50-ohm power resistors

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 Hah, I'm gonna need a bigger heat sink!  Running at full
 tilt my nanosecond 500V pulse generator will be dissipating
 700 watts, mostly in 50-ohm resistors.  I don't think banks
 of the 100W Caddock 9100 TO-247-package parts I've been
 using will be the best parts for the job.  So I'm looking
 for good heat-sink-mountable low-inductance (150nH max)
 50-ohm power resistors.  I'm finding microwave resistors,
 but I'm a bit worried about their fragility.  Should I be?
 I need two-terminal series, rather than termination types.


--  
 Thanks,
    - Win

Re: 50-ohm power resistors
Am 15.09.2016 um 14:49 schrieb Winfield Hill:
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< http://www.emc-rflabs.com/Passive-Components/Resistors

Beware, some may contain BeO.

regards, gerhard

Re: 50-ohm power resistors
Is there anything to be gained by looking at dummy
loads for RF amplifiers?

I suspect the frequency/power is outside the normal
radio ham experience, but some of them are surprisingly
exploratory and inventive.

Re: 50-ohm power resistors
On Thursday, September 15, 2016 at 10:49:59 PM UTC+10, Winfield Hill wrote:
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Have you looked at the Vishay range? At Cambridge Instruments we bought them in TO-3 and TO-220 packs - amongst others. They do seem to offer a range of relatively low-thermal resistance styles.

They are planar parts, so the inductance won't be all that high.

http://www.vishaypg.com/foil-resistors/

None of the data-sheets I looked at said a word about inductance, so you'd probably need to contact them directly to find out anything specific.

--  
Bill Sloman, Sydney


Re: 50-ohm power resistors
On 2016-09-15 05:49, Winfield Hill wrote:
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Have you considered liquid cooling? Back in my more active ham radio  
days I built a 50ohm resistor that could pack away a kilowatt for 10min  
or more, cooled by cheap oil for agricultural equipment. That lasted  
about 30 years when the bucket rusted out and leaked. Threw it out, and  
in hindsight I shouldn't have.

Lately I needed load resistors again and resorted to just dumping them  
into a pan with filtered water. That works but strangely the  
gold-anodized kind of power resistors leaves some funky discoloration  
and weird floating matter in the water.

--  
Regards, Joerg

http://www.analogconsultants.com/

Re: 50-ohm power resistors

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Soup for robots!

Re: 50-ohm power resistors
On 9/15/2016 10:41 AM, Joerg wrote:
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Liquid cooling can be great and it does not need to be expensive.  If  
you get one of the liquid cooling heat sinks for CPUs and use an  
inexpensive heater core you can get rid of a lot of heat quickly.  One  
guy cooled his PC by running the liquid out to a barrel in his garage  
and didn't have to use a radiator at all.  The tubing and barrel was  
enough to cool 100 watt CPU.

Connect Peltier devices to generate power to drive a fan, lol!

--  

Rick C

Re: 50-ohm power resistors
On 2016-09-15 14:20, rickman wrote:
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I learned a lot about liquid cooling of liquids when I started brewing  
again. At the end of boiling the wort you have to get several gallons of  
it from 210F to under 90F as fast as possible. Do it too slowly and  
infection can creep in, ruining the beer. If you don't get it cool  
enough but start fermentation anyhow the yeast dies. I found the  
swimming pool to be an excellent location to do the heat exchange.


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:-)

--  
Regards, Joerg

http://www.analogconsultants.com/

Re: 50-ohm power resistors
On 16/09/16 07:20, rickman wrote:
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I loved this idea, until I realised that the extra thermal impedance  
might be a problem. But a dummy load that cools itself only when needed  
and without a separate power source, brilliant!

Re: 50-ohm power resistors
On 9/15/2016 6:02 PM, Clifford Heath wrote:
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I was totally kidding, but if designed properly it could be practical.  
The fan uses just a fraction of the power being dissipated so the  
Peltier devices can interfere with just a fraction of the power being  
dissipated.  Or, if it didn't need to be a purely resistive load, the  
full load could be the DC motor driving the fan, lol.

--  

Rick C

Re: 50-ohm power resistors
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just build a chimney.
--  
This email has not been checked by half-arsed antivirus software  

Re: 50-ohm power resistors
On 9/16/2016 5:37 PM, Jasen Betts wrote:
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Chimneys suck for cooling a heat sink.  I tried detecting an air current  
in a "chimney" above a 100 watt light bulb and it was virtually  
undetectable.  The chimney was a three foot section of 4 inch dryer vent.

--  

Rick C

Re: 50-ohm power resistors
On 9/17/2016 12:28 AM, rickman wrote:
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What did you use to detect the air current? I tried such an experiment  
long ago using a strip of toilet paper attached to one side of the  
chimney. Had some very visible movement of the T.P.

Re: 50-ohm power resistors
On 9/17/2016 4:39 AM, John S wrote:
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If you have to use an air current detector to see such small currents,  
you don't have nearly enough air to make a significant difference in  
heat removal.

--  

Rick C

Re: 50-ohm power resistors
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Well, that's blatantly, on-the-face wrong.  Perfectly still air is a  
fantastic insulator.  We fill our homes' walls with the stuff!

Vacuum is a good number of times even better (but with an even greater  
tempco), but that's a luxury reserved for "keeping hot things hot and cold  
things cold".

Tim

--  
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Contract Design
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Re: 50-ohm power resistors
On 9/18/2016 2:28 AM, Tim Williams wrote:
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Yup, and slowly moving air is right behind still air for insulating.  
Read nearly any spec sheet on an air cooled device.  It will give  
thermal ratings for various ft/s which are far above speeds that require  
tissue paper to measure.

--  

Rick C

Re: 50-ohm power resistors
On Saturday, September 17, 2016 at 11:27:35 PM UTC-7, Tim Williams wrote:
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Heat, though, moves by conduction (insulators move less than conductors) and
by convection (moving air removes heat by heat capacity, irrespective of its heat conductivity).
If you make good air contact with fins, conduction doesn't determine the
efficacy of your convective heat removal.

Filling your house walls with anything that does NOT convect, is a general win.
Adobe houses, with no air-fill in the walls, are said to be quite comfortable.

Re: 50-ohm power resistors
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Well-- that's more of a filtering effect.

Dense walls (adobe, brick, concrete, etc.) are always said to "retain heat  
well".  This is a bug sold as a feature.  Don't you love marketing?..

It's doesn't work out so well when the average outside air temperature goes  
high or low.  I've lived in an old brick house before: the walls had a  
thermal time constant of about 18 hours, so when the weather suddenly turns  
hot, you get about a day's respite while the walls remain cool.  Then it's  
sweltering for at least two days after that (one for the hot day following,  
one for the day after cool weather returns, where the walls are still hot).

It's even worse in winter (up here, where winter is a thing), because  
there's never a temperate day.  You're just burning the furnace non-stop,  
and never making any progress.

And you have to burn the furnace non-stop, otherwise it takes >4 hours to  
warm up the place when you get home.

In a desert environment (where adobe originated), the average temperature is  
probably just fine, it's the swings that kill you (sweltering direct  
sunlight, to near-frosty nights).

Adobe and brick also have a lot of porosity, so it's not entirely fair o say  
they aren't air-filled.  Still, they're in the 1 W/(m*K) range, so you need  
to use ridiculously thick walls to get any useful insulation value.

I've heard of adobe construction where the outer wall is built over hay  
bales.  Seems a rather perishable choice, should any water leak in there...  
but again, in a desert environment, that'll probably never happen.  That  
makes a thermal CLC filter, with a high impedance (low conductivity)!  So  
the indoor temperature is extremely stable, /and/ central heating/cooling is  
effective (though it still has to run 24/7).

The ideal home insulation is light (so it heats and cools rapidly), and low  
conductivity.  It's hard to beat wood and drywall with fiberglass or foam  
packing: the materials are light, so you can turn off the HVAC while you're  
away, to save cost (i.e., reduce the average interior-to-exterior  
temperature drop, which is the figure costing you money).  And turn it back  
on and have things come to equilibrium within a reasonable time frame (an  
hour or so).

Tim

--  
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Contract Design
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Re: 50-ohm power resistors
On 20/09/16 10:37, Tim Williams wrote:
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Straw bales in the UK, not normally known for being dry :)
http://www.bre.co.uk/filelibrary/pdf/projects/low_impact_materials/IP15_11.pdf

And, from a less disinterested source:
http://www.strawcottage.co.uk/homegrownhomeresearch.html

I'd be concerned about vermin; straw is a food source.


Re: 50-ohm power resistors
On Tuesday, September 20, 2016 at 5:59:50 AM UTC-4, Tom Gardner wrote:
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I know someone with a straw bale house.. been up for ~5 years, so far so good.

Straw is the left over stalk after the wheat is harvested.  
(it might be some other grain.)  
Typically it's not food.  (though mice like to make homes in it.)  
Hay is cut and dried grass.. it is food.  

George H.  

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