Resistor wattage selection

HI,

I am bit confused as to determine the wattage for a resistor. They come in

1/4, 1/2, 1, 2 watt etc.. I want to to a 80ohm resistor between a 300v dc supply and lets say ground. I'll have 3.74amp and 1125 watts. How do I choose the wattage of the resistor ? The difference in the resistor is almost none, so a 4 watt resistor ?

Ken

Reply to
Ken O
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What do you mean by "the difference in the resistor"?

If you put 300V across an 80Ohm resistor, it'll dissipate 1125 Watts. So with a safety margin you'll need a 2kW resistor. I imagine that that would be one expensive beast!

Reply to
Greg Neill

It doesn't matter. Any of those will instantly go up in smoke and make an impressive flash.

Why do you want to burn up all that power?

John

Reply to
John Larkin

He could get a toaster at a garage/yard sale for almost nothing. ;-)

Cheers! Rich

Reply to
Rich Grise

Ha! :-)

Reply to
Greg Neill

You mean dissipation not difference don't you ?

And the important thing you neglected to mention is that your 300V isn't DC but pulses.

The average power dissipated will be energy in pulse ( J ) * repetition rate ( Hz ) .

Graham

Reply to
Eeyore

"Eeyore" wrote in message news: snipped-for-privacy@REMOVETHIS.hotmail.com...

That would mean an even higher energy output than previously thought.?

k
Reply to
Ken O

The average power dissapated is the area under the power/time graph ;) (Integral..)

Reply to
pbdelete

obviously less

Reply to
Ken O

Considerably less than DC which is why your 4W resistor doesn't burn up.

Graham

Reply to
Eeyore

ground.

At those power levels you need a heat sink on the resistor. For example....

Lets say you pick a resistor rated at 2KW with a 60C max case temperature. If the air blown over the heat sink is at 20C the temperature difference is

60-20 = 40C. Therefore you need a heat sink with a thermal resistance of 40/1125 = 0.036 C/W or better.
Reply to
CWatters

Tdiff = Tcase_max - Tambient Rsink = Tdiff / Pout = (Tcase_max - Tambient) / Pout [°C/W]

Guess this will require liquid cooling. Calculating how airflow affects cooling or a thermal setup inside a passivly cooled box is more messy.. ;)

Reply to
pbdelete

No he doesn't. He hasn't told you the whole story. It's not DC.

Graham

Reply to
Eeyore

Hmmmmmmmmmm!

I does, however, sound very similar to DC, don't you think?

Don

Reply to
Don Bowey

Read his other posts !

The veil of secrecy wil be lifted.

Graahm

Reply to
Eeyore

you need a 1125 Watt resistor. (possibly less if it's only connected for short durations and infrequently)

if yoy can find 3 120V 540W heating elements and connect them in series that'd probably do what you want.

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   Jasen
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Reply to
jasen

Since toasters average about 8 to 10 ohms, about 10 of them in series should do for the required 80 ohms.

Actually, he can buy heating element wire, and put them in a series and parallel configuration as required. This is crude, but it should work.

The proper answer is to invest in a proper 2 kW resistor. This is going to cost a fair number of dollars.

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JANA
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Reply to
JANA

My Method for High Power Resistors.

To boost a 4 Watt to 10 Watts I simply Setup up a Plaster of Paris or Clay Box around the Resistor, and let it setup or bake it in the oven.

For larger resistors I use Ni-Chrom wire, cut it to length needed for the correct resistance and set it in Plaster of Paris. Much Cheaper then buying New.

Of course this is not recommended for some>> HI,

Reply to
AA

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Please bottom post.

I suggest that you\'re the one who\'s confused.
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Reply to
John Fields

John,

I am absolutely not confused.

According to the CRC handbook of chemistry and physics the k value for air is 0.0262. For Clay it is ~0.25 or 9 to 10 times higher. With Plaster of Paris k value around 0.42 or 16 times higher.

Air is very efficient thermal insulator and in general most solids are much more efficient thermal conductors. Air flow can improve its conductivity, but that requires constant power.

The material is not preventing the heat from escaping, it is providing a larger surface area for its conduction into the surrounding body (air). Because the material is more efficient at conducting the heat away from the resistor then air, and the substantial increase to the resistors overall surface area, more heat can and will be dissipated. Period.

As stated above the thermal conductivity of Plaster of Paris k value around

0.42. The heat generated by the wire is absorbed into the Plaster of Paris and dissipated by air from the structure. Plaster of Paris will decompose if to much energy is being attempted to be dissipated over to small of an area. Other conductors can also be used such as SS wire. Thus my suggestion "Of course this is not recommended for someone without knowledge of electronics as confusion often leads to pain." Perhaps this was not sufficient. I should have stated "Of course this is not recommended for someone without knowledge of Thermal Dynamics as confusion often leads to pain." as it is apparent you should have a capable grasp of electronics by now and should be familiar with the High Wattage Ceramic Resistor by now.

Later, AA

Reply to
AA

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