MOV(zener type) or TVS

[anishkgt]

Hi All,

Trying to figure out which of the two i should go for, TVS(zener type) or MOV. Would it be necessary to to protect an inductive load such as a Transformer. Don't have circuit yet but preparing and this is something i came across.

[tszaboo]

It depends. TVS diode us usually for ESD events, such as touching a connector and having an arc from your finger. MOV is for the 230V input protecting from a spike because your washing machine has a noisy engine. While they are similar, they serve a different purpose. For inductors, sometimes, you only need a simple diode. What is the application? Submit the schematic.

[Rerouter]

Generally a transformer would need a very large spike to do any damage to itself (insulation breakdown) this is where the MOV would come in, and clamp those input spikes, however they have a limited lifetime, each spike they soak up reduces there tripping voltage, until eventually they drift into your average supply voltage and detonate. (lets say after a few years, if specced low, or a decade or 2 if specced higher), also something to beware is the current spike from the MOV can sometimes induce its own voltage spike elsewhere, just something to be aware of.


TVS diodes are intended for very very fast surges that dont hang around for too long, but i have used them as zener clamps before and they have been quite reliable when paired with a PTC thermistor (initial resistance was inside spec of zener, ptc then relaxes away from the spec when it heats up). These would be more intended for after your bridge rectifier if you wanted to soak up some spikes, but these should be only used on supplies with a limited amount of output power to stay inside the SOA (Safe area of operation), as described in the datasheet, as while they can take say a 4000W spike for a few nanoseconds, they can only probably handle 5W for a minute. equally they don't have a sharp turn on voltage, they will be conducting a fair way before the knee voltage, and once it reaches the knee its still not fully on. so make sure your within your limits for the amount of current you need to sink

The ideal for mains supply with a power transformer is that as your transformer acts as a low pass filter, you can likely use a zener and a big meaty BJT as a shunt regulator that only turns on when the threshold voltage is exceeded, a shunt regulator vs a crowbar means your device stays working through the spikes,

[T3sl4co1l]

A transformer is a big lump of metal.

It takes a lot of energy to heat up a lump of metal, enough to cause problems (insulation breakdown, metal melting).

There's not really much you need, or want, to help it out.  It's pretty good at handling the nastiness of the outside world.

You definitely don't want a TVS, which will not handle lightning surges.  (Not the typical small ones, anyway.  However, the really big ones, that can handle it, are extremely expensive!)

You can put a (regular) TVS across the low voltage DC supply, but it is unlikely to ever be needed.  Losses in the transformer, and the value of the filter capacitor, make the circuit already fairly tolerant of mains transients.

Tim

[anishkgt]

You definitely don't want a TVS, which will not handle lightning surges.  (Not the typical small ones, anyway.  However, the really big ones, that can handle it, are extremely expensive!)

are you referring at the mains or the load ? the mains http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail&itemSeq=226152786&uq=636286269277736394 is pretty big.

Just one more thing, what are the reasons that T1/T2 would switch on continuously without a trigger pulse?

[T3sl4co1l]

Referring to D13.

T1/T2 will turn on from:
- OPT1 enable (intended function)
- OPT1, T1, T2 dV/dt exceeded
- Inductive load (critical turn-off dV/dt exceeded)
- Excessive Vin (exceeds V_DRM)

Tim

[anishkgt]

Thanks a lot tim. Was helpful.

what would be a better MOV here for the mains of not a zener ?

dV/dt for the 2N6404G is 50V/uS
critical turn-off dV/dt exceeded - what does it mean ? how does it relate to my project ? As far as i understand does it mean the load did not switch OFF at 50V (just a random guess)
Excessive Vin - 600V

If am not mistaken it could be the dv/dt that i should look. How do i verify these with an Oscilloscope ? Hope this beginner is not bothering you with a lot questions ?