Stepper Motor Problem

Hello. I am driving a unipolar stepper motor from a PIC 16F877. The PIC outputs the sequence on its pins correctly and the pulses are observed to be correct on the oscilloscope (meaning that they have a maximum value of 5 volts). The drive circuit of the motor is also correct. If I apply 5 volts to the gate of any of the 4 MOSFETs, that phase draws the correct current and the shaft moves to that phase. The problem happens when I connect the gates of the MOSFETs to the PIC pins. There is suddenly a voltage drop that causes the pulses to have a maximum value of 2 volts or something instead of 5 volts, which is not enough to turn the MOSFETs ON and hence operate the motor. I have pull-down resistors on the gates of the MOSFETs as well in order to turn a MOSFET off when the pin is floating. The strange thing is that the motor was operating just fine before. This just happened today. Can anyone help me locate my problem? Thanks a lot.

Nichola V. Abdo.

You know what's coming next.

Schematic.

Also what is trigger voltage for mosfets gates?

It sound like too much current asked of PIC.

Use amplifying transistor?

What value your resistors from output pins?

Did it work when you only had one mosfet connected or all?

Measure the gate current of the FET, if the voltage from the PIC is being pulled down then excessive current is being drawn somewhere!.

Do you have series resistors from the PIC pins to the gates?, personally I would prefer them!.

Well the pull down resistors are 1 kilo ohm resistors. I tried series resistors but nothing happened. The maximum voltage I can get is 2.2 and the gates need 3 volts to operate.

YAN-1 said:

Well the pull down resistors are 1 kilo ohm resistors. I tried series resistors but nothing happened. The maximum voltage I can get is 2.2 and the gates need 3 volts to operate.

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So what current is being drained from the PIC pins?.

1K also seems rather low for the pulldowns?, at 5V out from the PIC it's wasting 5mA for no good reason.

Well with no series resistors, the there is around 23 mA drained from the pin. I just don't understand why this should be any different from when I operate the MOSFET from the 5V supply directly. Either way the 5mA will be drawn. I even tried it from the 5V supply now and the current drawn was 52mA.

YAN-1 said:

Well with no series resistors, the there is around 23 mA drained from the pin. I just don't understand why this should be any different from when I operate the MOSFET from the 5V supply directly. Either way the 5mA will be drawn. I even tried it from the 5V supply now and the current drawn was 52mA.

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It sounds like you have duff MOSFET's?, I don't use FET's much, but AFAIK they aren't supposed to draw gate current!.

Do you have protection diodes across the motor windings?.

Yes I have the diodes. The MOSFETs I am using are the IRF530. THE CIRCUIT WAS FUNCTIONING BEAUTIFULLY JUST YESTERDAY :cry:
This is weird. If I try to operate the gates through opto-couplers, current is still drawn and the FETs don't switch on. And even when all gates are pulled to ground, the power supply indicates 0.018 Amps being drawn somewhere (the rated current of the windings is 2 Amps).

Nigel Goodwin said:

YAN-1 said:

Well with no series resistors, the there is around 23 mA drained from the pin. I just don't understand why this should be any different from when I operate the MOSFET from the 5V supply directly. Either way the 5mA will be drawn. I even tried it from the 5V supply now and the current drawn was 52mA.

Click to expand...

It sounds like you have duff MOSFET's?, I don't use FET's much, but AFAIK they aren't supposed to draw gate current!.

Do you have protection diodes across the motor windings?.

Click to expand...

Well not entirely true, you need current to initially charge the gate region (can be 12A peak with exponential decay)

Styx said:

Well not entirely true, you need current to initially charge the gate region (can be 12A peak with exponential decay)

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But it's only to charge a small capacitance! - or are you suggesting that drawing 23mA of gate current is normal?. Bear in mind, we're talking about direct drive from a PIC pin! - maximum capability 30mA or so.

Yes I am saying it is normal, if anything 23mA is too low.

There should be next to no continous current draw BUT I would expect near on an amp (for the time it takes to saturate the FET) to charge the gate, IF the PIC is incapable of supplying such a charge then the FET will turn-on/turn-off slowly and it will sit in its active region for longer then it shouuld and get cooked

I resorected this thread since it seems related to the other thread.
Since we do not know what his switching freq is (such low power he could be switiching at 100kHz, the PIC output pin would be getting a massive hammering)


All that is needed is a NPN-PNP psuh-pull to provide some current-capability from the PIC pin to ensure that the FET receives the charging current it needs



I have an IGBT at work which get hit with 12A peak with a 1us time constant (hitting it with +15V to -15V), yes that is an IGBT, but the same theory applies to a FET since the input to an IGBT is just a FET, the current level will be lower and the time shorter, but charge is needed, and the more charge you can give it in a shorter period ensure saturation ocurrs quicker, limiting this is very bad for FET