DigiKey resistors in dummy loads

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From: okas_rp%ncsd.dnet@gte.com
Date: Fri May 06 1994 - 09:38:18 EDT


Hello All,

Stephen, KB8PWA, asks:

>I noticed that Digikey has metal oxide film power resistors. The description
>says, "non-inductive design", but there is no note on inductance beyond that.

My $0.02:

I've built a dummy load composed of these devices and I have had very
satisfactory service from it. SWR is under 1.2:1 up to 440 MHz, which is the
extent of my abilities to test it. Basically, there are 16 of the 2W devices
in series/parallel for a total of 32W dissipation in dry air.

My version is built into a 1 qt. paint can. An SO-239 is installed on the lid
and two brass strips extend down from the mounting tabs into the can. 4 strings
of resistors straddle the brass strips. The hot end of the strings is connected
to the center pin of the SO-239 while the cold end is attached at the far end
of the strips. Each resistor string sits about 1/8" above the brass strip
ground plance.

For QRO applications, the can may be filled with mineral oil. The designer
claims it can dissipate over 100 watts for short durations.

I just got my May-June DigiKey catalog and the resistors in question appear
on page 205. The 2W Yageo devices are what I used. They're $0.23 each in
under-10 quantities; i.e. really cheap. I used 47 Ohm devices, though 51 Ohm
parts are obviously a tad better choice.

As a side note, the Ten-Tec kit catalog has a 300 watt dry dummy load for a
mere $22. Something to thnk about...

73,
Bob - N3MBY


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