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How to choose TVS diodes to dispel flyback surges from relay coils?

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calzap:
In the past, I've used simple diodes for DC relays and MOVs for AC relays.  These appear to have disadvantages.  A simple diode can slow the opening of the contacts leading to arcing and premature wear.  With repeated conduction, MOVs can eventually fail open.

So, I thought I'd give bidirectional TVS diodes a try on some 24VAC relays and contactors.   The devices to be protected are reasonably robust ... voltage monitors, LEDs, and timing relays.  I'm guessing a good choice of reverse standoff voltage spec for a TVS diode is about twice AC RMS voltage ... so about 50 V in this case.  But what about the peak pulse power spec?  Again guessing ... maybe 5-10 times inrush VA of the relay.  Largest coil in present design has inrush VA of 70.  So maybe use a Littefuse P6KE series TVS diode.  These have a peak pulse power rating of 600 W.  The P6KE62CA has reverse standoff V of 53, breakdown V of 59-65, and max clamping V of 77.

Advice appreciated.

Mike in California

Wolfram:
For inductive loads, the peak dissipation the TVS will see is the coil current at the moment of turn off, multiplied by the clamping voltage. The energy the TVS needs to dissipate is approximately equal to the energy stored in the coil, 0.5*L*I^2 . Note that inductance will be different when the relay armature closes the magnetic circuit of the coil, so the coil inductance when the relay is closed is good to use as a worst case value. For the voltage rating of the TVS, you need to make sure that it is comfortably above the maximum supply voltage. Note that the circuit that drives the coil will see the sum of the clamping voltage and your supply voltage, which would be 24 + 77 = 101 V with the P6KE62CA.

jonpaul:
Use ordinary diode, 1N4004 for small relays, not a transient absorbing Zener

Jon

MagicSmoker:
MOVs eventually fail from the cumulative energy dissipated in them, so most will handle millions of "surges" from a relay/contactor opening.

As for selecting a bidirectional TVS diode for clamping instead, I generally go with twice the supply voltage or half the switch voltage rating, whichever is less. Even a SMA package TVS is going to be robust enough for all but the largest relays/contactors, but if you want to take the belt-and-suspenders approach then just ensure the peak current rating of the TVS exceeds the current drawn by the relay/contactor. Unless you are using an SOD-123 package TVS to protect a huge 3ph. motor starting contactor I doubt there will be an issue.

T3sl4co1l:
Yeah, won't take much, a P6KE will certainly do.  Voltage rating should be high enough not to run afoul of line swell + modest surges, for which double is probably alright.

Noteworthy that larger surges will still be absorbed by it; if this is 24VAC from a modest sized transformer, that should be okay as the leakage inductance of the transformer acts to limit peak surge current.  That's where you'd need to match up peak power ratings.

For the coil voltage itself, the pulse length will be quite long (some ~ms), and the peak power quite low (about 2 * V * I, as you guessed), giving plenty of time to absorb that energy.

Tim

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