David,
The way I calculate it if you were to use a 100 ma meter you would need a total
of 15k resistance for the whole string. 1500 / .1 amp (100 ma) = 15000 ohms.
That would require a total resistor wattage of 150 watts.
However if you were to use a 1 ma meter rather than the 100 ma meter your
calculations would be ok for the resistance. 1500 / .001 amp (1 ma) = 1500000
ohms. (ohms divided by 1000000 = meg ohms) The total wattage of the resistor
string would be 1.5 watts. 1500 x .001 amps (1 ma) = 1.5 watts.
Another way to do it is remember that when you divide by ma your answer comes
out in k ohms. So 1500 / 1 ma = 1500 k ohms. And 1500 k ohms is 1.5 meg ohms.
You divide k ohms by 1000 to convert to meg ohms.
73
Gary K4FMX
David & Wendy Dodds wrote:
> 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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