Re: LF: Dummy loads for LF

[Rik Strobbe <rik.strobbe@fys.kuleuven.ac.be>]

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
>