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Rik and others,
I understand that "resistance wire" has a fairly
neutral temperature coefficient. Tungsten (light bulbs) do vary
significantly with temperature, but not so with nichrome.
73, Bob ZL2CA
----- Original Message -----
Sent: Tuesday, February 01, 2005 3:45
AM
Subject: Re: LF: Dummy loads for LF
Hello Bob, Gary, Markus and
others,
over the past days a lot of household equipment has been
suggested as 136 kHz dummy load.
I wonder what the "cold" resistance of
some of these devices are. Most metals have a positive temperature coefficient
of about 0.3 to 0.5 % per degree centigrade (iron : 0.5%, copper : 0.4%,
aluminium : 0.4%).
For oil jacket heaters the temperature at full power
may be only 40 degrees above room temperature (= 20% increased resistance) but
for devices like toasters the change very large.
So a 1000 W toaster (@ 220
V) will be about 50 Ohm when loaded with 1kW but probably much less if loaded
with only 100 Watt.
73, Rik ON7YD
At 13:09 29/01/2005
+1300, you wrote:
Gary,
Markus and others,
Oil jacket
heaters are also fairly good dummy loads for LF. A 1200 watt 230 volt
heater has a resistance that is a bit under 50 ohms, but there is some
inductance, so simple compensation with around 2200 pF (depends on specific
heater) can neutralise the reactance at 136 kHz. Polystyrene or
polypropylene HV capacitors are recommended. The low Q compensation
also raises the effective resistance by way of the series to parallel
transformation (consider it to be a form of L match, with series RL and
parallel C).
I have found
that 1200 watt heaters with two elements have lower inductance (with both
switches on, elements are presumably in parallel) than a heater with a
single element.
The 1200 watt
rating of an oil jacket heater is of course a steady state rating, so you
wont "blow it up" like you could with a load that is only good for short
term use.
You can use a domestic
heater fed directly via its mains plug (with clip leads), it does not need
to be dedicated to LF :-) If you want to, you can make a simple
interface box, with SO239 to mains socket, and put the compensating
capacitor inside the box.
73,
Bob ZL2CA
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