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capacitor replacement

andy6384

Jul 3, 2014
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I'm repairing a power supply from an old 1980's vintage trs-80 computer, a capacitor has gone. It's across a transformer so possibly just a filter.

Apart from polarised types like electrolytics, do I have to replace it with one of the same type
(ceramic for ceramic, metal foil for metal foil etc), or as long as the value and voltage are the same then
any will do.
I think the blown one is a metal foil type. I have some modern polyester ones, same value and voltage. They
are tiny in comparison though, maybe 10% the size. Are these OK?
 

Harald Kapp

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Hello and welcome to our forum.

A cpacitor acrosss a transformer is very likely a Y-capacitor and shall be replaced only by another Y-capacitor No other type of capacitor must be used!
It is not important from which materials the capacitor is made as long as it is rated as Y-capacitor. You will find "Y" printed on the capacitor (see sample images in the Wikipedia rticle). If there is no marking, the capacitor is not Y-rated.
 

KrisBlueNZ

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Right, as Harald says, capacitors designed for direct connection across the AC mains have special characteristics. They are divided into four classes: X2, X1, Y2 and Y1. See https://en.wikipedia.org/wiki/Film_capacitor#AC_voltage_and_current. You can use any of those types for connection across the AC mains; X2 will be cheapest.

Here's Digikey's selection table: http://www.digikey.com/product-sear...0002,fff80010&stock=1&quantity=1&pageSize=250

Use the capacitance and AC voltage fields to narrow down the options. This will give you an idea of what you're looking for even if you don't buy one from Digikey.

If you don't know the value of the old one, it's reasonable to assume 0.1 µF with an AC voltage rating of 160V for 115 VAC mains, or 275V for 230V mains.
 

Arouse1973

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Can I just add something minor. If the capacitor is connected from the live to earth for common mode filtering it must only be a Y type. This is because they tend to be more reliable have higher reactance due to the value of the capacitor and will limit the current to earth which otherwise could trip an RCD. The X and Y caps are also self healing I believe and have good mains spike tolerance as a result. But the ceramic versions do not.
Adam
 

davenn

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I would like to suggest ... everyone is jumping the gun here

to the OP, what transformer ? close to the mains input ??

how about some sharp and well lit pics of the area of the circuit board concerned

cheers
Dave
 

(*steve*)

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I agree with Dave.

And as for a comment about reliability of Y type capacitors. They're no more reliable, it's just that they are far less likely to fail short circuit.

Anyone want to see an X rated cap that's failed short circuit (like they're NOT supposed to do either)?
 

KrisBlueNZ

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Yes, Dave and Steve are right, I did jump the gun, and if the capacitor is not connected between Phase and Neutral, or across the primary of the mains transformer, as described in post #1 ("across a transformer"), then Adam and Harald are right that it needs to be a Y-rated cap, not X-rated.
 

Harald Kapp

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Care to tell a dumb non-native speaker what "to jump the gun" means?
o_O
 

KrisBlueNZ

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Come on Harald, you don't want us to send you a lmgtfy link do you?

To jump the gun is to "act before the proper or appropriate time". I guess it comes from the starting gun in a race. If you start running before the gun is fired, you would be "jumping the gun".
 

davenn

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yup exactly, Kris

and in this case, the Op was getting all sorts of specific advice before the true nature of the situation was clarified ;)


Dave
 

davenn

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so what's the German equivalent saying, Harald ?

is there something similar ?


Dave
 

Harald Kapp

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Something like "einen Frühstart hinlegen" (to start too early) which pretty much matches what Kris' said in post #9.
 

Arouse1973

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I agree with Dave.

And as for a comment about reliability of Y type capacitors. They're no more reliable, it's just that they are far less likely to fail short circuit.

Anyone want to see an X rated cap that's failed short circuit (like they're NOT supposed to do either)?

Steve maybe you could have proposed a better wording for my comment instead of the comment you posted.

I also found some information about X and Y capacitors. Maybe robustness is a better word but they do seem to be tested to enhanced reliability standards.

Extract :http://powerblog.vicorpower.com/2013/06/what-are-y-capacitors/
When does capacitor reliability become critical to safety?

Line filter capacitors are classified either as X-capacitors or Y-capacitors. X-capacitors are connected between line and neutral, to protect against differential mode interference. Their failure does not create conditions for dangerous electric shock, although it can create a fire risk. However Y-capacitors are designed to filter out common-mode noise, and are connected between line and chassis; if they short-circuit, they create a risk of shock to the user.
How are Y-Capacitors designed and deployed to ensure safety?
Y-capacitors are designed to enhanced electrical and mechanical reliability standards. Capacitance values are also limited to reduce the current passing through the capacitor when AC voltage is applied, and reduce the energy stored to a safe limit when DC voltage is applied. Capacitors must be tested to applicable standards to qualify them for use as Y-capacitors.

Extract : http://www.vishay.com/docs/26033/gentechinfofilm.pdf
Y-Capacitors
Capacitors for suppression of asymmetrical interference voltage, and are located between a live wire and a metal case which may be touched. High electrical and mechanical reliability to prevent short-circuits in the capacitors. The capacitance value is limited, in order to reduce the AC current flowing through the capacitor. By following these technical requirements, it is intended that its failure will not lead to the risk of electrical shock, making the device with Y capacitor (in conjunction with other protective measures) safe to human beings and animals.

Extract : http://www.tecategroup.com/capacitors/applications/ac-safety-capacitors.php

Image_005.gif
T1= 10 µsecond, T2=700 µsecond in Telecom applications (IEC 60950)
T1= 1.2 µsecond, T2=500 µsecond in Mains power applications (IEC 60384-14)

Both X and Y capacitors have the same potential for failure short circuit but Y capacitors are subjected to higher surge voltages during approvals. So I suppose if they were used in place of X types then they would indicate a possible increase in reliability.

Adam
 

(*steve*)

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This probably needs to go somewhere else, but X and Y rated capacitors are designed so that a dielectric failure won't cause a short.

So whilst they might suffer failures, the failure mode (for metalised film types) is supposed to result in a small vaporized section of metalization, rather than a continuing short. The obvious safety implications are that failure does not (or should not) result in a hazardous fault.

The testing as shown above does not require that the capacitor remain undamaged, just that a bad failure mode does not occur. The small (self healing) failures result in a gradual loss of capacitance.

The wikipedia page states: "To receive safety approvals, X and Y powerline-rated capacitors are destructively tested to the point of failure. Even when exposed to large overvoltage surges, these safety-rated capacitors must fail in a fail-safe manner that will not endanger personnel or property"

The document you linked to above notes that following the surge testing (obviously not tests to destruction) the capacitor must "still perform reliably in the circuit, under AC voltage conditions". It doesn't say they need to be undamaged or have the full original capacitance.

In some respects you could say they are more reliable, and it was perhaps incorrect of me to say they are not more reliable. But the primary concern is failure modes and their impact on safety.
 

Arouse1973

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I never mentioned their failure mode only that Y capacitors should be used from Live to Earth, that's all.
 
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