# [Amps] peak voltage rating for load cap

Joe Subich, W4TV lists at subich.com
Sat Aug 19 08:26:58 EDT 2017

```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 1200V capacitor will do the job with adequate headroom., when
> 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.
>
> 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
>>