----- Original Message -----

From: Rik Strobbe

To: [email protected]

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