Electronics > Beginners

Fly back diode


Hi, I'm doing a project using some pneumatic valves that are controlled by selonoids, the selonoids are activated by some digital signals from a robotcontroller. All outputs are optically isolated outputs. The manual to the robotcontroller states the following:
--- Quote ---External relay coils, solenoids, and other units that will be connected to the controller must
be neutralized. The figure below illustrates how this can be done.
The turn-off time for DC relays increases after neutralization, especially if a diode is
connected across the coil. Varistors give shorter turn-off times. Neutralizing the coils
lengthens the life of the switches that control them.
--- End quote ---

So I need some sort of fly back diode to protect the controller, right?
The cables to the valves got some electronics in them, does this act like a fly back diode? or do I need to add some diodes. Please se picture below. Tnx  :)

Are you going to be driving the solenoid(s) with both polarities?
That circuit looks to be designed so it will snub flyback for both polarities using two diodes. The forward voltage of one and the breakdown of the other (which swaps around when the polarity is reversed)

When you need to use both polarities like that things are a little more complex as you can't just snub any reverse voltage with a single diode that's forward biased for any flyback voltage. You need to spec the diodes for breakdown as well.

If you just want to drive a solenoid on/off (no reverse polarity) you can put a general purpose power diode across it, eg 1n4007 for a small solenoid or a larger general purpose diode if it's a big solenoid. Of course, there are diodes specially designed for the task if you want it to be a bit more robust/professional.

(incase you don't know what i mean my driving a solenoid with reverse polarity.. Some solenoids have two coils so you can invert the power and drive them back to rest position faster, instead of relying on a spring)

Hi, tnx for replying..

I'm only going to turn the solenoids on and off, no reverse polarity. Then I just put a diode across the solenoids... :) tnx

I am very familiar with the type of suppression you indicated.  We use them on solenoid vales all the time.  The back-back zeners shown make it polarity insensitive, in other words you can wire it without paying attention to polarity.  Using zeners will also make the drop out time of the valve faster and more consistent than just with a simple diode.  Of course, it costs more.  If you are doing induatrial wiring, the suppressor shown is a cheap, simple solution.  If you are DIY, a 1n4004 or similar just wired across the coil, with attention to polarity work normally work just as well.



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