Thread Starter


Joined Jun 8, 2007

I am going to be controlling a switched reluctance motor using a 555 timer in monostable mode which will be triggered by a DSP. I will use a comparator against a reference voltage to ensure the MOSFET that will be driving the motor does not exceed a designed value.

My general flow therefore is:

DSP -> Comparator -> 555 -> Motor

Which will loop back to the comparator to ensure no over current.

My only problem is hooking up the comparator and 555... I believe my MOSFET will need a driver but I havn't a clue. I've seen some images of two transistors - One PNP and one NPN which will ensure that switching on and off is fast but I do not understand them nor do I find one that is consistantly used (Often different numbers of transistors are used).

Could anybody please advise on how I will drive my mosfet (PS. I can not simply use a MOSFET driver IC).



Joined Apr 20, 2004
If you can arrange to have a circuit hold the gate at the same potential as the source, then the device will be off. And if your circuit can raise and maintain the gate voltage at 10 volts above the source (anything from 10 - 16 volts is ok), then the device will be in full conduction. Given the lack of information in the above posting, this is pretty general but accurate.

By the way, "ensure the MOSFET that will be driving the motor does not exceed a designed value" is impossible to interpret.


Joined Dec 27, 2007
It seems a bit strange to me that you have the DSP's power and you're combining such a high level device with really low-level components.

You may or may not need a mosfet driver, it really depends on the MOSFET itself along with how fast you are switching. If it is a logic level mosfet, then you can drive it directly. If it is a typical FET, then you will need to provide the necessary voltage levels. If you are switching it off and on very fast, then you need the push-pull arrangement to source/sink the current fast enough. You must consider the gate's capacitance and the fact that you will be needing to switch it on and off x-amount times per second.