Replacement Capacitor Question

[Mooly]

They should be fine in that case.

All they are doing (mostly I would think) is making sure the device meets standards for radiated interference. You could argue that fitting leaded parts will alter that design aspect anyway due to lead inductance, which leaves the door open to just leaving them out altogether or perhaps fitting a standard sized filter cap across L and N at the mains input. That's something only you can decide.

 

[AndrewT]

Thus the standard voltage of primary rectifiers found in ac adapters is 400Vac.

Your answer, to use 400Vac rectifier in a 250Vac supply seems to be confirming the 3times for PIV rating and half that for the individual diodes of a discrete rectifier.

Did you mean to type 400Vac? or should it have read 400Vdc?

 

[AndrewT]

................All they are doing (mostly I would think) is making sure the device meets standards for radiated interference. You could argue that fitting leaded parts will alter that design aspect anyway due to lead inductance, which leaves the door open to just leaving them out altogether or perhaps fitting a standard sized filter cap across L and N at the mains input. That's something only you can decide.

The small film or ceramic cap will attenuate the voltage spikes that come in riding on the mains.

Those spikes use up the life of subsequent capacitors.
Earlier attenuation improves the life of subsequent capacitors.

 

[Mooly]

Andrew, a 400vac bridge was the once standard industry standard fitment for mains use but reliability concerns and the now easy and cheap availability of 600, 800 and 1000 volt devices mean these are more likely to be seen. Mains supplied bridges made up of 1N4004 diodes were commonplace not so many years back.

Most small to medium SMPS I've worked on use a choke input before the bridge but its not mandatory.

 

[DQ828]

Thanks everybody, I continue to learn at the hands of enthusiasts.

 

[AndrewT]

Andrew, a 400vac bridge was the once standard industry standard fitment for mains use but reliability concerns and the now easy and cheap availability of 600, 800 and 1000 volt devices mean these are more likely to be seen. Mains supplied bridges made up of 1N4004 diodes were commonplace not so many years back.

Most small to medium SMPS I've worked on use a choke input before the bridge but its not mandatory.

if you both mean 400Vac rating for the bridge rectifier, then that is equivalent to ~550Vdc.
This is well below the three times that I thought was the accepted PIV requirement.

250Vac requires ~750V PIV rating.
That would be roughly equivalent to 530Vac
So having to change from 400Vac to >=600Vac does seem to fit with using 3times AC voltage for the PIV rating.

But back to my question:
Are the individual diodes inside a bridge rectifier each exposed to half the PIV rating?

 

[AndrewT]

................mains input to an SMPS. L and N via a choke and applied to the bridge.

...................More usually no choke and via a current limiting Power NTC.

.............Most small to medium SMPS I've worked on use a choke input before the bridge but its not mandatory.

I have looked inside about a dozen different type/duty smps and none had a choke in the feed to the bridge rectifier (usually a discrete rather than an integrated).

Many had a filter with a common mode choke, but that is an interference attenuation device, not a current limiting device.

 

[Mooly]

How did we get on to current limiting and inrush limiting ?

I looked at the photo and saw what appears to be a standard input arrangement consisting of a small common mode choke, which I simply abbreviated to choke. If there are class Y caps from L and N to ground (and the grounded equipment casing if applicable) then they also supress common mode noise. Caps directly across the line (L to N) supress differential mode noise.

 

[29285]

Your answer, to use 400Vac rectifier in a 250Vac supply seems to be confirming the 3times for PIV rating and half that for the individual diodes of a discrete rectifier.

Did you mean to type 400Vac? or should it have read 400Vdc?

I mean 400V blocking voltage, which is for my knowledge the normal specification of rectifiers. This is the ac peak value which equals rectified peak voltage.
Today I had a look at rectifiers offered by digikey. Obviously they mostly sell 600V-types nowadays.

Each diode of the bridge rectifier is exposed to the rectified voltage across the bulk cap, which equals the amplitude or peak value of mains voltage.
Assuming 230Vac this is about 310V.

 

[29285]

How did we get on to current limiting and inrush limiting ?

good question - considering specs of manufacturers inrush current must be limited to something between 30~200 amps to protect the rectifier. This can be achieved with an ntc inrush limiting resistor.

Without that series resistor of several ohms, there is little chance to survive a standard surge test.

 

[AndrewT]

How did we get on to current limiting and inrush limiting ?

Because you said the smps that you were familiar with had a choke input

I looked at the photo and saw what appears to be a standard input arrangement consisting of a small common mode choke, which I simply abbreviated to choke. If there are class Y caps from L and N to ground (and the grounded equipment casing if applicable) then they also supress common mode noise. Caps directly across the line (L to N) supress differential mode noise.

It now appears you were referring to interference attenuation.

 

[AndrewT]

I mean 400V blocking voltage, which is for my knowledge the normal specification of rectifiers. This is the ac peak value which equals rectified peak voltage.
Today I had a look at rectifiers offered by digikey. Obviously they mostly sell 600V-types nowadays.

Each diode of the bridge rectifier is exposed to the rectified voltage across the bulk cap, which equals the amplitude or peak value of mains voltage.
Assuming 230Vac this is about 310V.

and the rectifier needs a PIV of 2times that 310Vpk that you have stated.
400V is not enough for 220Vac mains supply.
240Vac mains supply needs ~720V PIV rating. One would use two 400Vdc diodes in series, or a bridge rectifier with double that PIV rating.

The 3times comes from {sqrt(2)*V + sqrt(2)*V }* Mains tolerance of ~106%
i.e. 2*sqrt(2)*1.06 = 2.998

 

[Mooly]

A 400 volt bridge is specced for up to 280 vrms input. Look up some official manufacturer data sheets. These were standard fitment in VCR, satellite receivers etc etc.

 

[AndrewT]

That makes it quite clear.
stating both the DC output rating and the AC input rating leaves no ambiguity.
Whereas stating 400V confirms nothing.

half that for the individual diodes of a discrete rectifier.

But back to my question:
Are the individual diodes inside a bridge rectifier each exposed to half the PIV rating?

Back to the question.
for a discrete diode set forming a bridge rectifier, what rating is required for each diode?

 

[DQ828]

While your here maybe you could help me with this issue

Noise issue in DAC/Preamp

If you've ever turned on your amp and forgotten to connect the other end of the RCA cables to a source that's the noise I have, that sort of buzzing/humming noise.
The really annoying thing is I had the Dac/Preamp working fine & I pulled it apart to paint the custom case which is mostly make from timber. Now I have put it back together it plays music but has all this noise it didn't have before.

I have checked things generally and cannot find an obvious problem.

Is there a testing regime I should follow to find the problem, in the past I'd disconnect things randomly but I was hoping there might be a more professional approach that somebody could advise me to follow.

 

[AndrewT]

Insert a shorting plug to the input/s. Measure the output noise+hum.

A couple of your cheapest/scrap RCA plugs can be used to create a pair of shorting plugs.
Just solder a copper link across the hot to cold inside the plug and screw back on the insulating cover.

You can be more sophisticated and use a resistor to create dummy loading plug.

 

[DQ828]

Here comes dumb questions.

What will that do for me?

What should I use to do the measurement?

How should I do the measurement?

 

[Mooly]

Back to the question.
for a discrete diode set forming a bridge rectifier, what rating is required for each diode?

For 240 volt mains a 400 volt PIV rating is required for each diode.

The full wave bridge sees 680 volts peak to peak across its input. If the bridge output is run open circuit or run resistively loaded then you see 340 volts peak half wave pulses across each of the four diodes. If the bridge has a reservoir cap then each diode sees a 340 volt peak/peak sine across it.

 

[AndrewT]

Thanks.
This is what I have thought for a long time. But not until I see the discussion does it occur to ask for confirmation.

 

[Mooly]

While your here maybe you could help me with this issue

Noise issue in DAC/Preamp

If you've ever turned on your amp and forgotten to connect the other end of the RCA cables to a source that's the noise I have, that sort of buzzing/humming noise.
The really annoying thing is I had the Dac/Preamp working fine & I pulled it apart to paint the custom case which is mostly make from timber. Now I have put it back together it plays music but has all this noise it didn't have before.

I have checked things generally and cannot find an obvious problem.

Is there a testing regime I should follow to find the problem, in the past I'd disconnect things randomly but I was hoping there might be a more professional approach that somebody could advise me to follow.

Could the problem be stray pickup from a radiated field from a transformer ? See if moving or rotating the units affects the hum/buzz.

These things can be difficult to diagnose. Common problems could be a mains ground loop where two or more items are mains grounded and with those grounds also being connected internally to the audio circuitry within each unit.

Its really a process of elimination.