I have a question regarding the VB05-T2S05 AC-DC converter (datasheet).
The datasheet is in Chinese (lcsc part) so sorry about that. I had to use Google translate to read it.
The device has an input range up to 264VAC. The datasheet recommends a MOV across the ac inputs of the following characteristics: Varistor, 561KD14.
Schematic:
In a datasheet for a MOV with the same model number a clamping voltage of 925v is given.
Isn't that a bit high?
No. That's the 50 amp worst-case clamping voltage, the key value is the maximum allowable voltage - that must be higher that the mains voltage or else it will conduct and burn out in normal operation. A MOV is for killing transient voltage spikes (which can be several kilovolts on raw mains), but its not perfect, its not a substitute for full conditioning, just a way to protect the semiconductor devices from instant death during a thunderstorm or when a fridge motor cuts out.
These devices cannot handle much continuous power (but can dissipate many kW for a very short time). Any hint of conduction is normal operation will heat them dramatically, leading to failure (and removing their protective function). Typically they fail short-circuit so it is mandatory to have a fuse before the MOV.
Typically, when a MOV does its job,
it goes out in a fireball and throws
soot all around it! A messy and smelly
condition! But, well worth it, because
it saves your circuitry from the same!
Herb
