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Old 25th Feb 2019, 9:38 am   #1
G4XWDJim
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Default Dummy Load possibilities

Would anyone like to speculate what wattage I might be able to use this resistor to measure in a ventilated case with no forced air movement.

It says 50 ohms on the back away from the camera, is 0.96" diameter and has 5mm wall thickness.

Jim
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Old 25th Feb 2019, 10:39 am   #2
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Default Re: Dummy Load possibilities

Morganite type 701's are 10W resistors, so mounted as you indicate - 5 - 8W safely for a reasonable length of time, 10W in short bursts - letting it cool afterwards.
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Old 25th Feb 2019, 12:28 pm   #3
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Default Re: Dummy Load possibilities

Thanks for that Terry. I hadn't found any info despite trying.

I was hoping it would be 20 watts which would be more helpful. Maybe I shall have to try dunking it in oil or fan cooling it. Some experimenting needed.

Jim
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Old 25th Feb 2019, 8:08 pm   #4
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Default Re: Dummy Load possibilities

I was recently looking at an American website, dealing mostly with spares and mods for Kenwood HF rigs, but also presented this design for a 100W dummy load https://k4eaa.com/dummy.html. May not be a professional grade unit, may be adequate for us hams? There was a guy selling 20 off of the 3W resistors, online at a good price.

B
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Old 25th Feb 2019, 8:19 pm   #5
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Default Re: Dummy Load possibilities

While dunking it in oil to increase heat-conduction might at first glance be a good idea, be aware that some oils can be distinctly problematic and can - over time - cause the resistor to change value in inconvenient ways.
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Old 25th Feb 2019, 9:36 pm   #6
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Default Re: Dummy Load possibilities

Those tubular ones aren’t particularly temperature stable. I accidentally put 100W through one in a Marconi power meter and it read 28 ohms for a good ten minutes afterwards! Whoops.

RS sell Caddock 50 ohm 1% TO220 packaged resistors which are good for loads in HF bands. About £11 a go though!
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Old 25th Feb 2019, 9:38 pm   #7
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Default Re: Dummy Load possibilities

Quote:
Originally Posted by G6Tanuki View Post
While dunking it in oil to increase heat-conduction might at first glance be a good idea, be aware that some oils can be distinctly problematic and can - over time - cause the resistor to change value in inconvenient ways.
I accept that this going down the cheap and cheerful path, but noting the guy's comments, "I've used it for about a year now. A few hundred of your service rigs that you have sent to me have been loaded up into this dummy, and it is still like the day it was built. I know, because I had to take it apart to take these photos! It still measures 49.9 Ohms, even after all those rigs, all that power!"

So seems like its been in pretty much in almost daily use for a year. It would be necessary to keep an eye on it and bin if it goes bad.

B
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Old 25th Feb 2019, 9:45 pm   #8
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Default Re: Dummy Load possibilities

I've been using a "47 ohm" resistor of this type for testing a DC-AC inverter. It was carrying 165W for about 5 minutes and the resistance measured 43 ohms at the end of the run, temperature around 232°F.
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Guy
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Old 25th Feb 2019, 10:03 pm   #9
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Default Re: Dummy Load possibilities

That's pretty good!

Another option (and a shop with lots of other toys): https://qrp-labs.com/dummy.html
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Old 25th Feb 2019, 11:14 pm   #10
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Default Re: Dummy Load possibilities

Quote:
Originally Posted by Terry_VK5TM View Post
Morganite type 701's are 10W resistors
I think that's an error.

My catalogue says they are rated at 90W @ 20 degrees C, and I have certainly seen them in dummy loads with intermittent ratings above that figure. The rating is for vertical mounting (which gives better cooling) and is limited by a maximum surface temperature of 220 degrees C.

Probably good for a few hundred watts of SSB, which has a low duty cycle.
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Old 26th Feb 2019, 6:07 am   #11
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Default Re: Dummy Load possibilities

Does your catalogue mention whether those figures are resistance specific across the whole range of resistances by any chance?

I was using the figure we were told when we used them back in my old Post Office tech trainee days, so your catalogue is probably better info than what I was relying on.

Re oil for cooling, transformer oil if you can get it or mineral oil (baby oil) would be OK.
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Old 26th Feb 2019, 7:24 am   #12
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Default Re: Dummy Load possibilities

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Originally Posted by m0cemdave View Post
I think that's an error.

My catalogue says they are rated at 90W @ 20 degrees C
Thanks for this info Dave, would you mind please including a scan of the page you have. I’ve spent some time searching online with no success.

When you compare the size of a 2W resistor with this 701 you would expect a lot more than 10W.

I made a dummy load into a small can about 35 years ago which worked ok although I never stressed it but I’m worried now that the transformer oil I used then has nasties in it.

My main use of a 50 ohm DL would be to tune the pi tank of a transmitter to 50 ohms prior to connecting and setting up the ATU. I only have interest in low frequency AM and CW these days and have no SSB gear.

Jim
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Old 26th Feb 2019, 2:03 pm   #13
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Default Re: Dummy Load possibilities

Hi All,
If you want to go down the oil-cooled route you might want to consider silicone oil. This used to be difficult to find in sensible quantities but is now widely available as treadmill oil - Ebay 173792548861 for instance (no connection to me) This oil has a viscosity of 350cst. which is a little more viscous than the oil used, for instance, in cooling the Redifon solid state linear amplifiers (200cst) but I can't see the increase in viscosity being a problem for something which is a simple shape like a resistor in a can. Bit different if you need the oil to work it's way in amongst lots of small components etc.
The electrical properties of silicone oil are very good - better than mineral oil, I believe - and it does not have any of the harmful side effects associated with some of the other candidate oils.
cheers
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Old 26th Feb 2019, 3:48 pm   #14
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Default Re: Dummy Load possibilities

Engine oil will be fine and cheap.
 
Old 26th Feb 2019, 6:00 pm   #15
G4YVM David
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Default Re: Dummy Load possibilities

Could you wrap it in copper and solder on fins? Even if they were delicate fins you could then put it in oil and see how you go. I used cooking oil in one many years ago...never seemed to do any harm but it was not one of these types.
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Old 26th Feb 2019, 6:34 pm   #16
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Default Re: Dummy Load possibilities

Jim, my 50 watt Murphy 618 has a similar looking dummy load resistor, but half the length. It is rated at 35 watts. I'd put power on and increase in steps until it got too hot to touch.
73, Andrew
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Old 26th Feb 2019, 6:37 pm   #17
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Default Re: Dummy Load possibilities

Jim & Terry:

Here is a link to the relevant page on their online catalogue:

http://www.hvrint.com/rodtube.htm

HVR is the current name for the old Morganite company.
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Old 26th Feb 2019, 10:08 pm   #18
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Default Re: Dummy Load possibilities

Thanks Dave,

Useful info. I was looking under their former name.

Jim
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Old 27th Feb 2019, 12:29 am   #19
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Default Re: Dummy Load possibilities

Thanks for that.
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Old 27th Feb 2019, 12:38 am   #20
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Default Re: Dummy Load possibilities

Quote:
Originally Posted by merlinmaxwell View Post
Engine oil will be fine and cheap.
Not totally sure about that. The paraffin oil showed on the 'Kenwood' webpage is a pure hydrocarbon whereas engine oil contains a whole cocktail of inorganic additives thrown in to add one property or another. In terms of both dielectric and chemical compatibility properties, the engine oil might pose issues?

B
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