Non Inductive Resistors for Dummy Loads

[wb1aij]

http://www.vishay.com/docs/50051/lto100.pdf

The link above will bring you to a data sheet of thick film, non inductive, power resistors. Should be good for a dummy load & the price is low; a couple bucks each for the 100W series. I am thinking of building a 1000 watt dummy load with 10-500 ohm resistors in parallel. Anyone out there ever used these for this purpose that could comment either pro or con?

[KA2QFX]

I have used them for balancing resistors in hybrid power combiners for HF (75m) where their size and mounting are very convenient.  
I would be concerned however of their capacitance to case at  higher frequencies. If you wanted to use these up to 10m or beyond I think this would be an issue.   I'd also be concerned about physical layout on a heat sink for both dissipation and impedance. 1000 watts is a lot of heat to disperse.

A good dummy load will use a cylindrical resistor in a tapering coaxial cylinder so the characteristic impedance of the coaxial dimensions will match the resistor's impedance to ground as it approaches it's grounded end.

To minimize any -j phase shift you might consider using 10 - 500 (did I say that!?) 5.0 (five) ohm in series to reduce the net capacitive effect by having some R in series and a reducing voltage gradient along the load.  

I'd be interested in what Frank GFZ might have to say on this.

Mark

[WA1GFZ]

I would use a bunch of Caddock resistors (or Rf resistors) on a heat sink to avoid the long leads. Build a double sided PC board to sit in the middle with resistors set in a circle. Have the center pin of the coax connector come through the middle of the board from a plate mounted above the heat sink Or come through the heatsink if you hog out some fins . center pin to a small common plane to each resistor. Ground side of the resistor goes to the bottom ground plane of the board bolted to the heat sink with spacers like say a #6 nut.  GFZ in w9 land
Or buy a Bird load cheap.

[W1RKW]

I considered building a big DL but I stumbled across this dry 2KW baby for $50.00 at a hamfest.

[Opcom]

I put one together using eight thin film units, 500W each, on a 40 lb copper heatsink. There are pics somewhere on here. Even at 500W it gets pretty warm after some time and I had to put a high volume low speed fan on it.

A thermal alarm and fan switch would be wise when using these small-area type resistors because they have definite spec for temperature. 175 deg C seems like a lot but it can easily be exceeded with high power levels because the device can get hotter much faster than the heat sink can remove the heat from it. (like testing a 2-transistor 100W linear amp -the devices run much hotter than the heatsink) There was good advice on here in that thread about using copper spreaders or outright copper heatsinks because aluminum is not nearly as conductive to heat.

Note the derating curve for the resistor here starts at 25 deg C and is 50% at 100 deg C. These inexpensive ones must be derated. The ones I chose were expensive but go to 100 deg C before derating. Not good or bad just something to consider.

Here
http://amfone.net/Amforum/index.php?topic=22823.0

dang smart people here keep me from blowing things up!

[k4kyv]

I have built several DLs using Glo-Bar resistors.  I have found that Glo-bars, like any other kind of composition resistor, tend to drift away from nominal resistance over time.

My main DL, made up of a dozen 18" long, 1" diameter 600-ohm glo-bars measures very close to 50 ohms with all resistors in parallel, and 450 ohms with three clusters of 4 each connected in parallel, connected in series.  The series-parallel configuration shows noticeable j factor on 40m, as could be expected. 

I made up a smaller one with 8 ea. 5-watt 50-ohm glo-bars connected in series plus a couple more large 50-ohm composition resistor (most of the glo-bars had drifted below nominal resistance).  That one showed less j factor but still enough to screw up measurements on 40m. I made yet another one using a 350-ohm glo-bar that had drifted to 390 ohms, in series with a 50-ohm one that  had drifted to 60 ohms, to make exactly 450 ohms.  It shows negligible j factor.

[W4AMV]

http://www.vishay.com/docs/50051/lto100.pdf

The link above will bring you to a data sheet of thick film, non inductive, power resistors. Should be good for a dummy load & the price is low; a couple bucks each for the 100W series. I am thinking of building a 1000 watt dummy load with 10-500 ohm resistors in parallel. Anyone out there ever used these for this purpose that could comment either pro or con?

I requested samples of the 50 ohm 100 W version. As there is NO HF data on these from the manufacturer... he was more than happy to swap devices for data.

[K5UJ]

One of my DLs is four 200 ohm globars.  I think each dissipates 200 or 300 w.  I put a fan on them and they do okay for my power level.

[W4AMV]

The Vishay 100 W model LTO100F47R00FTE3 is quite good. Measured here to 500 MHz. Where the low frequency R value is 47 ohms and the little bit of self inductance can be tuned out to reduce the SWR to almost unity.

[W4AMV]

Someone asked... what if you add nothing to the R. No tuning no compensation.