I have built a driver board for alternately energizing two separate coils. The gate is opto-isolated from the rest of the circuit and a PLC is used to turn the opto-isolators on and off (there to two circuits on the board to drive either coil). After overloading the mosfets and essentially destroying one, I replaced both and find that at the end of one shutting off the other one immediately turns on for a very short period of time. Could it be the capacitance on one gate discharging slower than the resistor allows and turning on the other mosfet. Stumped on this one, any help would be appreciated.

LaurenceR

 

I'd be very surprised if anyone has any idea what the problem could be. I'll try to help, but I'll need more. A better description to start ( what does it mean to say "I replaced both and find that at the end of one shutting off the other one immediately turns on for a very short period of time." That doesn't make any sense to me ) Also, a schematic showing the entire circuit including all power supplies. And, if we can't track it down with that, a circuit layout diagram.

 

Thanks a lot for taking a look at this. Attached is a copy of the layout with some additional information. Please let me know if you need more.

Could be something with the LEDs I noticed that with no load attached both LEDs light up when one side is energized. putting it on a scope you can see a small spike on the off side just after the on side shuts off.

Before I totally confuse you here is what I am dealing with. I have two coils to drive. The PLC determines the rate between switching coils on and off alternately, and it also determines how long the coil remains on during the rate cycle. usually less than the rate time.

there was no spike coming out of the opto-isolators.

thanks again in advance.

Larry

 

Do you have a schematic? And a detailed description of the problem? Help me help you.

 

Thanks,
I'll get it.

 

The PLC turns on one coil, shuts it off, then turn on the other coil and shuts it off in the same amount of time. This repeats itself.
When one coil shuts off you see the led blink before it is turned on by the PLC. Small spike on the oscilloscope. Could LEDs be allowing crossover from fly-back voltage from coils. Not sure what is going on.
Thank you again,

Larry

 

Correction:
When one coil shuts off you see the other coil blink before turning on...

 

I would be mostly concerned with the power distribution and the coils causing ringing. One thing to try would be to isolate each driver/LED/coil on separate supplies and see if the issue goes away. If so, then you need to improve ground paths and consider adding snuffers at the coils. Physically separating the coils or shielding might also help. I've had coils wreck havoc on circuits. Make sure they are isolated as much as possible.

Sorry if that's too general. I do have a little trouble following your schematics and layout. But isolating your power as much as you can could make all the difference.

 

Thank you. I will check this out.

 

Interesting findings. After a more careful analysis of the scope tracing I found that the opposite mosfet was not turning on rather as you suggested there was a voltage being induced in the opposite coil when the other one turned on. I verified it by switching the polarity of one coil and found that the spike occurred at the end of the one cycle rather than at the beginning. This would indicate the voltge had to have a certain polarity to pass through the LED. Fortunately I don't have the problem I thought I might have had however your insight caused me to look at things I hadn't considered much before. I can see where these induced currents and voltages could cause a lot of trouble.
Thanks again for taking the time to help out.
I learn something every day.

Larry