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Protecting circuit from solenoid inductive spikes and noise

logiclevel14

I am looking for a device that will isolate my circuitry from inductive spiking from a pair of 12v solenoids.  Another issue is that the solenoids will be connected via 4 to 5 foot wires which may act as antennas and introduce noise into the ground circuitry.   I know that flyback diodes are commonly used to loop the spike back to the positive terminal of your power supply.  What I would rather use is some sort of isolation IC or other device and just totally isolates the solenoids from the other circuits.  One idea I have is to create an isolated 12v rail and have the flyback diode send the spike to that rail.  I have seen some dc to dc converters that provide isolation but I cannot find one that can supply 4 to 5 amps.  I understand that a small 12v battery cannot usually supply 5 amps but just to be sure, whatever isolation method I use, I would like it to be able to handle high amperages.   Thank you.  I attached a block diagram to give a basic idea of what I am doing.

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  • Is the spike your issue? Or is it NOISE when the relays switch?

  • in reply to ynot

    You would need the flyback diodes no matter what you do.  I built a circuit to control a commercial grinder. The circuit  would watch the RPM's. When the RPM's dropped to a set level it would stop the tub. Then reverse the tub until the RPM's came back up then stop, forward again. The speed of the tub was controlled by PWM. So I used a NPN 2N3055 and the Atmega to control the PWM solenoid. Forward and  reverse  I used relays. Noise from these little DEVILS will KILL YA! So after redoing the caps on my power leads I went with the MC1413 relay driver. This is where I solved my noise problems from the relays. I don't know exactly what your doing but I can guess from my own pain and misery. If your question is about flyback diodes they suppress any reverse voltage that may occur when the field collapses.  1N4007 would be fine. The diodes don't really isolate anything they just short. That is why I think your talking relay noise. I could be wrong.

  • in reply to ynot

    Actually I have not started thinking about the relays yet, thank you for the MC1413, I will look into this also.  By noise I am referring to the long wires acting as antennas and inducing electrical noise into my circuit.  I will have an enclosure with wires coming out of it (4 to 6ft) and pulling in a pair of car door actuators (solenoids).  I could install the flybacks right up next to the actuators but I want to do it on the PCB itself.  Even with the flybacks I am worried the spike may slam the battery or the other circuitry too hard.  I would like to be able to drive the car door actuators but keep them isolated from the rest of the circuits.  

  • in reply to logiclevel14

    The flyback diodes should be as code as possible to the coil leads, preferably directly across the coil. This burns off the energy by recirculating current through the coil without a large voltage spike. If you put the diodes at the circuit card, there will be large currents in the wire which can couple out to other circuitry and the resulting IR drop in the ground wire can affect your grounds. You can use Schotky diodes to minimize any voltage blip and you could use a clamping circuit like a zener to further minimize any disturbance. Of course low ESR capacitors on the supply will help too.

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  • in reply to logiclevel14

    The flyback diodes should be as code as possible to the coil leads, preferably directly across the coil. This burns off the energy by recirculating current through the coil without a large voltage spike. If you put the diodes at the circuit card, there will be large currents in the wire which can couple out to other circuitry and the resulting IR drop in the ground wire can affect your grounds. You can use Schotky diodes to minimize any voltage blip and you could use a clamping circuit like a zener to further minimize any disturbance. Of course low ESR capacitors on the supply will help too.

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