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I'm looking for a solution for "flyback" protection when driving a 2 wire DC latching solenoid. You cannot use the traditional diode solution as the polarity of the coil is reversed to open or close the solenoid.

My CCT uses two relays to switch the solenoid - the first (SPST) applies power to the second relays contacts (DPDT) which does the polarity reversal for the power to the solenoid coil.

Of course my relay coils already have conventional flyback diodes connected - what I'm looking to protect against is the back EMF from the actual solenoid coil itself.

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  • \$\begingroup\$ Why don't you use transistors to drive the main solenoid? \$\endgroup\$ – starblue Mar 5 '10 at 7:43
  • \$\begingroup\$ because i had suitable relays and not suitable transistors :-) The original cct i am using drove a 2 wire dc motor operated valve which only needed power in one polarity or the other for 1-2 seconds to change state. The solenoid is similar in that it only needs a short pulse of one polarity or the other to change state. It was deemed quicker to adapt the current cct that wait for parts to do something differnt. \$\endgroup\$ – Stocky6409 Mar 5 '10 at 10:46
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You can use four diodes to the supply rails like for motors in an H-bridge.

See for example here.

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    \$\begingroup\$ This is a good solution. With sufficient bypass capacitance (the voltage supply will most likely have enough, add more if you're worried), you'll be fine. You just need to make sure the energy in the solenoid's inductance can return to the power supply w/o raising the voltage to the point of causing damage. \$\endgroup\$ – Jason S Mar 5 '10 at 17:43
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There are a number of ways you could do this.

  • Two series connected zeners diodes. With the diodes connected with opposite polarity (either both anodes connected or both cathodes connected) the voltage will be clamped in both directions. The zener voltage should be at least a volt or two greater that the supply voltage used to drive the solenoid. Increasing the zener voltage up to the voltage rating of your relay contacts will permit a faster response from the solenoid.
  • Four diodes clamping to the power supply, as suggested by starblue. With this approach you will want to make sure that the power supply can sink the current supplied by the solenoid when it switches off.
  • An RC snubber. You will have to calculate or experiment to get a suitable RC combination that trades off the peak voltage and time constant.

My preference would be for the back to back zeners. Unlike the four diode clamp, it keeps the flyback current contained, away from the power supply. The advantage over the RC snubber is the the voltage is well defined and it is easy to specify.

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  • \$\begingroup\$ Cheers - i think i have some suitable zeners on hand - will look in the morning. Thanks \$\endgroup\$ – Stocky6409 Mar 5 '10 at 10:47
  • \$\begingroup\$ So to clarify your "back to back - series connected" comment: does that mean both cathodes together with anodes to each solenoid terminal? OR 2 zeners in parallel A to K and one junction to each solenoid terminal? \$\endgroup\$ – Stocky6409 Mar 5 '10 at 10:49
  • \$\begingroup\$ Yes, both cathodes together with the anodes to the solenoid terminal is right. I've tried to clarify the answer a little. \$\endgroup\$ – Clint Lawrence Mar 5 '10 at 12:37

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