Ceramic HV Capacitor

[enthalpy]

I have a circuit with a 0.02uF: 2kV ceramic capacitor which I suspect to be faulty. It test OK with a digital ohmeter but the voltage is very small and I have been told that a Megger across it would be a better test. Anyone, any other ideas?

[Skywave]

Yes - a high voltage source - ideally current-limited - and a means of measuring the consequent leakage current would be a good idea. But I have to ask - what led you to the conclusion that the cap. was faulty in the first place? You may be chasing a red herring here

Al / Skywave

[enthalpy]

Chasing a red herring? Could well be Skywave!

Now, let me put my cards on the table: I am a power engineer. Anything below 400A and 415V makes me confused!

This is a HV supply for a Telequipment ‘scope. The mains transformer smokes. I have removed all the secondary connections and reconnected them one by one after measuring the current. No smoke until the last winding.

This winding takes about twice its rated current.

In the circuit are a couple of resistors and this capacitor, It’s early days yet, I am closing in on the culprit!

[PJL]

A D54 by any chance? I have one at the next stage of failure - an open-ciircuit HT winding. I would take a look at the 3.4KV selenium EHT rectifiers as well. If you want another for spares send me a PM.

[enthalpy]

I’ve got two Telequipment D52 ‘scopes. On one the primary winding of the transformer is shorted and blows the input fuse immediately on switching on. The second one has the fault in the circuit connected to one of the secondary windings.

I also have a Heathkit IO 12U but that's another story!

[PJL]

No chance of getting rid of the D54 then...pity. I assume the PSU is the same so:

Whilst you are trying to find out whats wrong it might be a good idea to attach the EHT transformer connection through a string of series resistors (250V AC max on each so 5x100K should do) and disconnect the tube. I could not work out why they use a 2K rated component here as I make the voltage = 2x1.4x1150.

[Skywave]

Originally Posted by enthalpy

This is a HV supply for a Telequipment ‘scope. The mains transformer smokes. I have removed all the secondary connections and reconnected them one by one after measuring the current. No smoke until the last winding.
This winding takes about twice its rated current.
In the circuit are a couple of resistors and this capacitor, It’s early days yet, I am closing in on the culprit!

Ah-Ha! So, presumably it is only when the "last winding" is connected to its load that the problem becomes manifest, yes? And, presumably, this wndg. is to a half-wave rectifier + filter to produce the EHT direct from the mains transformer? And the suspect capacitor is part of this filter?

OK - so I'm guessing a bit here - a more detailed description of the cct. arrangement where this cap. & transformer features will help.

One final thought - when you say that the "winding takes twice its rated current", (I assume you mean that "the load on the sec'y. wndg. takes twice . . . .") how do you know what the "normal" current should be?

Just trying to help . . .

Al / Skywave.

[PJL]

Skywave, it uses a multi-tapped secondary the last of which is 1070V for the EHT which is doubled by the 2K capacitor mentioned and a pair of selenium EHT rectifiers. The first obvious problem with the design is 2KV rated capacitor driving the doubler then there's the rectifiers. Seems a common fault on the D52 & D54.

[Skywave]

Originally Posted by PJL

Skywave, it uses a multi-tapped secondary the last of which is 1070V for the EHT which is doubled by the 2K capacitor mentioned and a pair of selenium EHT rectifiers. The first obvious problem with the design is 2KV rated capacitor driving the doubler then there's the rectifiers. Seems a common fault on the D52 & D54.

Thanks for the enlightenment. I wasn't aware that the Item was a D52 / D54.

Anyway, with that cct. config., there doesn't seem much in the way of components to check out - so I'll bow out now and leave the diagnostic work on this Item in the hands of those More Knowledgeable About It than I.

However, useful Thread 'though. Never know when I just might come across one of these myself.

Good luck with it, enthalpy!

Al / Skywave

[enthalpy]

I think I've done it! I found a de-coupling (?) capacitor connected between one end of the CRT heater, through a 100K resistor to the chassis. I changed the capacitor: the current in the winding fell to about 600mA: no smoke and a ragged sine wave appeared on the screen! I'm not sure why the capacitor is there (to remove HF?) and several times throughout the investigation my brain began to hurt! It's not easy for a power engineer! One 'scope down, one more to go!

[Skywave]

Originally Posted by enthalpy

I think I've done it! . . . etc . .

Ah - Ha! And well done!

But - I've just got to ask - does this imply that the suspect 0.02 uF 2Kv ceramic capacitor was innocent of all charges?

Al / Skywave.

[PJL]

That's a good result. It would be interesting to know which part failed. I can see an R306 of 100K but no capacitor. I guess the 600mA is the primary AC current?

[enthalpy]

You two, 11:32 pm and 10:48 pm, don’t you ever sleep!

Yes, Skywave, the original 0.02uF was not the one which I thought was causing the problem. From what you wrote, I am concluding that ceramic,
2 kV capacitors are not likely to fail. True?

PJL, I haven’t the wiring diagram to hand so I cannot confirm R306. The capacitor looked as if it had been added. It was soldered professionally (better than mine) and looked to be the same age as the other components, but somehow was out of alignment with the other compnents on the connection board. I wonder if it was a modification by the manufacturer?

I think I should stick to crystal sets in future!

[dsergeant]

2kv ceramic caps are widely used in TVs. They very frequently go short circuit (often the case is split and discoloured).

Dave

[Skywave]

Originally Posted by enthalpy

Yes, Skywave, the original 0.02uF was not the one which I thought was causing the problem.

OK - any capacitor of any type of construction will fail if stressed with enough over-voltage. We all know that. It's just that in my experience, that item seemed an unlikely (but not impossible) culprit for the original fault as described, taking into account the other components that were likely to be in the cct. (I wasn't aware, at the time of my first posting, that the equipment in question was a D52 'scope).

Hence, my original comment about "chasing a red herring": we've all been there.

Incidentally - and since you asked - yes I do sleep when I can - but occasional bouts of insomnia have troubled me all my life - and continue to do so.

Al / Skywave.