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Old 3rd August 2004, 02:10 PM   #1
alecwek is offline alecwek  Denmark
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Unhappy power ratings of resistors

how does one tell the power ratings of a resistor i need bleeders for the filter capacitors of a power supply...i have millions of resistors but for the life of me cant figure out the power ratings...is there a way u can see it on the body of the resistor...i have a few where there are no colour bands on thye resistor where the resistance and the power rating is written on the body....but they are not what i need...is there a way to tell the ratin
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Old 3rd August 2004, 02:27 PM   #2
Netlist is offline Netlist  Belgium
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Hi alecwek
I was writing an attempt to explain but I found this:
http://www.tpub.com/neets/book1/chapter3/1-8.htm
Couldn't do any better

/Hugo
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Old 3rd August 2004, 03:00 PM   #3
alecwek is offline alecwek  Denmark
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umm..i kinda know what power is.....
so baically a 10 watt resistor would be the size of a coke can,....
ok then how about this....i ve got 2200uf capacitors that i need to dicharge when the amp is turned of their max rating 50volts.....does the resistance really matter.....1round 5k is ok right....but what power rating should they be....
though,thanks ...netlist
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Old 3rd August 2004, 03:54 PM   #4
Netlist is offline Netlist  Belgium
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If you know what current will go trough that bleeder you can calculate the power dissipation. Double that value for safety margins and there you are. Something like 5W me thinks.
It could be the size of a small coke can though
No serious, a 5W resistor is only about 2cm or a small inch long.
Google for a picture.

/Hugo
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Old 3rd August 2004, 05:01 PM   #5
6SN7GT is offline 6SN7GT  United Kingdom
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Assuming the bleed resistors are always in circuit and I guess we are talking PSU reservoir caps, you won't want too much current flowing through them.

1mA is a good value, so 47kΩ sounds about right. This only needs to be 1/4W.

OK, so you reckon 10mA bleed is OK, then a 4k7 will do and it'll be 1W.

Which you choose determines how long you want to wait for the volt to die away

Quote:
how does one tell the power ratings of a resistor
That's a very good question.
For your average colour band type, power rating isn't stated, you "just know" by looking at their physical size.

To help, here's a piccie of some different ratings.

From left to right:-
1/4W metal film, 1/2W metal film, 1W, 2W, 3W carbon film.

Hope this helps
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File Type: jpg resistors.jpg (54.6 KB, 57 views)
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Old 3rd August 2004, 10:30 PM   #6
alecwek is offline alecwek  Denmark
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YOU KNOW....WHEN I FIRST POSted...sorry...anyway...i thought the answer would be in the colour of the resistors because i noticed that most of my big resistors..in size not ohms....were green the other smaller ones were blue or red or...well i was hoping there was a sure fire way of telling....guess that would make life to easy ....AND WE JUST CANT HAVE THAT CAN WE
thanks anyway...ill print out the image...be quite handy to have around when im looking for a certain resistor...thanks....
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Old 3rd August 2004, 10:45 PM   #7
cpemma is offline cpemma  United Kingdom
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Quote:
Originally posted by alecwek
....i ve got 2200uf capacitors that i need to dicharge when the amp is turned of their max rating 50volts.....does the resistance really matter.....1round 5k is ok right....but what power rating should they be....
Everything in electronics is Ohms Law. If it's not Ohm it's Hype.

50V, 5k, so W=V*V/R = 0.5W
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Old 4th August 2004, 04:44 PM   #8
6SN7GT is offline 6SN7GT  United Kingdom
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Quote:
Everything in electronics is Ohms Law. If it's not Ohm it's Hype.
Very true.

Agreed, actual power requirement is 1/2W (ish), and adding a safety factor, that means 1W if metal film and 0.66W if carbon film.

I.e. know your actual power dissipation and always overrate so the device lasts for years.
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Old 4th August 2004, 05:18 PM   #9
alecwek is offline alecwek  Denmark
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Unhappy THE SAGA CONTINUES

ok all those self problaimed experts here is one for you.....there is a schematic linked of what im talking about....i had a transformer....about37Vac linked to a full wavbridge that leed to 2200uf capacitors rated at 50v...ok and 5k draining the caps....so when i turned the whole thing on ...the ground to positive rail measured 14volts dc...then 13.9...then 13.8.........when it finally got down to around 10volts dc the damn caps blew up.....lucky for me i had baracaded myself behind a shelf......(to get to the switch...hmmmm thank u lord...) anyway pop!!!and suddenly the caps went to cap heaven.....why....why did they have to leave me??????????
i got a little carried away with the schematic...sorry....
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Old 4th August 2004, 05:20 PM   #10
dhaen is offline dhaen  Europe
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Default Sorry to complicate things....

An important consideration is temperature de-rating.
What this means is that the 1 watt resistor you chose to dissipate 600mW, won't be adequate if the ambient temperature is raised. Most de-rating can be accommodated by just choosing a resistor of double the rating. However, in a really warm environment - near other heat generating components, or in an enclosed space, you have to consult the manufacturers' data. They will usually publish a simple graph of dissapation vs temperature.
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