[TowerTalk] Film Power Resistors

Bryan Swadener bswadener at yahoo.com
Sat Dec 5 01:41:15 EST 2015

Yes Hans,

The Caddock (et al) thin film resistors start to derate at 25C. 
In the RF Lab of the world's largest OEM of sealed CO2 Lasers, 
we used the MP-series resistors in low power applications. 
At 100C, 50% capacity is A LOT to give up. Also important 
when developing a load (or anything that needs to dissipate 
significant power) is the temperature rise at a given dissipation. 
When I developed my forced air DL, I used a LM35DT precision 
temperature measurement IC (0.1% accuracy & linearity) to measure 
the heatsink temperature between two of the four 400W resistors. 

I used a 660W 40.68 MHz RF source. At that power level, 
extrapolating to 1500W, I had sufficient overhead. 
VSWR is well under 1.5:1 past 50 MHz, as measured on a HP 8711C 
network analyzer. I didn't bother sweeping any higher. 

vy 73 es gl, 

Bryan WA7PRC 
Photos & more info: http://www.tinyurl.com/wa7prc-dummy 

> Date: Fri, 4 Dec 2015 18:20:52 -0500
> From: Hans Hammarquist <hanslg at aol.com>
> To: towertalk at contesting.com
> Subject: [TowerTalk] Film Power Resistors
> Caddock, MP1000 100W resistor is derated to 50% @100 C. They don't like 
> "short" overloads so causation is recommended. I use 10 pcs in a 
> "linked" configuration That results in a low SWR up to 150 MHz. 
> It's more a matter of wire layout than limitations in the resistors.
> Hans - N2JFS

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