MOVs

I think you're mixing up MOV and NTC Thermistor (surgistor or inrush current limiter). The MOV is an overvoltage limiter and goes ACROSS the power line. The NTC thermistor goes in SERIES with the AC power and limits the initial inrush current. You'll need to trace out the schematic of the power line filter to see which one (or both) you're dealing with.

In normal use, I've never seen MOV's or surgistors blow. I've seen pleny of them blow up when the AC line conditions are all wrong, such as lightning hits, unbalanced phases causing the voltage to increase, power supply overload causing the surgistor to draw too much current, and such. I've also seen a few design errors, where the surgistor specs were inproperly selected.

I can guess my way through your problem, but I'm busy/lazy and would prefer you to do the dirty work. Do some measurements such as a scope picture of the inrush current, normal operation, extreme operation, etc with a line current sensor. Get the numbers off the MOV and surgistor and I'll see if they were properly selected. Read some design articles on selecting inrush current limiters.

Incidentally, most (not all) of the devices I've seen that have inrush current problems also use slow blow fuses.

Good luck.

--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
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Skype: JeffLiebermann     AE6KS    831-336-2558

Thanks for that correction, Jeff. Yes, in my haste I got the wrong term for the component in question. Must be my advancing age. :(

rosoft Help

got to

** Going short circuit is likely the result of one of the capacitors across the AC line failing - generally these are "class X" types which have a limited life expectancy. The other possibility is the "common mode" choke but that is very unlikely. ** Very unlikely too. The surge current may be quite large but also very brief, so not enough energy to damage copper winding wire which in any case would blow open. ** So the equipment IS rather old ? Like 20 years + ** Small SMPSs can use a resistor in series with the supply to limit surges - say 5 or 10 ohms, wire-wound. Otherwise it is normal to use an NTC thermistor with similar cold resistance values.

The resistor or NTC limits the surge current to a value that is tolerated indefinitely by fuses, diodes and switches used in the PSU - ie something like 20amps peak for a couple of milliseconds.

PSU makers also have to consider the situation where multiple supplies are on the same AC circuit and all switched together creating a very large surge that may trip the breaker.

.... Phil

Yes, most like a cap.

Almost always. Typically 70s & 80s. I can't deal with SMDs anyway.

Noted, thank you.

" The resistor or NTC limits the surge current to a value that is tolerated indefinitely by fuses, diodes and switches used in the PSU - ie something like 20amps peak for a couple of milliseconds. "

** The surge occurs soon as the PSU is switched on - the magnitude depends on the timing relative to the AC wave. My comments referred to the worst case surge, when the switch on is at or near an AC voltage peak. When this happens and if the total series resistance is say 5 ohms and the main electro is 220uF, the time constant is 1mS.

Remember, the context was all about *SMALL* SMPSs.

.... Phil

I suppose the advantage of the resistor solution - apart from the fact that I already have lots of suitable ones in my junk box - is that the PSU remains protected even when the power supply is temporarily interrupted, say when someone turns it off and on again over a short interval.