Hello,

I am trying to speed control of a DC motor. Motor is 220V DC approx. 100W. I have completed the logic part and now 8x51 is outputting controlled PWM wave which goes to a ULN2003 darlington driver input.

The output of ULN2003 is driving a Power mosfet gate. Power mosfet i am trying to use is 2SK794.

The problem I have is ground potential difference. The logic controller is powered by a seperate step down 12+12 transformer with centre tape output. Then full wave ractifier and a 1000ufd capacitor. From here it goes into 7805 input. the 5volt output goes to 8x51 and the 12V unregulated is used to power uln2003 and with a resistor it drives the gate of mosfet.

The DC motor is powered by a seperate bridge ractifier (and a 220ufd capacitor used to filter it) via mosfet. I get around 166V DC after filtering which is sufficient to drive the DC motor I am using. Motor field is connected to direct 166V DC via a relay again controlled by MCU via uln 2003. Motor armature should be controlled by PWM.

DC motor A+ is connected to the +166VDC and A- is connected to the Power mosfet Drain, Power mosfet source is connected to the Gnd of 166VDC bridge ractifier.

Mosfet gate is connected to output of ULN2003 and also connected to 166VDC ground via a 1K resistor.

When i try to connect uln2003 output to the mosfet gate, mosfet keeps continously ON and I see hi voltage are present on logic board as well.

Can some one help me solving this issue, which appears to me, a ground potential difference.

Thanks,

Rauf