> can anyone point me to a source of resistors other than mouser and
>digi-key???
Dale makes high-ohm spiral-film resistors. I tested tw5 Dale, 55M-ohm.
5Wers yesterday that performed well at 15kV (4W). The max V ability film
resistor I have been able to find at Digi-Key and Mouser is 2W, 1M -
which = 1414V.
> I can't believe there aren't more resistors than what they
>list!!! I need some 750k -3w-either ww
Wirewound resistors much over 100k-ohm tend to be semi-reliable.
High-value WW resistors usually carry a V-rating that limits power-rating
to less than the rated dissipation. For example, a 200k-ohm, 100W
resistor dissipates 100W @ 4472V, however, such a resistor typically
carries a max V rating of less than 4472V. In other words, the "100W"
rating is unsliced bologna at 200k-ohms. On the other hand, spiral-film
resistors seem to able to handle more V. I've used 400M-ohm, 5W
spiral-film resistors that could handle 40kV. To further complicate
matters, there are two V-ratings. One is end-to-end V-max and the other
is case-to-gnd V-max. In the example with the 400M unit, even though the
ene-to-end rating is 40kV, the case-to-gnd rating is probably <400V since
the spiral film is coated with a thin layer of paint so as not to
increase thermal resistivity between the cooling-air and the heat source.
Fortunately, there is a simple test for determining the V capability of
a resistor. Using a DMM and a HV multiplier probe, measure applied V
versus current flow. For a specific number of ohms, the ratio of V/I
should be constant over the range of expected voltages. When the ratio
of V/I (ohms) begins to decrease, the resistor is beginning to break down
internally. A classic example is the Heath SB-220. The HV meter
multiplier resistors are 3, 4.7M-ohm, 1W-rated carbon-comps. Around
1000V appears across each resistor. According to ohms-law, the
dissipation in each "1W" R is 0.47W, so all should be well, Mr. Murphy.
However, 1W carbon-comp resistors are max-rated at about 350V, so they
slowly change value with 1000V and eventually fail. A number of Alpha
amplifiers use 220k-ohm, 1W rated resistors to equalize the V across the
string of 450V electrolytic filter caps. Apparently, whoever designed
this amplifier did not need to read the fine print in the resistor mfg's
rating/specs. In time, the 220k resistors change value and unequalize
the distribution of potential across the filter caps.
cheerz
>or mof. these need to be at least 500
>v rated, to be used in the hv metering ckt. of an amp!!! thanks to all!!!!
> carl / kz5ca
>
>
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- R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K,
www.vcnet.com/measures.
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