250v Capacitor instead of 160v

[dayanpad]

Dear All,

please see the attached picture and please explain the red rectangular area capacitor is used 250v , But we can replaced 160v capacitor

is there any special reason to use 250v cap instead of 160v cap

Please advice
Thanks in advance

 

[MaxHeadRoom78]

The rest of the circuit is needed but any resultant DC is going to be 1.4 x the ACV.
Max.

 

[Nigel Goodwin]

It perhaps gives a greater margin of safety, but 160V should be perfectly fine - it's most like 250V simply because they use it elsewhere, and it's cheaper to order more of one value than two different values.

 

[dayanpad]

Dear Nigel
Thanks for the reply
Grater margin of safety only for capacitor but i think rest of parts will damage any high spicks

Please advice

 

[AnalogKid]

For decent reliability, a capacitor should be rated for twice the peak voltage across it.

Also, that is one strange power supply. What is it for?

ak

 

[spec]

Dear All,

please see the attached picture and please explain the red rectangular area capacitor is used 250v , But we can replaced 160v capacitor

is there any special reason to use 250v cap instead of 160v cap

Please advice
Thanks in advance

Hy dayanpad,

The peak voltage on your capacitor will be 1.414 * 110V = 155.54V. Allow 10% for mains fluctuations etc =171.094V and best to run a capacitor at no more than 0.66 of rating =256.50 so the 250V capacitor is the correct rating.

The other point though, is the current rating of the capacitor. If this is lower than the original the new capacitor may over heat and even explode. The resistor in series with the mains supply helps reduce the ripple current though.

So my advice, as the other members have said- get the proper component. I just thought you would like to see the calculations that led to the 250V rating.

 

[zahwi]

What an electrolytic cap doing on a transformer winding?
Seems like you will have to replace it many times in the future.

 

[spec]

What an electrolytic cap doing on a transformer winding?
Seems like you will have to replace it many times in the future.

Well spotted zahwi- I hadn't noticed that the capacitor is an electrolytic or the 10uF value. It is possible that the capacitor is a resovour. 160V at 10uF is quite a lot of energy. The rectifier diode/ diodes may be elsewhere. Some commercial circuits are very strange. I cant belive that capacitor has 115V RMS across it because it would explode pretty soon.

PS I changed my previous post because of your observation

 

[zahwi]

it because it would explode pretty soon.

Good, hi tech Christmas cracker.

 

[spec]

When I was a student we thought it was great fun to connect a 16uF electrolyetic across the 240V, 50Hz UK mains supply- you should have heard the bang and seen the mess- ally foil and electrolyte all over the lab. Our instructor wondered why his capacitors kept disappearing.

 

[ronsimpson]

that is one strange power supply. What is it for?

What an electrolytic cap doing on a transformer winding?

This circuit is in a monitor or TV set.
This is the high voltage/horizontal section.
I can't see every thing but:
The "115V" at the bottom left side is probably power into this section. So there is 115V on the cap. (not AC voltage, yes on AC current) and probably much AC current.
There is a chance that the "115V" is an output. But there would need to be a diode to ground off one of the transformer taps.

 

[dayanpad]

Hy dayanpad,

The peak voltage on your capacitor will be 1.414 * 110V = 155.54V. Allow 10% for mains fluctuations etc =171.094V and best to run a capacitor at no more than 0.66 of rating =256.50 so the 250V capacitor is the correct rating.

The other point though, is the current rating of the capacitor. If this is lower than the original the new capacitor may over heat and even explode. The resistor in series with the mains supply helps reduce the ripple current though.

So my advice, as the other members have said- get the proper component. I just thought you would like to see the calculations that led to the 250V rating.

Hy spec
Thanks for the detailed reply

In the above attached circuit the 115v is DC voltage from the power supply. therefore is it necessary to consider 1.414 ( Sorry for half image of the CCT)
Please advice further
Thanks in advance

 

[dayanpad]

This circuit is in a monitor or TV set.
This is the high voltage/horizontal section.
I can't see every thing but:
The "115V" at the bottom left side is probably power into this section. So there is 115V on the cap. (not AC voltage, yes on AC current) and probably much AC current.
There is a chance that the "115V" is an output. But there would need to be a diode to ground off one of the transformer taps.

Hi rons
You are correct ...this is TV set H section
115V goes to H out via FBT
Frequency at this area 15625Hz
Therefore can we replaced 160V cap instead of 250v

Please advice
Thanks in advance

 

[ronsimpson]

If I was designing the circuit I would measure the capacitor current and pick a cap that handles that AC current. That is because we plan to make millions of TVs and want a low failure rate.

In the repair world there is just one set. Voltage wise I think 160V is fine. Because I don't really know the current I would choose a low ESR capacitor (high current cap), just to be safe.

There is a change the label "115V" is not right.
I think you should replace the cap then measure the voltage just in case.
It might be that the higher voltage cap has a higher current rating and that is why it was chosen.
It also might be that this spot has problems with bad caps so some production engineer increased the voltage rating thinking it would help the failures.
I would not get the "smallest" capacitor you can find. Size matters.
This TV worked for years and even if you choose the wrong part and you only get one more year........

 

[spec]

Hy spec
Thanks for the detailed reply

No worries dayanpad- nice to have another member who says thanks and supplies info

In the above attached circuit the 115v is DC voltage from the power supply. therefore is it necessary to consider 1.414 ( Sorry for half image of the CCT)
Please advice further
Thanks in advance

About the 160V capacitor:

It is certain to be an aluminium foil, wet electrolytic type, in which case, all of the following will apply.

rons has all bases covered, but I would say that, with the info that you have supplied, the 160V replacement capacitor would be fine. There is just one caviat- under start-up conditions, there may be higher voltages on the cap, but unlikely to exceed 160V or cause a problem.

Although components have ratings, like the 160V of your replacement capacitor, the ratings are what the manufacturer can meet with a massive producttion run, sometime millions. The actual peformance of a specific component can be way better than the specification. Typically a manufacturer will achieve a below spec rate of 1% to 3%. This means that the buyer may get a below spec device right out of the box. They do not normally check each capacitor but, instead, do batch sampling. I would expect, statistically, that your replacement capacitotr will take around 200V in practice.

If you really want to be fancy, put a resistor, say 5K6, and a DC ammeter in series with the cap and put 160V DC across the circuit. Leave the cap for 10 minutes or longer. The current on the meter will probably settle down to around 100ua and should not be more than 500uA, say.

Never go near energy components, like batteries and caps, when they are an unkown quantity, and especially have your eyes protected and away from the cap. Think about the cap exploding; just a precaution, but it costs nothing to play safe.

Increase the voltage and see what the meter reads. The current should shoot up with a voltage increase as the cap charges up, but then fall back to almost nothing. When you reach 200V, with no dramatic increase in current, leave the capacitor for 30 minutes or more at 200V. If, after that time, the current has not increased notably, you will have a capacitor that will withstand 200V. On the other hand, if the current starts to grow rapidly at the quescent state, that is the limit of your capacitor's voltage withstanding. You may get short bursts of current as the capacitor reforms with increses in voltage.

The manufacturer's 200V rating will apply at some specified temperature: the higher the temperature the less voltage the cap can withstand without breaking down so, unless that area of the TV is very hot, that will be in your favour.

Another thing is that electrolytic capacitors are adaptive and form a film proportional to voltage (simplifying greatly). Any areas that will not stand a particular voltage will spark across (simplifying again) and destroy the offending foil area. This is how some capacitors are made in fact.

Large energy electrolytics have a very loose specifications for their capacitance value, typically -50% +100%, so they are not prcision devices- just blood and thunder. They will normally take quite a bit of abuse.

That's the voltage side sort of covered, but not the current side.

High energy caps have a ripple current rating. This just means the current flowing through the cap. It is limited by two factors: The foil and internal connection by instantanious current capability (think fuse) and the heating effect caused by power dissipation in the capacitor (long term) which heats up the liquid electrolyte and causes expansion and even gassing- some electro types have vents, and nearly all have blow off plugs at the end to release pressure in the event of over gassing due to excessive heat.

Capacitors will normally take massive currents for short periods, as in a conventional rectifier circuit without an inductor. The formula for the heating effect is (ripple current squared times ESR). ESR is the effective series resistance of the capacitor and is affected by various things including temperature.

Rons' method of measuring the ripple current with a DC ammeter in series with the capacitor input is, by far, the best method. But if that is not possible, you can measure the AC voltage on the capacitor, which will give you an idea of the ripple current. Use a scope, or if no scope, an AC voltmeter in series with a 100nF (say) solid capacitor. The AC voltmater must be capable of handling the the freqencies involved though.

The other aspect about ESR is that, if too high, it can actually stop the circuit from working or make it work inefficiently, especially in modern high frequency switch mode power supplies.

Electrolytic capacitors are lifed items and need to be replaced typically after 2,500hours to 5,000 hours. This time is very much reduced by high temperatures and high currents (which cause high tempersture in the can)

If your capacitor has any meaningfull markings on it, if you like, post it exactly as printed and I may be able to check specifically what the capacitor it is.

I have finished now, but one last word about safety: make sure you leave a resistor (100K say) across the cap for a few minutes to remove all the charge. It is no good just discharging a capacitor to zero volts and then removing the resistor because the voltage can recover by dialectric absorbsion, an effect that is not fully understood.

Hope you are not too bored- I have done quite a bit with capacitors for specialist applications.

Must have some breakfast now- just woken up

 

[Nigel Goodwin]

There is a chance that the "115V" is an output. But there would need to be a diode to ground off one of the transformer taps.

No chance at all

It's a pretty standard old TV LOPT stage, the 115V comes from the PSU to feed the LOPT.

 

[spec]

No chance at all

It's a pretty standard old TV LOPT stage, the 115V comes from the PSU to feed the LOPT.

Silly me But we didn't have that info when I posted. Many SMPSUs do put the rectifying diodes in stange places though. But, I must admit that I was half asleep/distracted by global warming when I first posted in this thread. The 115V threw me too- I was thinking US/Can AC mains supply.

LopTee, that takes me back- always a source of trouble. As a nipper I used to wonder what these LopTees were that my dad spoke about to his mates- they must be some very advanced magic device going by the name!

You would think that the poor old LOPT would have enough on its hands driving the CRT line san coils, without having to generate the EHT and other assorted voltages for the set. Can't seem to find the LopTee on our plasma set- where are they normally fitted these days?

 

[Nigel Goodwin]

Silly me But we didn't have that info when I posted.

We did

It's blatantly obvious from the circuit he posted

Can't seem to find the LopTee on our plasma set- where are they normally fitted these days?

Assuming that's not a sarcastic question?, there obviously isn't one as there's no CRT so no line or frame stages (Plasma and LCD aren't even scanned devices).

 

[spec]

We did

It's blatantly obvious from the circuit he posted

Ah, but 'we' are much clever than me!

...sarcastic...?...

Incorrect word usage and punctuation

...there obviously isn't one as there's no CRT so no line or frame stages (Plasma and LCD aren't even scanned devices).

Are you having a bad day?

(hope you post again Nigel, and tell me some more- not sarcastic)

 

[ronsimpson]

Above there was a question about how can there be a capacitor on a transformer pin?
AND
There was a question about 115V in OR out.
Also there are many things missing in the schematic.
On the left is a typical horizontal/ HV section in a TV. 115V is from the supply.
On the right is a typical "boost" method where 80V is from the supply. When the transistor is closed the tapped primary creates the 115V. In this case C4 is worked hard!