Author Topic: How to choose TVS diodes to dispel flyback surges from relay coils?  (Read 11396 times)

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Online wraper

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Re: How to choose TVS diodes to dispel flyback surges from relay coils?
« Reply #25 on: April 21, 2020, 02:28:20 am »
With repeated conduction, MOVs can eventually fail open.
Exactly opposite, clamping voltage drops after absorbing large transients and it may eventually fail short. The only time when it fail open is when there is such high current flowing through it that it literally explodes.
 

Offline ffrige

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Re: How to choose TVS diodes to dispel flyback surges from relay coils?
« Reply #26 on: April 21, 2020, 02:44:22 am »
So, can I just put a bi-directional TVS to ground (parallel to the driving FET) instead of a regular diode plus a Zener to Vcc? As long as the TVS clamping voltage is high enough (e.g. Vcc+24V) then it should work, right? The TVS would give me the benefit of additional ESD protection on the motor's terminal, which the Zener would not provide being much slower to react.
 

Offline T3sl4co1l

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Re: How to choose TVS diodes to dispel flyback surges from relay coils?
« Reply #27 on: April 21, 2020, 05:31:42 am »
In fact, clamping from GND is slightly better than clamping to +V, as it's insensitive to supply bypass / wiring length.  Place the TVS right by the switch.

Tim
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Offline ffrige

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Re: How to choose TVS diodes to dispel flyback surges from relay coils?
« Reply #28 on: April 21, 2020, 06:07:39 am »
@Tim: great, thank you!!
 

Offline trobbins

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Re: How to choose TVS diodes to dispel flyback surges from relay coils?
« Reply #29 on: April 21, 2020, 10:58:47 pm »
With repeated conduction, MOVs can eventually fail open.
Exactly opposite, clamping voltage drops after absorbing large transients and it may eventually fail short. The only time when it fail open is when there is such high current flowing through it that it literally explodes.
This only applies to a low impedance, continuous power source (such as mains) feeding the MOV.  It doesn't typically apply to energy limited applications like this for control side, and across relay coil situations - the MOV doesn't enter a degrade mechanism from each 'hit'.
 

Offline trobbins

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Re: How to choose TVS diodes to dispel flyback surges from relay coils?
« Reply #30 on: April 21, 2020, 11:22:53 pm »
From the linked article by TE, the closest performance to an suppressed coil would be a coil with MOV, with the TVS taking slightly longer to open, as the MOV provides a soft clipping action as any voltage transient increases in magnitude, rather than a sharper clip like a zener.
 

Offline T3sl4co1l

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Re: How to choose TVS diodes to dispel flyback surges from relay coils?
« Reply #31 on: April 21, 2020, 11:32:41 pm »
MOV datasheets typically provide a plot of expected lifetime versus current or energy, with an asymptotic infinite life curve given.  Relay coils store very little energy as surges go, and even a SMT chip MOV should do fine.

TVS vs. MOV doesn't really mean anything: if you're depending on the higher voltage drop of the MOV, you can just choose a higher rated TVS.  The TVS will give a squarer flyback voltage, giving slightly shorter discharge time for the same peak voltage.

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 


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