>A better way is to use a HV NPN transistor, with the collector
>grounded. The base goes to a pots wiper, the hot end of the pot goes
>to a regulated source higher than the grid requires. ...snip...
NPN and PNP HV transistors suffer from 'secondary breakdown effect' when
medium voltages and currents co-exist (as is the case in regulator
service). One design solution to 'secondary breakdown effect' is to
derate the dissipation to about 1/8 and parallel additional transistors.
Another solution is to use a power FET transistor. They are virtually
immune to 'secondary breakdown effect'. Motorola and International
Rectifier make a 6A, 600v, 150w FET that appears to perform
satisfactorily in HV regulator service.
R. L. Measures, 805-386-3734, AG6K
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