[Elecraft] Dummy loads for dummies

[Don Wilhelm]

Clay,

Unless things have changed since I tested the Caddock Thick Film Power 
resistors of other than 50 ohms, I would not parallel 20 1000 ohm 
resistors to produce a 50 ohm load.  Values other than 50 ohms are not 
guaranteed to be non-reactive.

I would put 4 strings of 4 50 ohm resistors in series in parallel. Each 
resistor mounted with leads as short as possible.

I don't know about the dissipation characteristics of those resistors 
mounted in oil, and I suspect it depends on the heatsinking ability of 
the resistor mountings when immersed in the oil.
20 100 watt resistors *should* produce a 2000 watt load, but as I 
indicated, it depends on the dissipation of the individual resistors and 
how fast they can transfer their heat to the oil.  It may be necessary 
to attach the resistors to a heat sink and immerse those heat sink 
assemblies into the oil.

For all those who are looking for a suitable dummy load for high power, 
you usually do not have to be concerned about measurement quality 
impedance.  Something adequate to provide a suitable load for the 
amplifier may be all that is needed.  Having a precision 50 ohm load is 
a requirement for measurement, but not for routine amplifier tuning and 
general operating procedures.

73,
Don W3FPR

73,
Don W3FPR

On 12/27/2016 7:16 PM, Clay Autery wrote:
> Makes me curious about designing with these TO-220 resistors in the 1000
> Ohm size of appropriate power rating...  Parallel 20 of them like folks
> do on the axials that they put in the can type dummy loads with oil.
>
> Options:
>
> 1) 20 in parallel on a finned copper or bare aluminum sink of sufficient
> size and then entire assembly in a oil wetted container.
> 2) Same, but only the finned side of the finned/pinned heat exchanger in
> a coolant bath.
>
> I'd like to have a rugged, high power, key down load built from easily
> obtainable and replaceable parts.