Help identifying MOV

[melmsmidth]

Hello everybody!
I have a Furman Power conditioner that randomly jumps into protection mode. I think the problem can be this MOV (labeled as MOV1 on the board) but I can't identify the value, I have little experience with this kind of components.
It is placed between the two input mains wires (230V here) and it is connected to a 10AMP resetteable fuse on one of the lugs.
Can anyone help me with the value?
Thanks in advance!

 

[Khron]

I have certain doubts it's the MOV. When those go, they go with a pop/bang, and there's plenty of soot around (which is why they're sometimes heatshrunk).

What devices do you have plugged into that? Or does it do that even with no load connected?

 

[melmsmidth]

The MOV was heatshrunk indeed. It does it with no load connected so the problem is inside the Furman.

It first stopped working going always into protection mode, and I changed a cap near the relay that I think starts the protection (you can see the info here No Power FURMAN PL-8 II Power Conditioner Repaired | Electronics Repair And Technology News). It came back to life but whith this issue that randomly goes into protection mode.

I read somewhere that sometimes the MOVs can fail intermittently before they go bad and that's why I thought about replacing it. It's a cheap part but I can't identify the value.

Any idea on where to look at? Maybe the resettable fuse?

 

[Walrus]

As it is across the mains, just take it out and see if the problem still occurs.

 

[Walrus]

Also, that capacitor, I would think, is in line with the mains live as a dropper resistor.
You did replace it with a Y rated capacitor didn't you?

 

[Khron]

The MOV may well be a 20mm diameter (going by the V20K marking), and is that "420" on the second row? That would likely be the clamping voltage.

Also, that capacitor, I would think, is in line with the mains live as a dropper resistor.
You did replace it with a Y rated capacitor didn't you?

You'd probably want an X2-rated capacitor there, not a Y1/Y2 one.

I could be wrong on this, but i kinda recall hearing years ago that X-class are supposed to fail open-circuit (since they're meant to be connected between live and neutral), while Y-class are supposed to fail short-circuit (meant to go between either of the mains connections, and safety-earth).

 

[Walrus]

The website I just read says the opposite. It could be wrong....
https://www.allaboutcircuits.com/technical-articles/safety-capacitor-class-x-and-class-y-capacitors/

 

[melmsmidth]

Well, as you said it's not the MOV. The original (and replacement) cap was metallized polyester, 270nF 630V and seems to be ok too.

I think the problem lies somewhere else and now the Furman goes into protection mode inmediately. Maybe the Relay or something around but this is out of my knowledge so I presume I will let it go. I don't need the Furman since I already bought another one and I think I can salvage the chasis to some other project

Thanks anyway for your help!

 

[Khron]

The website I just read says the opposite. It could be wrong....
https://www.allaboutcircuits.com/technical-articles/safety-capacitor-class-x-and-class-y-capacitors/

I stand corrected

Well, as you said it's not the MOV. The original (and replacement) cap was metallized polyester, 270nF 630V and seems to be ok too.

I think the problem lies somewhere else and now the Furman goes into protection mode inmediately. Maybe the Relay or something around but this is out of my knowledge so I presume I will let it go. I don't need the Furman since I already bought another one and I think I can salvage the chasis to some other project

Thanks anyway for your help!

Hate to say "i told you so", but...

Now i'm just getting curious what sort of "protection circuitry" resides in there

 

[melmsmidth]

Hate to say "i told you so", but...

Yes, of course you were right, I have a lot to learn yet

About the protection circuit, I can only say that there's a relay that must be activated so the hot (live) goes to next stage (filtering). Seems like the Neutral goes straight to power filtering and then the outputs.

There's a diode bridge, transistor, etc. that seems to provide the power to the relay and the "protection ok" "extreme voltage" leds. I think the combination with the MOV and a 10 amp circuit breaker (and something else of course) detects peaks or overvoltages and prevents the relay to activate so there is no Hot connection. There is a second MOV further, this one with three legs, but it seems to be ok too at first sight.

Right now, something is wrong with the relay or around it, it doesn't get activated (no click when turning on the unit as it used to do).

But as I said, this is way over my knowledge, know the basics but I'm a builder and sometimes I'm smart enough to repair some of my studio stuff...

 

[melmsmidth]

Well, I had to give it a last chance.

There is a trimpot inside, near the relay, labeled "TRIP POINT". No idea of what it does, just moved it a bit back and forth several times and back to it's original position and now it seems to be working properly... so far.

Maybe it was adjusting voltage for the relay to turn on and had some dirt. Who knows.

Sorry I'm not smart enough to give you useful information, I have a lot to study yet!

 

[MisterCMRR]

Since MOVs generally fail by becoming open (or nearly open) circuits, and the unit tries to determine if the MOV is electrically present in the circuit to light the "protection" indicator. Since an MOV has appreciable capacitance, like any capacitor connected across the AC line, a small current flows in it. The sensing circuit likely puts the diode bridge in series with the MOV, along with a resistor and transistor to sense if the MOV current is there. Removing the MOV from circuit should extinguish the indicator since that would be the same as a blown MOV. I'd guess that the trimpot adjusts the sensing threshold. Perhaps the pot itself had developed a bad wiper contact over time (it happens) and just moving it cured the problem. Of course, without tracing out the actual circuit, this is just guesswork on my part. But I do know MOVs pretty well - I'm designing a high-end surge protector (series-mode, as Surge-X) and voltage regulator for a well-known Canadian company.

 

[kags]

I had a Furman do the same thing and it was related to the soft-start board. Ultimately reseating connectors and cleaning contacts solved the intermittent problem. Furman sent me a full set of schematics at no charge.

 

[melmsmidth]

Since MOVs generally fail by becoming open (or nearly open) circuits, and the unit tries to determine if the MOV is electrically present in the circuit to light the "protection" indicator. Since an MOV has appreciable capacitance, like any capacitor connected across the AC line, a small current flows in it. The sensing circuit likely puts the diode bridge in series with the MOV, along with a resistor and transistor to sense if the MOV current is there. Removing the MOV from circuit should extinguish the indicator since that would be the same as a blown MOV. I'd guess that the trimpot adjusts the sensing threshold. Perhaps the pot itself had developed a bad wiper contact over time (it happens) and just moving it cured the problem. Of course, without tracing out the actual circuit, this is just guesswork on my part. But I do know MOVs pretty well - I'm designing a high-end surge protector (series-mode, as Surge-X) and voltage regulator for a well-known Canadian company.

It makes perfect sense, I think your guesswork is quite good since all the elements are on the board and connected as you say, it's for sure something similar.

Thanks for the very good explanation.

Furman sent me a full set of schematics at no charge.

I will try next time if nedded. Good to know, thanks.

 

[MisterCMRR]

Great, glad I could help in some small way!

 

[MisterCMRR]

I'd be interested to see the schematics if you'd be willing to share ...

 

[pucho812]

Would it be possibly to put schematics in the tech docs section?