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

[Peter Gottlieb]

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
>>>>> 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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