Return-Path: Received: (qmail 29968 invoked from network); 31 Jan 2005 14:35:00 -0000 Received: from unknown (HELO ptb-spamcore02.plus.net) (192.168.71.3) by ptb-mailstore01.plus.net with SMTP; 31 Jan 2005 14:35:00 -0000 Received: from mailnull by ptb-spamcore02.plus.net with spamcore-l-b (Exim 4.32; FreeBSD) id 1CvcbT-000EdO-8f for dave@picks.force9.co.uk; Mon, 31 Jan 2005 14:32:39 +0000 Received: from [192.168.67.3] (helo=ptb-mxcore03.plus.net) by ptb-spamcore02.plus.net with esmtp (Exim 4.32; FreeBSD) id 1CvcbR-000Eco-SP for dave@picks.force9.co.uk; Mon, 31 Jan 2005 14:32:34 +0000 Received: from post.thorcom.com ([193.82.116.20]) by ptb-mxcore03.plus.net with esmtp (Exim) id 1Cvcdq-00086r-QK for dave@picks.force9.co.uk; Mon, 31 Jan 2005 14:35:03 +0000 Received: from majordom by post.thorcom.com with local (Exim 4.14) id 1Cvccb-0008Hl-UL for rs_out_1@blacksheep.org; Mon, 31 Jan 2005 14:33:45 +0000 Received: from [193.82.116.30] (helo=relay.thorcom.net) by post.thorcom.com with esmtp (Exim 4.14) id 1Cvccb-0008Hc-Fp for rsgb_lf_group@blacksheep.org; Mon, 31 Jan 2005 14:33:45 +0000 Received: from spoetnik.kulnet.kuleuven.ac.be ([134.58.240.46]) by relay.thorcom.net with esmtp (Exim 4.43) id 1CvccY-00060u-UT for rsgb_lf_group@blacksheep.org; Mon, 31 Jan 2005 14:33:45 +0000 Received: from localhost (localhost [127.0.0.1]) by spoetnik.kulnet.kuleuven.ac.be (Postfix) with ESMTP id 4488C33E67 for ; Mon, 31 Jan 2005 15:33:35 +0100 (CET) Received: from octavianus.kulnet.kuleuven.ac.be (octavianus.kulnet.kuleuven.ac.be [134.58.240.71]) by spoetnik.kulnet.kuleuven.ac.be (Postfix) with ESMTP id 8F59333E81 for ; Mon, 31 Jan 2005 15:33:34 +0100 (CET) Received: from dell-rik.fys.kuleuven.ac.be (pc-10-33-165-177.fys.kuleuven.ac.be [10.33.165.177]) by octavianus.kulnet.kuleuven.ac.be (Postfix) with ESMTP id 199E2AED80 for ; Mon, 31 Jan 2005 15:33:34 +0100 (CET) Message-Id: <5.1.0.14.0.20050131153515.0270c180@u0019445.kuleuven.be> X-Sender: u0019445@u0019445.kuleuven.be X-Mailer: QUALCOMM Windows Eudora Version 5.1 Date: Mon, 31 Jan 2005 15:45:34 +0100 To: rsgb_lf_group@blacksheep.org From: Rik Strobbe In-Reply-To: <002d01c50596$d697c0f0$e901a8c0@bob2l2u6k2n1g3> References: <142.3e3d8156.2f2c1586@aol.com> Mime-Version: 1.0 X-Virus-Scanned: by KULeuven Antivirus Cluster X-SPF-Result: relay.thorcom.net: 134.58.240.46 is neither permitted nor denied by domain of fys.kuleuven.ac.be X-Spam-Score: 0.5 (/) X-Spam-Report: autolearn=no,HTML_20_30=0.504,HTML_MESSAGE=0.001 Subject: Re: LF: Dummy loads for LF Content-Type: text/html; charset=windows-1252 X-Spam-Checker-Version: SpamAssassin 2.63 (2004-01-11) on post.thorcom.com X-Spam-Status: No, hits=0.0 required=5.0 tests=HTML_MESSAGE autolearn=no version=2.63 X-SA-Exim-Scanned: Yes Sender: owner-rsgb_lf_group@blacksheep.org Precedence: bulk Reply-To: rsgb_lf_group@blacksheep.org X-Listname: rsgb_lf_group X-SA-Exim-Rcpt-To: rs_out_1@blacksheep.org X-SA-Exim-Scanned: No; SAEximRunCond expanded to false X-Spam-Filtered: by PlusNet SpamCORE (v3.00) Content-transfer-encoding: 8bit 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