
any idea on resistor values?
 
It looks like it takes 10 volts to fully turn on the MOSFET. It has a gate threashold voltage as high as 5.5v but at a Ids current of only 50ua so its more than 5.5 for sure.
10v to turn on MOSFET + voltage wasted in the voltage divider + 0.65v Vbe = much more than 5v is required at the base of the BJT to it turn on.

Use a logic level MOSFET and drive it direct from your 5 volts.
Buy a MOSFET driver IC.
Add a PNP BJT between the NPN and the MOSFET.
Drive the MOSFET off the collector of the NPN and invert your 5v logic signal into it.
 
I don't understand the emitter follower driving the Mosfet gate. The Mosfet needs at least 10V Vgs for good turn-on and you are only applying a few volts, which will barely start to turn on the Mosfet.

Also, that particular Mosfet is a poor choice. It has a high on-resistance of 2.9Ω max. which means its "on" voltage (even with Vgs=10V) can be as high as 5.8V with the 2A motor current, which will then dissipate 11.6W of power. You want a Mosfet with a much lower on-resistance, perhaps around 0.2Ω or less.
 
yes, i know, the mosfet choice was very bad. we purchased 25 though so we want to use them. the actual application is for H-Bridges but there is too much voltage drop between them because of the high resistance. Is there any way that this circuit would work with the given mosfet? what if the motor only drew 1A?
 
I assume this purchase was for another application, since it's a very poor choice for this one.

If you limited the current to 1A then you simply use Ohm's law to calculate the transistor drop. The worst-case on-voltage of each transistor would be the on-resistance of 2.9Ω times 1A = 2.9V. Thus in a H-bridge the total worst-case transistor drop would be 5.8V. I assume you have studied Ohm's law?
 
You can parallel MOSFETS and split the load through them. Just make sure you can drive the extra gate capacitance.
 
The transistor is configured as an emitter follower, so the emitter voltage will follow the base voltage minus the Vbe drop. Therefore you only have 4.3 volts to the gate of the FET. Change the transistor to a switch. This will invert your input so a 0 volt signal will turn on the FET instead of the +5. Don't know if that will work for you or not.
 

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hmm ok it seems that i am in trouble then.
well will mtp3055v mosfets be any better?
they have low resistance (0.15 ohms)
and a high enough voltage and current rating.
and at vgs = 5v, it can take upto 4A through ds.
does that sound right?
thanks
 
jcvince said:
.
well will mtp3055v mosfets be any better?
they have low resistance (0.15 ohms)
and a high enough voltage and current rating.
and at vgs = 5v, it can take upto 4A through ds.
does that sound right?
Click to expand...
You looked at the "typical" curve on the datasheet. But some are good and some are much worse than "typical". You cannot buy a "typical" one. Maybe the last production run had a bad yield and none are as good as "typical".

It is guaranteed to turn on with an on-resistance of 0.15 ohms only when the gate-source voltage is 10V and it is cool.
If you want it to turn on with a gate-source voltage of only 5V then buy a "logic-level" Mosfet.
 
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Do you mean the mtp3055vL? That is a logic level MOSFET which has a maximum on-resistance of 0.18Ω at 5V Vgs at room temperature.
 
crutschow said:
Do you mean the mtp3055vL? That is a logic level MOSFET which has a maximum on-resistance of 0.18Ω at 5V Vgs at room temperature.
Click to expand...
I didn't look at its datasheet but if it is 'logic-level" then it will work fine.
 
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