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Please explain

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bristol188

New Member
Can somebody please tell me the difference between a Mosfet and a Mosfet Driver. And what sort of driver do I need to use between a radio control receiver output (pwm) and a Power Mosfet to drive a small motor?
Thanks to everybody.
Rog.
 

Hero999

Banned
A MOSFET is a type of transistor, a MOSFET driver is a circuit which drives power in and out of the a MOSFET's gate large capacitance to enable it to switch on and off very quickly.
 

Sceadwian

Banned
You don't just need a driver, you need a decoder. Servo signals are 1.5ms center and 1ms and 2ms to either extreme repeated approximately 50 times per second. Best case scenario (2ms signal) this will only give you a 10% duty cycle, worst case (1ms signal) it will give you a 2.5% duty cycle, and the frequency isn't fixed (it's slightly variable depending on all of the other channels aggregate duty cycle plus the framing pulse) This is not a signal that can be used to drive a Mosfet directly.
You either need to use a pulse stretcher which coverts that short servo pulse into a simple PWM frequency (0-100% duty cycle at extremes) or use a micro controller to read the pulse length and drive the mosfet for you. Mosfet drivers usually have two purposes, one is for driving a high side mosfet because voltages greater than VCC are required to get a high side FET to switch on so a charge pump is usually used. The other is if you want to minimize FET transition losses by creating very sharp edged switching transients from perhaps a softer logic edge to avoid operating in the FETS linear range, which creates heat. Logic levels FETS are very common and for common voltages can be driven directly from the I/O line of a micro controller or from discrete logic.
 

gramo

New Member
Here’s an example of a FET drive circuit;

irl2203JPG-1.jpg

Click here to watch this circuit in action

Notice that the two transistors act like a push-pull switch, driving the FET on and off. This is required for FET's driving large currents, not all circuits need this, but as the example shows above, it can also be used to drive FET's with logic voltages/signals (like a 5V PWM)

To control the speed of the motor, simply change the PWM duty cycle. PWM (Pulse wave modulation) is a simple way of saying on time:eek:ff time. So if you had a 50% duty cycle, the period of on time would be 50% and off time would be 50%. 100% = 100% on 0% off

The frequency of these cycles would need to be considered, and a quick check of the datasheet of your FET will let you know how fast your PWM needs to be.
 

bristol188

New Member
Thank you all for that I'm fully explained. I mean that sincerly. (bows head and retrets backward)
Roger
 

firewater

New Member
For the newbie trying to understand schematics could you tell where the positive and negative of the 12 volt and 5 volt TTL go to please.
 
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