> Hi Rich,
>
>
> // We sell 1n5408, 1000piv, 200A peak, 3A avg. diodes for 17-cents each
> or $14.40 per hundred. . . When mounted on perfboard, the junctions run
> cooler than epoxy-potted diode-blocks because cooling air has better
> access.
>
> ::Price is right, for sure; although the JTX1N5554 NOS surplus mil-spec
> devices, which are more modern design and have about 1/10th the reverse
> leakage current of 1N5408's (due to junction design, which is epitaxial
> vs. double diffused), are also around $.17/ea or so. I buy them for that
> price from Apex. They're factory-sealed in serialized containers,
> although as with any "old stock," one is never totally sure of
> availability!
>
> Re the cooling issue, though: As you know, axial lead rectifiers are
> cooled via conduction, via their leads. Cooling the diode "bodies" does
> essentially nothing, the thermal resistance from junction to air is
> extremely high. The thermal resistance from junction to lead, however, is
> very low. The leads must be cooled. Flowing air across the leads isn't
> an effective way to cool diodes because the leads have such little surface
> area (.040" diameter, typically), and if the leads are "short," this
> situation is even worse -- now, there's essentially _no_ surface area.
>
> So, the K2AW modules are actually very well cooled because they are die
> cast in a thermoset (thermally conductive) epoxy compound (Emerson-Cuming
> 2850FT, I believe -- you can find the compound on the net) which has a
> _much_ lower thermal resistance than air, and is actually nearly as good
> as many metals. That's why the exterior surface of the modules get "hot."
> It would be a _bad_ thing if they did not.
>
> 73,
>
> Steve WB2WIK/6
>
>
>
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