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thermal resistance, dummy loads

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  thermal resistance, dummy loads Christopher Howard <christopher@librehacker.com> - 2026-04-23 13:33 -0800
    Re: thermal resistance, dummy loads joegwinn@comcast.net - 2026-04-23 18:32 -0400
      Re: thermal resistance, dummy loads Christopher Howard <christopher@librehacker.com> - 2026-04-23 15:15 -0800
        Re: thermal resistance, dummy loads joegwinn@comcast.net - 2026-04-24 18:12 -0400
    Re: thermal resistance, dummy loads john larkin <jl@glen--canyon.com> - 2026-04-23 17:18 -0700
    Re: thermal resistance, dummy loads Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2026-04-24 00:42 +0000
    Re: thermal resistance, dummy loads "Edward Rawde" <invalid@invalid.invalid> - 2026-04-23 20:44 -0400
      Re: thermal resistance, dummy loads Jeff Layman <Jeff@invalid.invalid> - 2026-04-24 08:46 +0100
    Re: thermal resistance, dummy loads Jan Panteltje <alien@comet.invalid> - 2026-04-24 07:32 +0000
    Re: thermal resistance, dummy loads liz@poppyrecords.invalid.invalid (Liz Tuddenham) - 2026-04-24 09:28 +0100
    Re: thermal resistance, dummy loads legg <legg@nospam.magma.ca> - 2026-04-24 11:18 -0400
    Re: thermal resistance, dummy loads Simon Simple <nothanks@nottoday.co.uk> - 2026-04-27 23:34 +0100
      Re: thermal resistance, dummy loads Jan Panteltje <alien@comet.invalid> - 2026-04-28 06:44 +0000

#743286 — thermal resistance, dummy loads

Hi, I'm wondering what could be used as a dummy load for testing low
voltage, high wattage DC power supplies. E.g., I have a 12V 500W DC PS I
wanted to test under heavy load. I see 50 ohm dummy loads for ham stuff
are easy to get, but that would be too high a resistance for something
like 12V.

I see 1 ohm, 200W resistors are easy to find, like the HS200 1R J. But
in the fine print you've got to provide a correctly rated heatsink. When
I did the calculations for maximum total thermal resistance, assuming
200W power dissipation, and some non-conservative values of standard
room temp and a max junction temp of 200 ℃, I'm looking at less than 1 ℃
/ W total thermal resistance, and the heatsink ends up being five times
the cost of the resistor.

So I'm wondering what is the practical way forward. Like, put the
resistor in mineral oil? Or get some nichrome...?

-- 
Christopher Howard

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#743287

On Thu, 23 Apr 2026 13:33:01 -0800, Christopher Howard
<christopher@librehacker.com> wrote:

>Hi, I'm wondering what could be used as a dummy load for testing low
>voltage, high wattage DC power supplies. E.g., I have a 12V 500W DC PS I
>wanted to test under heavy load. I see 50 ohm dummy loads for ham stuff
>are easy to get, but that would be too high a resistance for something
>like 12V.
>
>I see 1 ohm, 200W resistors are easy to find, like the HS200 1R J. But
>in the fine print you've got to provide a correctly rated heatsink. When
>I did the calculations for maximum total thermal resistance, assuming
>200W power dissipation, and some non-conservative values of standard
>room temp and a max junction temp of 200 ?, I'm looking at less than 1 ?
>/ W total thermal resistance, and the heatsink ends up being five times
>the cost of the resistor.
>
>So I'm wondering what is the practical way forward. Like, put the
>resistor in mineral oil? Or get some nichrome...?

At that voltage and impedance, boiling ordinary tap water will do just
fine.  Could be ten 10-ohm wire wound resistors in parallel to spread
things out.

How long is the load used for per run?  If it's a long game, arrange
things so the vapor rises into an ordinary small radiator, condenses,
and runs back into the container with the resistors - a crude heat
pipe.

Joe

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#743289

joegwinn@comcast.net writes:

> How long is the load used for per run?  If it's a long game, arrange
> things so the vapor rises into an ordinary small radiator, condenses,
> and runs back into the container with the resistors - a crude heat
> pipe.
>
> Joe

Could you clarify what is meant by "ordinary small radiator"? Maybe link
to a product example?

-- 
Christopher Howard

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#743334

On Thu, 23 Apr 2026 15:15:48 -0800, Christopher Howard
<christopher@librehacker.com> wrote:

>joegwinn@comcast.net writes:
>
>> How long is the load used for per run?  If it's a long game, arrange
>> things so the vapor rises into an ordinary small radiator, condenses,
>> and runs back into the container with the resistors - a crude heat
>> pipe.
>>
>> Joe
>
>Could you clarify what is meant by "ordinary small radiator"? Maybe link
>to a product example?

I was thinking of such things as automotive oil coolers for
transmissions and the like.  Available at the better junkyards.  Or:

.<https://www.carparts.com/oil-cooler?msockid=3f6e419375ce6a403452536974166bdb>

But as suggested one can use coolers intended as CPU coolers, if they
are big enough.  Or just cobble one together using ordinary copper
tubing and solder.  If a very long life is needed, the joints should
be brazed, not soldered.

Joe

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#743290

On Thu, 23 Apr 2026 13:33:01 -0800, Christopher Howard
<christopher@librehacker.com> wrote:

>Hi, I'm wondering what could be used as a dummy load for testing low
>voltage, high wattage DC power supplies. E.g., I have a 12V 500W DC PS I
>wanted to test under heavy load. I see 50 ohm dummy loads for ham stuff
>are easy to get, but that would be too high a resistance for something
>like 12V.
>
>I see 1 ohm, 200W resistors are easy to find, like the HS200 1R J. But
>in the fine print you've got to provide a correctly rated heatsink. When
>I did the calculations for maximum total thermal resistance, assuming
>200W power dissipation, and some non-conservative values of standard
>room temp and a max junction temp of 200 ?, I'm looking at less than 1 ?
>/ W total thermal resistance, and the heatsink ends up being five times
>the cost of the resistor.
>
>So I'm wondering what is the practical way forward. Like, put the
>resistor in mineral oil? Or get some nichrome...?

For temporary use, some wire in a bucket of water works well.

Or a power resistor in a bucket of water.

You could also dremel a resistror into a sheet of copperclad, again
underwater.

A big piece of wire can be strung up somehow and blasted with a fan,
or just let get hot in free air.

Chain link fence?

Yes, heatsinks are expensive. 

We use copper CPU coolers, with mosfets or TO-220 power resistors
bolted to them. That can dump maybe 300 watts.

https://www.dropbox.com/scl/fi/kinwcu0mapa5rf0edc0gm/G199.jpg?rlkey=rc324w0joiq9t6ooj4da4lgiz&raw=1

https://www.dropbox.com/scl/fi/dp7cl2lw26r20revt813d/R18_Test_1.jpg?rlkey=m4qkzrvxjdaeudwza8fsv3xr7&raw=1


John Larkin
Highland Tech Glen Canyon Design Center
Lunatic Fringe Electronics

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#743291

Christopher Howard <christopher@librehacker.com> wrote:
> Hi, I'm wondering what could be used as a dummy load for testing low
> voltage, high wattage DC power supplies. E.g., I have a 12V 500W DC PS I
> wanted to test under heavy load. I see 50 ohm dummy loads for ham stuff
> are easy to get, but that would be too high a resistance for something
> like 12V.
> 
> I see 1 ohm, 200W resistors are easy to find, like the HS200 1R J. But
> in the fine print you've got to provide a correctly rated heatsink. When
> I did the calculations for maximum total thermal resistance, assuming
> 200W power dissipation, and some non-conservative values of standard
> room temp and a max junction temp of 200 ℃, I'm looking at less than 1 ℃
> / W total thermal resistance, and the heatsink ends up being five times
> the cost of the resistor.
> 
> So I'm wondering what is the practical way forward. Like, put the
> resistor in mineral oil? Or get some nichrome...?
> 

Depends. If you’re doing a short-time test, e.g. load regulation,
efficiency, and current limiting, 
you can just let the resistor cool down between measurements. 

Otherwise, you could just dunk it in a bucket of water. 

Cheers 

Phil Hobbs 
-- 
Dr Philip C D Hobbs  Principal Consultant  ElectroOptical Innovations LLC /
Hobbs ElectroOptics  Optics, Electro-optics, Photonics, Analog Electronics 

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#743292

"Christopher Howard" <christopher@librehacker.com> wrote in message news:874il1tl9u.fsf@librehacker.com...
> Hi, I'm wondering what could be used as a dummy load for testing low
> voltage, high wattage DC power supplies. E.g., I have a 12V 500W DC PS I
> wanted to test under heavy load. I see 50 ohm dummy loads for ham stuff
> are easy to get, but that would be too high a resistance for something
> like 12V.

Consider filament lamps if you can still buy them, and if you can put enough of them in
parrallel to get the load resistance you want at 12V, and if the non-linear v/i
characteristic is not an issue.

>
> I see 1 ohm, 200W resistors are easy to find, like the HS200 1R J. But
> in the fine print you've got to provide a correctly rated heatsink. When
> I did the calculations for maximum total thermal resistance, assuming
> 200W power dissipation, and some non-conservative values of standard
> room temp and a max junction temp of 200 ?, I'm looking at less than 1 ?
> / W total thermal resistance, and the heatsink ends up being five times
> the cost of the resistor.
>
> So I'm wondering what is the practical way forward. Like, put the
> resistor in mineral oil? Or get some nichrome...?
>
> -- 
> Christopher Howard 

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#743296

On 24/04/2026 01:44, Edward Rawde wrote:
> "Christopher Howard" <christopher@librehacker.com> wrote in message news:874il1tl9u.fsf@librehacker.com...
>> Hi, I'm wondering what could be used as a dummy load for testing low
>> voltage, high wattage DC power supplies. E.g., I have a 12V 500W DC PS I
>> wanted to test under heavy load. I see 50 ohm dummy loads for ham stuff
>> are easy to get, but that would be too high a resistance for something
>> like 12V.
> 
> Consider filament lamps if you can still buy them, and if you can put enough of them in
> parrallel to get the load resistance you want at 12V, and if the non-linear v/i
> characteristic is not an issue.

+1

Filament car bulbs are still available, and will be considerably cheaper 
if you have a car scrapyard nearby. A selection of headlamp bulbs 
(55/65W) and stoplamp bulbs (21W) should allow for a wide range of loads.

Old car bulbs can be useful. Being bone idle, I'm using a 21W bulb with 
an old laptop power supply as a slow, low-power, spare car battery 
charger. The 21W bulb drops the power supply's 19V nicely, and when it 
gets dim I know the battery is charged.

-- 
Jeff

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#743295

>Christopher Howard <christopher@librehacker.com>wrote:
>>Hi, I'm wondering what could be used as a dummy load for testing low
>voltage, high wattage DC power supplies. E.g., I have a 12V 500W DC PS I
>wanted to test under heavy load. I see 50 ohm dummy loads for ham stuff
>are easy to get, but that would be too high a resistance for something
>like 12V.
>
>I see 1 ohm, 200W resistors are easy to find, like the HS200 1R J. But
>in the fine print you've got to provide a correctly rated heatsink. When
>I did the calculations for maximum total thermal resistance, assuming
>200W power dissipation, and some non-conservative values of standard
>room temp and a max junction temp of 200 ℃, I'm looking at less than 1 ℃
>/ W total thermal resistance, and the heatsink ends up being five times
>the cost of the resistor.
>
>So I'm wondering what is the practical way forward. Like, put the
>resistor in mineral oil? Or get some nichrome...?

Test by using / charging an empty car battery?

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#743297

Christopher Howard <christopher@librehacker.com> wrote:

> Hi, I'm wondering what could be used as a dummy load for testing low
> voltage, high wattage DC power supplies. E.g., I have a 12V 500W DC PS I
> wanted to test under heavy load. I see 50 ohm dummy loads for ham stuff
> are easy to get, but that would be too high a resistance for something
> like 12V.
> 
> I see 1 ohm, 200W resistors are easy to find, like the HS200 1R J. But
> in the fine print you've got to provide a correctly rated heatsink. When
> I did the calculations for maximum total thermal resistance, assuming
> 200W power dissipation, and some non-conservative values of standard
> room temp and a max junction temp of 200 ℃, I'm looking at less than 1 ℃
> / W total thermal resistance, and the heatsink ends up being five times
> the cost of the resistor.
> 
> So I'm wondering what is the practical way forward. Like, put the
> resistor in mineral oil? Or get some nichrome...?

Buy some slate bars (a good slate quarry will cut them to order) and
wind "Oxy-ferry" wire around them.  I made some large starting resistors
for a 5 kW motor this way in less than one morning's work.

Drill the bars at regular intervals and insert 0BA bolts with nuts and
washers, then wrap the wire in one direction until you come to a bolt.
Go half a turn around the bolt and then wrap in the opposite direction
until you come to the next bolt ...etc.   Power up each resistor
individually so that it runs up to red heat, which will oxidise the wire
and insulate the turns, then allow it to cool.  Connect the bolts
alternately positive and negative, so the resistors are in parallel,
until you have the resistance value you need.  

Relatively cheap to make and easily adjustable to suit your particular
requirements.  If the bolts are linked with crocodile clips, a wide
variety of loads can be arranged quite quickly.



[Do not be tempted to use laminated asbestos sheet instead of slate;
pockets of moisture will turn to steam and they will explode with quite
unbelieveable violence.]

-- 
~ Liz Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk

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#743307

On Thu, 23 Apr 2026 13:33:01 -0800, Christopher Howard
<christopher@librehacker.com> wrote:

>Hi, I'm wondering what could be used as a dummy load for testing low
>voltage, high wattage DC power supplies. E.g., I have a 12V 500W DC PS I
>wanted to test under heavy load. I see 50 ohm dummy loads for ham stuff
>are easy to get, but that would be too high a resistance for something
>like 12V.
>
>I see 1 ohm, 200W resistors are easy to find, like the HS200 1R J. But
>in the fine print you've got to provide a correctly rated heatsink. When
>I did the calculations for maximum total thermal resistance, assuming
>200W power dissipation, and some non-conservative values of standard
>room temp and a max junction temp of 200 ?, I'm looking at less than 1 ?
>/ W total thermal resistance, and the heatsink ends up being five times
>the cost of the resistor.
>
>So I'm wondering what is the practical way forward. Like, put the
>resistor in mineral oil? Or get some nichrome...?


If you get an old domestic heater, with a coiled wire element (~10R),
you can tap it at convenient points, rewiring sections to act in
parallel or series to get different fixed load effects.

Taps should be mechanical screws, washers, lugs, without insulating 
plastic, to run repeatedly without fuss.

You can configure/reconfigure at the remote, cool end of the wired
connections with croc clips or (if you want to get fancy) switches.
Add metering, to indicate what's actually happening, at the DUT.

Add a fan, if there isn't already one built-in, to increase 
service life.

Use on cooler days, or locate test area somewhere where you don't 
have to 'be' all the time.

RL

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#743481

On 23/04/2026 22:33, Christopher Howard wrote:
> Hi, I'm wondering what could be used as a dummy load for testing low
> voltage, high wattage DC power supplies. E.g., I have a 12V 500W DC PS I
> wanted to test under heavy load. I see 50 ohm dummy loads for ham stuff
> are easy to get, but that would be too high a resistance for something
> like 12V.
> 
> I see 1 ohm, 200W resistors are easy to find, like the HS200 1R J. But
> in the fine print you've got to provide a correctly rated heatsink. When
> I did the calculations for maximum total thermal resistance, assuming
> 200W power dissipation, and some non-conservative values of standard
> room temp and a max junction temp of 200 ℃, I'm looking at less than 1 ℃
> / W total thermal resistance, and the heatsink ends up being five times
> the cost of the resistor.
> 
> So I'm wondering what is the practical way forward. Like, put the
> resistor in mineral oil? Or get some nichrome...?
> 
Let's assume that your DC power supplies are powered from an AC mains 
supply.

So, as a load, use an inverter with a 12V input and a whatever-your- 
mains-voltage-is output. Use this to supply your DC supplies.

Now, even I know that free lunches don't exist, so we're gonna need some 
extra current added at the 12v end.  Best left an an exercise for the 
student.

-- 
SS

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#743494

>Simon Simple <nothanks@nottoday.co.uk>wrote:
>>On 23/04/2026 22:33, Christopher Howard wrote:
>> Hi, I'm wondering what could be used as a dummy load for testing low
>> voltage, high wattage DC power supplies. E.g., I have a 12V 500W DC PS I
>> wanted to test under heavy load. I see 50 ohm dummy loads for ham stuff
>> are easy to get, but that would be too high a resistance for something
>> like 12V.
>> 
>> I see 1 ohm, 200W resistors are easy to find, like the HS200 1R J. But
>> in the fine print you've got to provide a correctly rated heatsink. When
>> I did the calculations for maximum total thermal resistance, assuming
>> 200W power dissipation, and some non-conservative values of standard
>> room temp and a max junction temp of 200 ℃, I'm looking at less than 1 ℃
>> / W total thermal resistance, and the heatsink ends up being five times
>> the cost of the resistor.
>> 
>> So I'm wondering what is the practical way forward. Like, put the
>> resistor in mineral oil? Or get some nichrome...?
>> 
>Let's assume that your DC power supplies are powered from an AC mains 
>supply.
>
>So, as a load, use an inverter with a 12V input and a whatever-your- 
>mains-voltage-is output. Use this to supply your DC supplies.
>
>Now, even I know that free lunches don't exist, so we're gonna need some 
>extra current added at the 12v end.  Best left an an exercise for the 
>student.

Cool! just use your electric 230V or whatever heater as load!
I already have a 12V to 230V 2 kW pure sineave inverter,
and an electic heater! 
Will have to remember that! may come in handy!

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