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Old 08-04-2007, 03:42 AM #1
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Default resistor help

i wanted to know the difference between like a 1/4 watt 2.2k resistor and a 1/2 watt 2.2k resistor


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Old 08-04-2007, 03:51 AM #2
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Default Re: resistor help

good question . thats just the amount of power its safe to put through them i believe,... i think thats what it means lol
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Old 08-04-2007, 04:44 AM #3
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Default Re: resistor help

what about the difference, if something calls for a 50K resistor

can you use a 47K?
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Old 08-04-2007, 05:23 AM #4
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Default Re: resistor help

Quote:
Originally Posted by twisted_transistor
what about the difference, if something calls for a 50K resistor

can you use a 47K?
You CAN...but keep in mind, the values are noramlly fairly specific - you can fry some stuff by putting too much voltage, or current through them, so be careful, and if the schematic calls for a 50k, slap in a 50k :-) there are very few shortcuts in circuit design - some is blind luck, figuring stuff out, but for the most part, electricity is fairly unforgiving....
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Old 08-04-2007, 05:52 AM #5
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Default Re: resistor help

well the 1/4 can handle one quater of a watt, and the 1/2 can handle half a watt, pretty simple. but if you pass half a watt of power through a 1/4 resistor, it will get hot VERY fast
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Old 08-04-2007, 08:22 AM #6
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Default Re: resistor help

+1 on SenKat's post. I wouldn't skimp around with premade circuits because the difference of 3KOhms can be a HUGE difference in a circuit. It all depends on the circuit, so it could also be a very tiny change. It mostly depends on what that specific resistor is controlling.

As about the power rating...

Watts = Volts * Amps ... So if you are running a diode at 3V and it's using 500mA (0.5A), you're giving it 1.5 Watts. If this is a direct-feed kind of thing, then it's probably best that you get something that is rated up to 2W to be on the safe side.

Also keep in mind that the higher rating that the resistor is the more expensive and usually the bigger it gets. I've got some that are rated up to 50W and they're covered in a copper heat sink and all that... ;D

Hope that helps;
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Old 08-04-2007, 06:36 PM #7
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Default Re: resistor help

that works lol thank you guys for the help
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Old 08-05-2007, 01:35 AM #8
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Default Re: resistor help

Keep in mind that resistors come in various tolerances (10%, 5%, etc..). *If you have a selection to choose from, you can measure the resistors to get the closest to the value you are seeking. * (i.e. if you have a 47k 10% resistor, it could actually be anywhere from 42.3k to 51.7k)

As far as the power rating, you must choose the size by determining how much power will be loss in the resistor. Use the (simplified) formula:
P = V * I (voltage across the resistor x current flowing through it)
or
P = I^2 * R (square of the current x the resistance - note that this is essentially the same as the previous formula because V=I*R)


for example: *You have a 5 ohm resistor and have a 300mA current flowing through it.
R = 5 ohms, I = 0.3 amps

P = 0.3*0.3*5 = 0.45W *--> *you would need a *minimum* 1/2 W resistor (if you have space, use a larger one).

Always size up your resistor wattage rather than down, especially if they will be installed in an environment that dissipates heat poorly.


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Old 08-05-2007, 02:49 AM #9
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Default Re: resistor help

mine is a 5% 47k, i know that because of the bands
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Old 08-05-2007, 03:30 AM #10
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Default Re: resistor help

So it should be in the range of 44.65k to 49.35k.

Quote:
Originally Posted by twisted_transistor
mine is a 5% 47k, i know that because of the bands
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