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[SOLVED] TL494 based motor speed controller

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Full Member level 4
Sep 3, 2011
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Hello Everyone!

TL494 motor speed control.gif

I came across this motor speed control circuit.

I have tried it using only one MOSFET eliminating the Q2N2222 part. It works that way.I know that pin13 is mode control pin.

I want to know how 15A current limit is achieved using Q2N2222.

At 15A, the voltage dropped across R11 is 0.75V which is enough to make the transistor conduct. As it does, it pulls the voltage on pin 13 down to almost ground and turns the output signals off.

Thank for the reply

Voltage at pin 13=0V means parallel operation
Voltage at pin 13=Vref means push pull operation of the output transistors.

Even if transistor Q2N2222 conducts [governed by the voltage drop across R11],it will be parallel operation.So how the current limit will happen.

You are correct - I used a Motorola data sheet and it clearly has errors in it. The TI data sheet shows it's correct operation.

The principle is still there though, in push-pull operation there will always be one or the other FET conducting so in theory they could be working 50/50 giving full power to the motor. In parallel operation, the FETs would conduct at the same time and also stop conducting at the same time so the duty cycle could be lower.

the output-control input when grounded disables the pulse-steering flip-flop and inhibits its outputs. In this mode, the pulses seen at the output of the dead-time control/PWM comparator are transmitted by both output transistors in parallel. when in normal operation when the control is + the output is in push-pull operation, Under this condition, each of the output transistors is enabled, alternately, by the pulse-steering flip-flop
Hence when the 2N2222 conducts the power will be Halved
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