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MOSFET driver circuit ?

Thread starter #1
Hello guru's all.

I have a circuit (attached) that is giving me some issues. In the drawing, I show 2 circuit's and both work, but apparently, something is amiss.

To start with, the 555 timer circuit and the OR gate circuits both work fine. I show them so you will see what the control of the MOSFET is. The problem is really not evident to me yet because I built the circuit for a friend on the "other coast" and when he hooks it up to a pump (4 amps load) the pump turns on as it is supposed to when the 555 tells it to but when the 555 turns the MOSFET off, the pump is still trying to run, growling like its powered by 2 or 3 volts or some such. He is sending me one of these pumps so I can examine it closely but till I have it in hand, I am trying to figure out what is causing this.

He tried 2 pumps and 2 of the control circuits, same results. I built the Ckt. A (which is the circuit I built for him) and tested it again, using 6.5 amp auto bulbs as load and also a small fan as the load. Bulbs and fan all turn off completly...? Then I tried Ckt. B in the drawing just to see if there is really any need for the transistor to drive the MOSFET and the result's were very similar when I got to the load.

So, there are several ?'s I have. Does anyone have suggestions as to the circuit(s) design in general? Is there anything in these circuit's that jumps out at you that could improve the final design? Can you see any reason why the pump would stay on after the 555 tells it to turn off?

I think there may just be an issue with the pump selected, but if you can give me any ideas, it would be helpful and appreciated. And if I actually have the right idea going on with either design, I could use a shot of encouragement.

What else can I tell you...... oh, the MOSFET is an N Channel 35 amp device with .008 ohm drain to source resistance. I figure it would be more than enough for the application, but now that I think of it, perhaps the selected MOSFET is the problem???

Looking forward to hearing from you all and thanks ahead of time.



New Member
Hi electrookie,

I can't see any flaw in your design.

You might try a small modification and see if the MosFet switches off completely. Remove the gate pulldown resistor. Pull down is done with T2.

Addionally you might connect a fast recovery diode across the power transistor.



Last edited:
Thread starter #4
MOSFET motor drive next step

Thanks Boncuk. I figured if anything, the 1K resistor would be involved. I changed the schematic as per your recommendations and the pix is attached. On the original, also shown in the pix, I have the 1N4148 diode across the PNP. In your drawing, it is gone all together. I figure the fact it is not shown means it is not intended to be there now and that the NPN transistor is essentially taking its place, doing a better job!?!?!

Also, please clarify, you mentioned a fast recovery diode across the power transistor: are you referring to the MOSFET in this case?

As my name implies, I AM a rookie at this yet. I believe I understand everything you did not say as well as what you did say.

I was also pleased to hear that I got the design right, at least for the most part. I still have to wait till the pump is in hand so I can find out what is causing it to try to stay on. I will report back when that happens and I come back with more questions or final results.

In the meant time, anyone else care to review my design? I appreciate the help all. So thanks again.


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