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Ceramic disc caps

  • Thread starter Dave Plowman (News)
  • Start date
D

Dave Plowman (News)

Jan 1, 1970
0
I've had a few failures of these recently. Used round a bridge rectifier -
for RFI purposes, I'd guess. They were the normal sort of wax covered
types. In all three cases, one had shorted.

Is there anything better to use for this?
 
N

N_Cook

Jan 1, 1970
0
Dave Plowman (News) said:
I've had a few failures of these recently. Used round a bridge rectifier -
for RFI purposes, I'd guess. They were the normal sort of wax covered
types. In all three cases, one had shorted.

Is there anything better to use for this?

--
*If you think this van is dirty, you should try having sex with the driver*

Dave Plowman [email protected] London SW
To e-mail, change noise into sound.


Wax covered?
 
M

Meat Plow

Jan 1, 1970
0
I've had a few failures of these recently. Used round a bridge rectifier
- for RFI purposes, I'd guess. They were the normal sort of wax covered
types. In all three cases, one had shorted.

Is there anything better to use for this?

Move them away from any heat source? I've not known them to be a trouble
child. If you choose MLCC replacements be sure to choose the correct
class of dielectric. Class COG/NPO contain neodymium, samarium and other
rare earth oxides and have a negligible aging rate.
 
D

Dave Plowman (News)

Jan 1, 1970
0
Move them away from any heat source?

In these cases they were pretty low powered supplies - 100 mA or so and
quite open so heat shouldn't have been a problem. Although they were
mounted close to the diodes.
I've not known them to be a trouble child.

Not exactly that as they'd lasted quite a time. But like all these things
if slightly more expensive ones last even better it makes sense.
If you choose MLCC replacements be sure to choose the correct class of
dielectric. Class COG/NPO contain neodymium, samarium and other rare
earth oxides and have a negligible aging rate.

Thanks.
 
T

tm

Jan 1, 1970
0
Jeff Liebermann said:
Any chance it was an MOV (metal oxide varistor) instead of a disc cap?
Kinda difficult to tell from your description. "Normal"? "Wax"?


If the bridge has several diodes in series for each leg, you'll need
equalizing caps and resistors to make sure that the voltage is equally
distributed among the series connected diodes. That means they should
all be equal value.

If not, just get a higher voltage rating. Shorted disc ceramic
usually means that some high voltage punched through the dielectric or
that the plating has exploded and sprayed conductive crud around the
insides.

Or it might be a NTC surge limiter. If these are in series with the AC line,
it
limits the initial charging current of the DC input capacitors. They protect
the
line fuse.
 
M

Meat Plow

Jan 1, 1970
0
Any chance it was an MOV (metal oxide varistor) instead of a disc cap?
Kinda difficult to tell from your description. "Normal"? "Wax"?

I havn't seen too many MLCC's that have been waxed or potted but if they
vibrate in a audio detector or IF stage and introduce noise. Waxing them
would be one way to dampen vibration. Plus I think Dave
knows the difference between an MOV and a MLCC dontcha think?


<snip for brevity>
 
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