>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.
Rich---
R. L. Measures, 805-386-3734, AG6K
--
FAQ on WWW: http://www.contesting.com/ampfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|