resistor frequently blown

[Nigel Goodwin]

I stand by my original comments. This is not a power resistor, but it will blow with a lot of power if Line and neutral are reverse wired in house. even if TV is correct with 3 pronged plug.

The correct value resistor is FAR too high to blow, on the circuits I checked they were either 4.7M or 10M - so it certainly wouldn't blow, and is designed to be used either way round.

Not according to UL requirements. it is 3.5mA at 60Hz leakage to ground MAX per stationary unit.

Again, you're quoting the wrong thing - we were discussing a resistor between earth and neutral blowing - this isn't anything to do with UL specs (which apply to the unit), but to the possible voltage difference between earth and neutral, which can be a few volts and enough current to be useful (or in this case to blow a low value resistor incorrectly connected across them).

Be a bit more 'exciting' with the mains plug reversed though

They curbed using 2 pin plugs since VGA ports were added to TV's and monitors to reduce emissions.
This unit is 3 pin, unless a cheater cord was used.

It wasn't clear if the unit stopped working or it just started to fizzle. Nor was it clear what R values were originally there and replaced with same value and power rating and if in close proximity to a PTC which will regulate at 85'C during protection.


Burnt R can be 200K 1/4W

As I indicated as CCFL's age they draw more power which would only affect series power drop Resistors.

A small increase in CCFL current isn't going to blow a surge limiter, even a LARGE increase in CCFL current wouldn't, the worst possible case scenario is that the PSU would current limit.

But as someone who repairs such things professionally, it doesn't happen - even assuming you could get excessive consumption to that degree with a CCFL (which I seriously doubt) the inverter protection circuit shuts it down long before the PSU would get involved.

Putting 120V across a 10K R used to bleed a Y2 Cap would certainly do that and still work as a TV unit with SMPS isolation but noise interference. This could only happen if Line & Neutral were reversed unintentionally. ( DIY wiring by someone)

This is how I would show schematic. Feel free to correct any assumptions.

That's looks pretty well what we've all been assuming.

Incidentally, it's fairly common for such safety components to go to earth from the output of the bridge rectifier, making the leakage equal regardless of plug polarity.

NOTE: CX101 means it is an X rated Cap line to line.
CY101 means it is a Y rated cap. Common mode to ground

Burnt R can be 200K 1/4W
Cap must be film Y rated for the Y rated position.

The resistor should a similar special safety component, and not just a normal resistor - and certainly not just a 1/4K one.

CY101 makes the antenna slightly hot ( < 3.5mA)
edit... ( but only if you got no safety ground and a miswired AC outlet. )

It's common (and perfectly normal) to get a slight 'tingle' off class II AV equipment.

 

[Tony Stewart]

Nigel said...The correct value resistor is FAR too high to blow, on the circuits I checked they were either 4.7M or 10M - so it certainly wouldn't blow, and is designed to be used either way round.

Do we know what value was used that blows? If it was neutral as designed, it would never blow regardless , hence miswired. .. Tony

Again, you're quoting the wrong thing - we were discussing a resistor between earth and neutral blowing - this isn't anything to do with UL specs (which apply to the unit), but to the possible voltage difference between earth and neutral, which can be a few volts and enough current to be useful (or in this case to blow a low value resistor incorrectly connected across them).

I am correct on this matter. Leakage is performed by shorting line and neutral and measuring AC to gnd current. 3.5mA max for IEC and UL . It should never be a low value. If so , then improper repair and improper connections to AC are valid conclusions.

Be a bit more 'exciting' with the mains plug reversed though



A small increase in CCFL current isn't going to blow a surge limiter, even a LARGE increase in CCFL current wouldn't, the worst possible case scenario is that the PSU would current limit.

If it were a series Resistor of low value but not too low, as I suspected before I saw the layout, then a 10 Watt increase in CCFL is possible which could result in series loss. But my assumption was series and it now parallel

But as someone who repairs such things professionally, it doesn't happen - even assuming you could get excessive consumption to that degree with a CCFL (which I seriously doubt) the inverter protection circuit shuts it down long before the PSU would get involved.

Correct. assuming user put in wrong value ( plausible) in a series circuit that resulted in 40W loss and 40W Load. It would cause this result.
since it works from 100~240Vac a drop on input voltage of a series R is possible to burn with the wrong value. Most of the power consumed in an LCD is for backlight in this 40W PSU. again it is not in series, that much we know , but you still dont know value he used and what input was connected... line or neutral .. to analyze the fault that makes sense... as I have done.



That's looks pretty well what we've all been assuming.

Incidentally, it's fairly common for such safety components to go to earth from the output of the bridge rectifier, making the leakage equal regardless of plug polarity.

I know, They are designed to be equal to reduce the common mode leakage in half.

The resistor should a similar special safety component, and not just a normal resistor - and certainly not just a 1/4K one.

They use 3 SMD line to line, but Neutral to ground only needs 500V min which is std. for 1/4W . in case of ground fault.

It's common (and perfectly normal) to get a slight 'tingle' off class II AV equipment.[/QUOTE]

Every laptop gives a tingle if outside , especially on a bare knee or elbow in wet grass. , and they are isolated too with no earth connection to case. THe reason here is different. the SMPS switch rate coupling capacitance causes leakage across the transformer. But this is safer than if one had a ground fault condition with an earthed metal laptop and user between.

I recognize your experience in repairing consumer electronics. Perhaps you dont recognize my 35 yrs in R&D and Test Engineering... respectfully, Tony

 

[ngzek]

Are you saying you don't know which pin is line or neutral and how the CM filter works or how to ensure a safety ground is on a metal container cooler?

yet you are trying to repair this?

My confession to Tony Stewart I never think about our house cabling problem. I just start suspecting our building line when you talk about it. I do not know how to ensure a safety ground. Electronics become a must and become a new hobby while practicing my computer hw&n.

 

[Tony Stewart]

The filter cap is to reduce noise on AM radio, and also reduce lightning disturbance effects on TV and the R to prevent discharge to finger if touched just after unplugged ...

If you get thunderstorm, it may be safer to unplug TV,

For safety use 3 pin plug and check with meter.

I suggest a cheap Volt Ohm meter if you want to learn.
Neutral maybe only be a few volts from house wiring resistance and current.
Ground pin to chassis and other electrical grounded boxes...check voltage between cases first then if near 0, check ohms <1

so who cares, if TV works now, don't fix it.

 

[ngzek]

The filter cap is to reduce noise on AM radio, and also reduce lightning disturbance effects on TV and the R to prevent discharge to finger if touched just after unplugged ...

If you get thunderstorm, it may be safer to unplug TV,

For safety use 3 pin plug and check with meter.

I suggest a cheap Volt Ohm meter if you want to learn.
Neutral maybe only be a few volts from house wiring resistance and current.
Ground pin to chassis and other electrical grounded boxes...check voltage between cases first then if near 0, check ohms <1

so who cares, if TV works now, don't fix it.

Yes its working and I have a cheap volt ohm meter. Thank you Tony Stewart