Question: making a UNI-directional TVS

[EEVPiobee]

Hello,

I'm a bit of a hurry to close a project, and all I need is to put back a safety, unidirectional TVS to it... It will be 220volts 600w (or more) Problem is, I ran out and I don't (ideally) want to wait the whole week to order parts... Now I could have one BI-directionnal TVS with the ratings I want , 220vetcetc. (which is even surprising he has some TVS, here Local supplier means "I don't have a thing for you", even if he's supposed to be specialized... Anyways..)

now I've read the topic:

https://www.eevblog.com/forum/beginners/question-making-a-bidirectional-tvs/msg142102/#msg142102
"Question: making a bidirectional TVS"

And I'm wondering if the opposite would be possible  , but instead, using a standard Diode (rated well above 220V) to prevent any negative voltage on positive rail, -in parallel- with the 220v Bi-directionnal-TVS that would prevent over voltage

And if it IS, do I have to watch for something in particular? like in the specs of the regular diode? Leaking? (I doubt so) Aniything you could think of...

Of course my project Works, but in case of a lightning strike, I'd cry far more having to rebuild it fully instead of just changing a Fuse, a TVS and maybe few little components

[Psi]

It will work electrically to limit negative spikes to -0.8V  (-0.4V for schottky)
However a TVS diode is designed to handle very short ultra high energy spikes that would damage normal diodes.
So the protection using a generic diode will be less than with a proper TVS.

[EEVPiobee]

...So the protection using a generic diode will be less than with a proper TVS.

I see... but If the regular diodec is enough to blow the mains fuse (4 amps, regular glass fuse) ... Should this be ok? TVS is located not so far after the 4 rectifier diodes and beefy electrolytic filter cap... I'd get negative voltage in the unlikely event of bridge failing badly...

I could even put in a beefy diode like a 10A10 for instance?
http://www.wontop.com/pdf/10A05.pdf (datasheet)

[T3sl4co1l]

If the bridge is failing that badly, the fuse is blowing anyway, and reversed voltage is the least of your worries.

Last time I cooked off a diode bridge, only one diode actually failed shorted.  The opposing diode was equally vulnerable (mechanism was a thermal failure at rated current, so all four diodes were hot, going into the fault), and managed to survive fault current (probably one or several cycles at 60Hz with peaks to 1kA or so).

Moral of the story, most of the diodes will probably either 1. continue working during and after the fault, or 2. fail short, in which case the reversal is around 0V, even better than what the diodes would do.

I would just as well use an MOV on the AC line side, rather than a TVS.  The energy capacity is much greater, whereas a TVS dropping that much voltage is probably going to short out as soon as the voltage gets excessive (which still blows the fuse, but repair requires replacing more).

Tim

[EEVPiobee]

T3sl4co1l: so basically, you are suggesting me that I could just put in a bi-directionnal one, without having to care about reverse voltages on the DC part of my circuit... Right? It makes sense

I would just as well use an MOV on the AC line side, rather than a TVS. 

Yes, has one too!

(which still blows the fuse, but repair requires replacing more).

..for what I guess this is why many cheap consumers electronics (Mall-Wart not to name it)  are designed this way... Fails more easily, buy more cr*p from them

[T3sl4co1l]

TVSs are fine for what they're fine for... a common place I see them is a voltage clamp in a switching supply.  Instead of an RCD snubber or clamp, they use a 200V (or so) TVS, which sees pulses up to one amp or so, and not too much average power.  That's a perfectly fine application for them -- it's basically heat that fuses the silicon chip, so as long as the average power and pulse energy aren't too much, it'll last forever.

There isn't much silicon inside the things, and silicon is expensive, so they blow quickly under any kind of huge stress, like expecting them to handle power line transients.

The other crappy part about any kind of TVS is the curve.  A typical 1.5KE15 for instance is designed to protect 12V supplies, so (at 25C) it's guaranteed to not conduct (much... 1uA) at 12.8V, start drawing significant current by 14.3V (min) to 15.8V (max), and clamp 21.2V (max) during a pulse of 71.7A (peak).  So your circuit needs to withstand say 24V to survive that kind of fault.

Same goes for MOVs; a 240VAC device might be rated 370V (at the "significant current" point) and clamp a big pulse in the 500-600V range.  The difference is, MOVs are massive hunks of ceramic, so they can withstand gangbusters energy spikes without failure.  (They're notorious for failing anyway, because too-small devices tend to be used.  Damage is cumulative, with an exponential weighting, so you can use an MOV like a zener diode practically forever, but you'll only get a few zaps in the 100A+ range before even a large one gives up.)

But even if your switcher shuts down at that voltage (overvoltage limit..?), it has to withstand all that voltage, too.  So you can easily get fried transistors despite best intentions.  But hey, we're talking, like, lightning strike or something, so if it escapes without too much vaporization (besides the MOV), that's not doing too bad.

Don't worry about consumer stuff; it isn't designed to be repaired, so they don't bother with it at all (only a fuse, or fusible resistor, for required safety protection).

Tim