EEVblog Electronics Community Forum
Products => Test Equipment => Topic started by: vu2nan on January 02, 2020, 03:35:47 am
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Test equipment from the kitchen!
https://nandustips.blogspot.com/2011/02/handy-1-kw-hf-dummy-load.html
Regards,
Nandu.
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I like the idea of a one ring stove with a PL259 on it!
Makes you wonder what other home appliances could be adapted (I draw the line at fitting speaker terminals to my electric shower for testing car power amps)?
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This got me curious so I sat down and did the math.
1kW @ 230V means a 52.9 ohm load
Let's be generous and assume that it's non-inductive even though the element is wound into a two or three turn spiral, at HF this won't matter anyway.
52,9R + j0 is equal to an SWR of 1.058:1 or a return loss of 31dB which is damn good.
HOWEVER
All of this assumes that the heating element is a solid ceramic spiral. If It's a coil of resistance wire inside a ceramic cylinder then all bets are off.
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Would suggest asking the OP to comment on the inductive nature of the element - but I'm guessing as he/she's been banned after two posts then we won't get a response.
They do claim it works though. So perhaps they have non-wound elements in India. :-//
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The next time I service my coffee machine I'll try and remember to have a look at how high in frequency it's ~50 ohm element (also 1kW @ 230V) is resistive to.
I have also used various heating appliances as (DC / low freq) dummy loads (or capacitor bank charging resistors) - it's by far the cheapest way to get a high power load if the resistance range and lack of precision is OK for your application. In particular something like this: https://www.argos.co.uk/product/7254220 (https://www.argos.co.uk/product/7254220) works well (albeit in a fairly large form factor) - for 13GBP you get 3 different switchable resistance values (corresponding to 750, 1250 and 2000W @ 230V) and no AC fan or anything that will get unhappy running on DC (though I would try to avoid having the thermostat or switches break high DC voltage/current!).
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This got me curious so I sat down and did the math.
1kW @ 230V means a 52.9 ohm load
Let's be generous and assume that it's non-inductive even though the element is wound into a two or three turn spiral, at HF this won't matter anyway.
52,9R + j0 is equal to an SWR of 1.058:1 or a return loss of 31dB which is damn good.
HOWEVER
All of this assumes that the heating element is a solid ceramic spiral. If It's a coil of resistance wire inside a ceramic cylinder then all bets are off.
This assumes the resistance is stable with temperature. I guess it may be found, that the cold state resistance is lower and goes up quite a bit when heated.
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1. There is no question of the inductance of the heating elements being ruled out. The Pi output tank circuits in both the vacuum tube amplifiers enabled a proper match. That would not be the case with current 'no-tune', 50 Ω output solid state amplifiers.
2. Regarding temperature stability, since Nichrome (the material used for the element) has a very low temperature coefficient of resistance (0.0001), the change would not be significant.
Many thanks.
Regards,
Nandu.
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Those elements are always internally wound as a long coil of thin resistive wire, around a mandrel. Either it is left in place, like the old asbestos ones were, or it is removed and replaced with the typical fill of powdered magnesia, well compacted hydraulically to transfer the heat better. They are all slightly inductive, just depends on the individual manufacturer of the resistive element as to how large that coil is, and if the capacitance will resonate with it at a particular frequency.
You would probably be better off using a toaster, as there the resistive material is not wound as a coil, instead as a nearly non inductive flat plane. I just is not sheilded, unless you bond the case to the coax, and provide a conductive mesh across the top as shield.
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can someone Fedex me that stove? so i can verify it with my VNA. anyway, whats the BW for the Heathkit SB-200? i dont see it mentioned in the spec.
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I wonder if heating element for tank water heater commonly in use in USA would work? It surely needs to be water cooled, but total length is shorter than those cooking coils. I have no idea what's inside. Solid or coil....
I think SB200 was 3.5MHz to 21MHz, often modified to include 28MHz.
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I have several large/huge incandescent light bulbs, few 500 watts and few 1500 watts.
Back when I was active, I used to use 100 watt light bulb as dummy load without problems. Of course, those were tube days.
I do have those shown by OP but smaller. I have no ways to measure inductance.
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A 1kW stove heater measured 3.8uH inductance with resistance of 57.95 Ohm. That translates to the following impedance and VSWR at the lower HAM bands:
@1.8MHz :
z=58+j43
VSWR=2.2
@3.5MHz
z=58+j84
vswr=4.2
@7MHz
z=58+j123
vswr=7.1
So this heater can be used as a load at 1.8MHz band but not at other bands higher in frequency unless the tuner in the radio can compensate for the higher vswr.
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I wonder if heating element for tank water heater commonly in use in USA would work? It surely needs to be water cooled, but total length is shorter than those cooking coils. I have no idea what's inside. Solid or coil....
Water heaters use calrod elements, they're the same as the elements in ovens, stove burners and various other appliances. They have a nichrome wire packed in a white powdery substance into a metal tube. In pictures I've seen the nichrome has been coiled about 1/3 the diameter of the outer tube.
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:palm: it`s hard to see that after paying a RD1/50 :palm: