[Amps] HV Voltmeters

[David & Wendy Dodds]

Hi.

I hope you big guns will excuse a really basic novice question?

I've just finished building my first HT PSU. (Very proud of myself.) It produces 1150/1350v at 400mA. Not big by the standards of this list, but big enough for a 2c39 amp on 23cm.

The beast is working very nicely, with one exception, which is the HT voltmeter. It barely reads at all, when the Avo meter is twanging off the end-stop! The PSU is based on the N6CA design and I've followed his practice of hanging the meter off a string of resistors attached to the rectifier output. As I am using a 100mA FSD meter I calculated the multiplier resistors as needing to total 1M5, so I used 10 2W 150k resistors.

Can anyone tell me if I've fouled up the arithmetic? (we medieval history graduates aren't too hot at this theoretical stuff!!)

My thinking was as follows...

R(multiplier) = voltage / I(fsd) - R(meter)

therefore R(multiplier) = 1500v / 100mA - 0.7 Ohms

therefore R(multiplier) = 1M5

Power rating was calculated as follows....

Power rating = I(fsd) squared x R(multiplier)

therefore Power rating = 100mA squared x 1M5

therefore Power rating = 1.5 W

Am I talking nonsense here? The formulae are lifted from the RSGB Rad Com handbook, but maybe I'm not using them right....

73 de David GM4WLL


Please reply to: 
gm4wll@talisman41.freeserve.co.uk (Wendy)
gm4wll@qsl.net (David)          

See our web-site: www.qsl.net/gm4wll


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