# [Amps] peak voltage rating for load cap

Sat Aug 19 12:36:34 EDT 2017

```Hello all, power required is a lot less. The VSWR is excellent as it is
a mono band low VHF yagi. However sometimes things can go wrong and I
want the capacitor to survive my late night sleepy blunders!

I have a cap of the correct value with 0.02 inch spacing. I just 'looks'
too close spaced.

Regards

On 19/08/2017 14:26, Joe Subich, W4TV wrote:
> On 8/19/2017 7:42 AM, Alek Petkovic wrote:
> > I always thought, that for the loading capacitor, you just use the old
> > "Power = V squared over R" formula.
>
> That will give you average (RMS) voltage with SWR = 1:1.  For peak
> voltage, multiply by 1.414 (peak voltage of a sine wave):  316 * 1.414
> = 447 V.
>
>> your system is all tuned and matched.
> Multiply the peak voltage by the SWR for worst case peak voltage if
> the voltage node occurs on the capacitor - 316 * 1.414 * 3 = 1340V.
> A 1200 volt rated capacitor will be marginal in a *system* allowing
> for a maximum SWR of 3:1 at 2 KW - it will be barely adequate when
> the system is designed for 1500 W (1166 V).
>
> 73,
>
>    ... Joe, W4TV
>
>
> On 8/19/2017 7:42 AM, Alek Petkovic wrote:
>> I always thought, that for the loading capacitor, you just use the
>> old "Power = V squared over R" formula.
>>
>> You know R is 50 Ohms, and you supposedly know the power, so you can
>> work out the voltage from there.
>>
>> For example, for an output power of 2kW, V squared will be 100,000,
>> therefore V will be 316V.
>>
>> your system is all tuned and matched.
>>
>> 73, Alek. VK6APK
>>
>> On 19/08/2017 6:07 PM, Conrad PA5Y wrote:
>>> Hello what is the rule of thumb for conservatively estimating the
>>> required ratings for a load capacitor when used with a mono band
>>> Pi-L tank circuit?
>>>
>>> Anode voltage 2750V
>>>
>>> Tube design RL = 2600 ohms
>>>
>>> Q = 12
>>>
>>> I have searched on the internet an I cannot find any really
>>> convincing info, I have something that will stand 1200V but that is
>>> marginal and will not survive a fault I expect.
>>>
>>> Alternatively does anyone have a small vac variable around 40-60pF
>>> that is around 3 inches/76mm? That will obviously do the job.
>>>
>>> Also where can I buy a couple of Ohmite Z-50 chokes in Europe?
>>> Anyone got one or two that they can spare?
>>>
>>> Regards
>>>
>>>
>>>
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>>> http://lists.contesting.com/mailman/listinfo/amps
>>>
>>
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```