# Need help to understand circuit!!!

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#### pppopey

##### New Member
Some FET's can even switch well into the Mhz range.

If your using a frequency of 50hz at say 50% duty cycle then your pulse will be 10ms.

By using the chart in fig.8 of the data sheet,
This shows that the maximum amount of current that the fet conduct at about say 24v is 10 amps for that single instant.

#### pppopey

##### New Member

This is the schematic I made, I tried the previous schematic but it did not work, specially the drive circuit.

The mosfet was not switchign on properly n so was heating up. A friend of mine told me to put this circuit with the 2 BJTs. He did no know the theory behind it bt told me that i will work. And it did. I do not know how and why it works, nor do i know the calculations.

Could I please ask you to explain me a little how this circuit works and how the two BJTs work well for the Mosfet driving. By the way, the frequency I am operating is at 80Hz and the scooter is able to run well without the mosfet getting hot.

PS: R4 is 5K, not 2.5K

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#### ronv

##### Well-Known Member
When the output of the PWM goes + Q1 turns on and its collector goes to ground, turning on Q2. When Q2 turns on its collector goes + turning on the FET. I think I would add a 10K from Q2 base to emitter to help it turn off faster.

#### pppopey

##### New Member
When the output of the PWM goes + Q1 turns on and its collector goes to ground, turning on Q2. When Q2 turns on its collector goes + turning on the FET. I think I would add a 10K from Q2 base to emitter to help it turn off faster.

For Q1, did you mean the Emitter goes to ground?

When you say adding a 10K resistor from Q2 base to emitter, do you mean adding it between the 12V supply and emitter? Sorry, I am a little confused

#### ronv

##### Well-Known Member
From the schematic Q1 emmitter is attached to ground. So when the transistor is turned on the collector is also at ground or 0 volts.
Add the resistor from base to emitter. One end to the base the other end to the emitter.

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