New 'lytics blow the fuse

[glorocks]

I just replaced the filter caps in a tube crossover, and it now take the fuse out on power up. (yes, I put 'em in correctly)
I suspect the new caps have a much lower initial impedance and the inrush current is too much. I'm using Cornell-Dublier, and the values and voltages are correct.
Anyone out there have any suggestions?

[head_spaz]

If they are the same value as the originals, and installed properly - polarity wise - then you're probably right in assuming they are drawing more charge current.
You could try using a small wattage light bulb in series with the AC line to see if current limiting helps.
If so, you can build a soft start ciruit, or simply add a thermistor in the AC line, which is what I do. I get mine from old PC power supplies.

[glorocks]

Thanks- I hadn't thought about the light bulb, but it makes sense. I''l give it a try in a few minutes. If that doesn't work, I'll try to track down a variac.
So, I'm wondering if perhaps the lytics need to be "formed" on their first power up and will behave on subsequent power ups?

[454Casull]

Is the capacitance higher in the new caps?

[glorocks]

No- same value and voltage. The new caps are considerably smaller than the originals, but they read the correct capacity on a cap tester.
I rigged a supply to output the rated B+ (450 vdc) and hooked it up to a variac I borrowed. I'm raising the voltage 5 volts or so every couple of hours until I get to +450. Hopefully, that will form the caps and they'll behave after this treatment.
If you guys have and old HAM radio enthusiast in your neighborhood, get to know him. They know a LOT of tube and HV tricks.

[MCampbell]

Originally posted by glorocks
No- same value and voltage. The new caps are considerably smaller than the originals, but they read the correct capacity on a cap tester.
I rigged a supply to output the rated B+ (450 vdc) and hooked it up to a variac I borrowed. I'm raising the voltage 5 volts or so every couple of hours until I get to +450. Hopefully, that will form the caps and they'll behave after this treatment.
If you guys have and old HAM radio enthusiast in your neighborhood, get to know him. They know a LOT of tube and HV tricks.

By raising the voltage so slowly to the rating voltage of 450V, I believe that you are in effect forming the capacitor at each of the lower voltage(s). This may affect the performance of the capacitor. It's better to go straight in at the rated voltage and keep an eye on the leakage current over time. Once it steadies out, the cap is properly formed.

See this link for good general outline:
http://www.vcomp.co.uk/tech_tips/ref...eform_caps.htm

[AndrewT]

Hi,
presumably the mains transformer is an EI. This should not need a soft start but a small benefit will accrue if you choose to use one.

The slow charge circuit can be a Power Thermistor in the secondary circuit to limit the peak charging current. This will be very beneficial. Fit it.

A light bulb will not do, it is PTC, you need NTC.

[AndrewT]

Originally posted by MCampbell


By raising the voltage so slowly to the rating voltage of 450V, I believe that you are in effect forming the capacitor at each of the lower voltage(s). This may affect the performance of the capacitor. It's better to go straight in at the rated voltage and keep an eye on the leakage current over time. Once it steadies out, the cap is properly formed.

the high series resistance ensures that the capacitor is initially charged by a very low voltage and as the capacitor leakage reduces so the charge across the plates increases.
It is wrong to suggest that staged charging is less effective than gradually increasing charging.
Ultimately the di-electric film will be the same thickness and it will suit the final voltage.

I use much higher resistor values to achieve much slower re-forming.

[MCampbell]

Originally posted by AndrewT

the high series resistance ensures that the capacitor is initially charged by a very low voltage and as the capacitor leakage reduces so the charge across the plates increases.
It is wrong to suggest that staged charging is less effective than gradually increasing charging.
Ultimately the di-electric film will be the same thickness and it will suit the final voltage.

I use much higher resistor values to achieve much slower re-forming.

Andrew,

You may well be right that ultimately the dielectric layer will be the same thickness. What I am struggling with is the advantage of staging the voltage or of slow reforming. Certainly I have never seen it recommended by any manufacturer. Has your experience been that sow reaging will ultimately lead to a lower leakage current for the reaged capacitor. Or is there some other benefit that I'm missing?

Matthew

[AndrewT]

I find leakage currents after slow re-forming between ten times and one thousand times lower than specification.

I asked for comment a while back, but got no replies.