How to test large capacitors?

[TonyTecson]

if you are not installing the caps yet, i would suggest
that you put a shorting jumper accross the terminals to prevent build up due to DA....

 

[AndrewT]

And all good. The best of them had about .15V across the 28K resistor feeding the +ve terminal, so I make that to be roughly not very much in terms of leakage current. So all good there; thank you for the assistance!

leakage is usually specified as 0.0x CV after a few minutes from cold.
That x value can be anywhere from 1 to 5, but more usually 2 or 3

a typical 30Vdc 10mF capacitor must have leakage <0.03*30V*0.01F <9mA

I find that my double re-forming method gets to between 1/50th to 1/300th of the specified datasheet value.

 

[AndrewT]

Voltage across the cap? It would have been ~29.5V generally across the group.

Have I misunderstood the process? As I measured it I had way less than 1 mA flowing into the cap, hence the leakage must equal that amount. Or have I misconstrued?

the current flowing INTO the capacitor is made up from at least 3 parts.
a.) to increase the charge stored in the capacitor.
b.) to chemically reform the plates/electrolyte.
c.) to leak across the plates.
It is extremely difficult to separate out these three current components. Even a regulated DC supply has a little variation and that definitely affects current a.
Current b. eventually reaches zero. But how long?

 

[TonyTecson]

to me any cap that will not reform with less than 1mA is suspect, i will be careful about using them.....

 

[aspringv]

Well thanks again to you both for your help. I suspect I've got to a pretty good end result. Next stop is finding a spot for a slow start circuit in the case due to the ridiculous amount of PSU capacitance!

Cheers!

 

[AndrewT]

I recommend the Slow charge system shown by most NTC manufacturers.
i.e. place the NTC in the capacitor circuit.
If the added source impedance is a problem, bypass with a time delayed relay, or SS relay.

I do not recommend using a (primary located) soft start with extended time delay as a slow charging current limiter.

 

[aspringv]

Interesting. Why exactly? I see switching resitor circuits on the primary (and indeed the DiyAudio store!) , and NTC on the primary on most Pass schematics so I had inferred that this was best practice.

What is the benefit/reason to place the NTC on the secondari(es)?

 

[smichael]

Andrew,

when you discharge the capacitors are you then doing it through the resistor or "directly"?

Thanks

Steen

 

[AndrewT]

I did accidentally remove the tip of my screwdriver way back.

I use a resistor to discharge, whenever the voltage is higher than what DA recovery would restore to a previously discharged capacitor.

 

[AndrewT]

Interesting. Why exactly? I see switching resitor circuits on the primary (and indeed the DiyAudio store!) , and NTC on the primary on most Pass schematics so I had inferred that this was best practice.

What is the benefit/reason to place the NTC on the secondari(es)?

Because the NTC manufacturers show the NTC in series with the capacitor bank. All their calculation methods are for NTC in series with the capacitor bank.
All the datasheets that quote a maximum capacitance are for NTC in series with the capacitor bank.

Putting the NTC in the primary does not effectively set a predictable limit for the slow charge current from the secondary.
Someone could probably set up a model to predict the way a primary NTC feeding the transformer into the rectifier to slow charge the capacitors would behave.
It is too complicated for me.

Soft start on the other hand is to current limit the primary circuit of the transformer during the transformer start up. This is of short time duration, typically 10 or so cycles of the main's waveform. Typical time delay for the soft start bypass would be 100ms to 300ms.