[Milsurplus] When is a Cap Not a Cap? Redux

[C.Whitaker]

de WB2CPN
Not to belabor this line, but I have tried to use a large oil-filled 
capacitor
on 50 Hz AC instead of a conventional resistor to drop the line voltage
enough to run a tube filament.    Where the Xc would equal the R.
It didn't work.
The capacitor got hot.
I think it is the molecular activity in the dielectric material.
Air capacitors don't get hot.  I think.
EOT
(End Of Trivia)
73
Clete
++++++++++++++++++++++++++++++++++++++++++++++

On 9/18/2013 2:48 AM, Ian Wilson wrote:
> :)
>
>
> On Tue, Sep 17, 2013 at 5:14 PM, Peter Gottlieb <nerd at verizon.net> wrote:
>
>> Yeah, a LEC.
>>
>>
>>
>> On 9/17/2013 8:13 PM, Bob Camp wrote:
>>
>>> Hi
>>>
>>> …. and shortly after the cap cracks into several pieces ….
>>>
>>> Bob
>>>
>>> On Sep 17, 2013, at 8:05 PM, Peter Gottlieb <nerd at verizon.net> wrote:
>>>
>>>   When a cap turns orange that's resistive loss and then the phase angle
>>>> does match and then energy goes into the cap.
>>>>
>>>>
>>>> On 9/17/2013 7:57 PM, Bob Camp wrote:
>>>>
>>>>> Hi
>>>>>
>>>>> A cap designed to handle RF power is normally rated in terms of KVAR
>>>>> (kilovolt amperes reactive if I remember correctly). Strictly speaking it's
>>>>> not power, but energy. The phase angle (current to voltage) is wrong for it
>>>>> to be power.
>>>>>
>>>>> Simple approach is to strap a current probe on the beast and see how
>>>>> much current there is. For a big cap, that's going to be a healthy probe.
>>>>>
>>>>> Does it matter - well yes indeed it does. I've seen caps light up a
>>>>> nice bright orange (briefly) when things have gone wrong and the current is
>>>>> a tad high…
>>>>>
>>>>> The issue is obviously the loss (angle really isn't 90 degrees) in the
>>>>> cap. For small value caps (like a normal silver mica) you can measure the
>>>>> loss and figure out what's going on. The CK45 series cap mentioned isn't
>>>>> intended for high power RF use, so I'd bet it's pretty nasty compared to a
>>>>> normal COG / NPO part.
>>>>>
>>>>> Bob
>>>>>
>>>>> On Sep 17, 2013, at 9:35 AM, C.Whitaker <whitaker at pa.net> wrote:
>>>>>
>>>>>   de WB2CPN
>>>>>> So how does one measure the RF power going through a capacitor?
>>>>>> What are the parameters?
>>>>>> 73  Clete
>>>>>> ++++++++++++++++++++++++++++++**+++++++++++++++++++++
>>>>>>
>>>>>> On 9/17/2013 7:19 AM, David Stinson wrote:
>>>>>>
>>>>>>> Some time ago, we had a discussion about "when is 150 pFd NOT 150
>>>>>>> pFd."
>>>>>>> I had purchased some 100 and 150 pFd ceramics to us in a
>>>>>>> series-resonant LC circuit
>>>>>>> carrying about 10 watts of RF to an antenna.
>>>>>>> I believe it was this cap:
>>>>>>> http://www.digikey.com/**product-detail/en/CK45-**
>>>>>>> R3AD151K-NR/445-2715-ND/**1007601<http://www.digikey.com/product-detail/en/CK45-R3AD151K-NR/445-2715-ND/1007601>
>>>>>>> While 150 pFd silver micas from my junk box worked fine,
>>>>>>> these brand-new ceramics disappated more than half the power
>>>>>>> I sent through them.  The whole bunch was worthless for this
>>>>>>> application.
>>>>>>>
>>>>>>> I wanted to review that thread, but can't find it.
>>>>>>> Are there any thoughts on why this is the case,
>>>>>>> and how I might avoid this mistake in the future?
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