Hi all, I'm an electrician so actually only have a fairly basic grasp of electronic circuitry. I have a Panasonic LED tv which is blowing the fuse 10a on the power board as soon as voltage (240v, I'm in New Zealand) is applied. I've had a quick look over the board and there appears to be a dead short on this capacitor.
Doing a bit of painstaking Googling of the 3R9 code leads me to believe it is a 3.9pf capacitor, but is there any other information on the code below which I'm missing?

I can't seem to find an example of the exact cap anywhere online. Can anyone suggest a replacement?

Pic attached. Any help will be greatly appreciated!

Jesse

 

Welcome to AAC!

I can't seem to find an example of the exact cap anywhere online. Can anyone suggest a replacement?

It's probably not a capacitor. MOVs also come in that form factor.

 

Ah! OK, Thanks for that. I'll do some more googling!

 

I've had another check and located what is definitely a MOV further back near the fuse etc.
I'll add a couple of photos in the hope someone may have some ideas..

 

I think it is an NTC thermistor with a cold resistance of 3.9Ω, used as an inrush current limiter.

 

That part marked 3R9 is definitely broken. Notice the swelling in the vacinity of the crack? Maybe you had a nearby lightning strike.

If these are MOVs then you can temporarily remove them without much worry of further damage since MOVs should only be active under unusual circumstances, and see whether removing them solves the fuse blowing problem.


By the way, your photographs are way above average and that makes things a lot easier.

 

A MOV would be connected in parallel with the power input but this appears to be in series.

 

The ZNR is definitely a MOV type of surge/transient absorption device. So, the other device may be another MOV, not likely, but possible, or more likely a Thermistor to limit inrush current as Alberthall says.

 

I think it is an NTC thermistor with a cold resistance of 3.9Ω, used as an inrush current limiter.

Thanks for the reply. This may be taking me down the right path. Done some more googling, still can't find the exact same component with the codes, but do you think NTC or PTC?

Cheers

 

If it is indeed there to limit inrush then it must be NTC.

 

If it is indeed there to limit inrush then it must be NTC.

Ok, I will keep searching down this path!

 

I would suggest based upon you saying its in series with the input mains, its in the mains input area, and the picture, its a NTC thermister used as a surge protector.

That C with an M inside it, is the copy write for a Murata part,

These links might be of help.

https://www.digikey.be/product-detail/en/murata-electronics/NTPAA3R9LDNB0/490-3954-ND/966692

https://passive-components.eu/how-to-use-ntc-thermistors-for-inrush-current-limiting/

https://www.allelectronics.com/item/tnr-3.9/inrush-current-suppressor-3.9-ohm/1.html

"problem" is there are many similar devices,

Question might be why its gone, and the fuse point,
if it was dead open circuit, then the unit would not work,
If its gone short circuit, which is IMHO unusual, then the excess current at startup should be very short, a few mains cycles, and should not blow a fuse immediately,

But these things run HOT , 100 plus degrees, hotter in a fault condition,
could be that its failed short circuit, and something else is actualy causing the problem,

As a last option,,,,

You could take it out and see what happens. Unit should not start or blow fuse,
then try shorting the pads where it was, , suggest a bit of fuse wire, and stand back,
if it was a NTC , then the unit should start with the part shorted,

 

I would suggest based upon you saying its in series with the input mains, its in the mains input area, and the picture, its a NTC thermister used as a surge protector.

That C with an M inside it, is the copy write for a Murata part,

These links might be of help.

https://www.digikey.be/product-detail/en/murata-electronics/NTPAA3R9LDNB0/490-3954-ND/966692

https://passive-components.eu/how-to-use-ntc-thermistors-for-inrush-current-limiting/

https://www.allelectronics.com/item/tnr-3.9/inrush-current-suppressor-3.9-ohm/1.html

"problem" is there are many similar devices,

Question might be why its gone, and the fuse point,
if it was dead open circuit, then the unit would not work,
If its gone short circuit, which is IMHO unusual, then the excess current at startup should be very short, a few mains cycles, and should not blow a fuse immediately,

But these things run HOT , 100 plus degrees, hotter in a fault condition,
could be that its failed short circuit, and something else is actualy causing the problem,

As a last option,,,,

You could take it out and see what happens. Unit should not start or blow fuse,
then try shorting the pads where it was, , suggest a bit of fuse wire, and stand back,
if it was a NTC , then the unit should start with the part shorted,

Thanks for all that. I appear to be getting closer now.
Yes, my next question was going to be whether it's safe to remove it and then power the board up. Would shorting the pads with fuse wire not essentially do the same thing as this shorted component is currently doing? To me it appears to be in series...

 

Thanks for all that. I appear to be getting closer now.
Yes, my next question was going to be whether it's safe to remove it and then power the board up. Would shorting the pads with fuse wire not essentially do the same thing as this shorted component is currently doing? To me it appears to be in series...

What value would you expect to see across the terminals of a 'good' thermistor? It's just dawned on my that I'm saying it's shorted, but the reading I'm getting on my auto multimeter is 10ohm...

 

My guess from the marking would be 3.9Ω which is sometimes written as 3R9 (to avoid confusion caused by not seeing the decimal point) but note that its resistance will change a lot as the temperature changes.

 

My guess is a cap and a 3R9 would lend me to believe a 3.9 pF cap. 0.0039 nF but yeah, I am going with a cap. Cracked like it is a shorted cap. Could always remove it and power up and see what happens because popping a 220 VAC 10 Amp fuse takes a pretty good short. Anyway, that's my guess and I am sticking with it.

Ron

 

Re a capacitor, why would a cap have resistance, and why would a cap be in series with the power in ?

re its resistance, NTC resistors by definition have a resistance that changes with temperature.

 

Re a capacitor, why would a cap have resistance, and why would a cap be in series with the power in ?

re its resistance, NTC resistors by definition have a resistance that changes with temperature.

My bad, missed the part of i"n series with the mains". Saw immediately blows 10 amp fuse. Never did quite get how it is used in circuit.

Ron

 

I don't think it has been confirmed that is in series with the power input, but it does look to me that it is looking at the PCB picture.

 

It's more typical for a MOV to fail short than a resistor. If current isn't limited, they destroy themselves completely and then they're open circuit. Sort of like electrolytic caps. The initial failure is usually a short that often results in them opening.