Electronics > Projects, Designs, and Technical Stuff
How to choose TVS diodes to dispel flyback surges from relay coils?
wraper:
--- Quote from: calzap on February 05, 2020, 03:27:07 pm ---With repeated conduction, MOVs can eventually fail open.
--- End quote ---
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.
ffrige:
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.
T3sl4co1l:
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
ffrige:
@Tim: great, thank you!!
trobbins:
--- Quote from: wraper on April 21, 2020, 02:28:20 am ---
--- Quote from: calzap on February 05, 2020, 03:27:07 pm ---With repeated conduction, MOVs can eventually fail open.
--- End quote ---
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.
--- End quote ---
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'.
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