Quick wiring question, ohms and watts
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Lead Driver
Joined: Oct 2004
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From: Columbia, MO
Quick wiring question, ohms and watts
If I have 2 resistors, each 1/2 Watt & 470 Ohm. I'm trying to make it 1 Watt, 470 Ohm. Do I wire them in a series, or in parallel?
It's been a long time since my phisics classes. I can't remember which way increases the wattage and which way decreases the resistance.
It's been a long time since my phisics classes. I can't remember which way increases the wattage and which way decreases the resistance.
"Mifflin Clay"
Joined: Jul 2005
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From: Mifflin, PA
Neither- in parallel, the equivelant of 2 equal value resistors is value/2, in your case 235 ohms, but it is 1 watt. In series, they add, for a total of 940 ohms. T achieve 470 ohms at 1 watt, you need 4 of the 470 ohm resistore - in series parallel. (Parallel 2 sets, and put the sets in series). For a refresher course try this link, and scroll down some....
http://hyperphysics.phy-astr.gsu.edu...ric/resis.html
http://hyperphysics.phy-astr.gsu.edu...ric/resis.html
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Lead Driver
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From: Columbia, MO
Aarkie, yes, it would be easier, but Radio Shack was out of the 1 Watt.
Here's what I'm thinking:
Each bunch of 2 resistors in parallel would be double the wattage and half the resistance of just one resistor. So it would be 235 Ohms and 1 Watt. But when you wire the 2 bundles together in a series, the resistance jumps back to 470 ohms and stays at 1 Watt. Correct?
What I'm attempting to do is make a wire to replace the AIH (I know you can buy one, but I'm too stubborn if I can make it myself), so it won't throw a code. I had one made with a single 470 Ohm, 1/4 Watt resistor. But, I'm still throwing a code, so I think it burned up the single resistor. I was reading on some different forums of some guys experiencing similar problems. They went with a higher wattage resistor, still at 470 Ohms, and it seemed to cure the problem.
Ideally, I'd like to have a single 470 - 500 Ohm, 2 Watt, wire wound resistor that wouldn't be as fragile, given they sit right in the valley of these monsters.
Here's what I'm thinking:
Each bunch of 2 resistors in parallel would be double the wattage and half the resistance of just one resistor. So it would be 235 Ohms and 1 Watt. But when you wire the 2 bundles together in a series, the resistance jumps back to 470 ohms and stays at 1 Watt. Correct?
What I'm attempting to do is make a wire to replace the AIH (I know you can buy one, but I'm too stubborn if I can make it myself), so it won't throw a code. I had one made with a single 470 Ohm, 1/4 Watt resistor. But, I'm still throwing a code, so I think it burned up the single resistor. I was reading on some different forums of some guys experiencing similar problems. They went with a higher wattage resistor, still at 470 Ohms, and it seemed to cure the problem.
Ideally, I'd like to have a single 470 - 500 Ohm, 2 Watt, wire wound resistor that wouldn't be as fragile, given they sit right in the valley of these monsters.
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Lead Driver
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From: Columbia, MO
Originally Posted by Bob Ayers
The total power capacity will double in either case, 2 1/2 watt resistors in series will be 1W, or 2 1/2 watt resistors in parallel will also be 1W.
It seems that I'm getting conflicting answers from alchymist and Bob.
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From: Durham, NC
Originally Posted by jtharvey
So, in my drawing above, you're saying that that what I have wired up there will give 2 Watts (not 1) still at 470 Ohms?
It seems that I'm getting conflicting answers from alchymist and Bob.
It seems that I'm getting conflicting answers from alchymist and Bob.
"Mifflin Clay"
Joined: Jul 2005
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From: Mifflin, PA
Originally Posted by Bob Ayers
Yes, with the 4 1/2 watt resistor configuration you have drawn, the total power capacity will be 2 watts.
With all that said, why not figure out the max current draw at the max applied voltage, calculate the power rating required, and just buy a wirewound 470 in the desired rating? 2 or 4 or 5 or more resistors just increase the probability of a failure of one component. And, of course, you can use a resistor of a higher power rating if you wish - it's the minimum required rating thats important.
Senior User
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From: Ridgecrest, CA
Do not worry about having too much wattage rating, only too little. The only down fall to higher wattage resistors is their size. So the double sets of parallel 470 ohm 1/2 watt will do the trick. I prefer to use as few components as possible, the weakest link theory. Depending on what you're doing with these, be aware of surge current; i.e. fans, motors, etc. There's more tests you can do to be sure you need the wattage. Also remember that components dissipate power in the form of heat, so isolate from any low flammable areas.
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From: Durham, NC
Originally Posted by alchymist
Sorry if I created any confusion, your original request was for a 1 watt - circuit. You specified 1/2watt resistors to start with. Yes, the total rating on the 4 1/2 w resistors is 2 watts.
With all that said, why not figure out the max current draw at the max applied voltage, calculate the power rating required, and just buy a wirewound 470 in the desired rating? 2 or 4 or 5 or more resistors just increase the probability of a failure of one component. And, of course, you can use a resistor of a higher power rating if you wish - it's the minimum required rating thats important.
With all that said, why not figure out the max current draw at the max applied voltage, calculate the power rating required, and just buy a wirewound 470 in the desired rating? 2 or 4 or 5 or more resistors just increase the probability of a failure of one component. And, of course, you can use a resistor of a higher power rating if you wish - it's the minimum required rating thats important.
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Lead Driver
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From: Columbia, MO
Originally Posted by alchymist
Sorry if I created any confusion, your original request was for a 1 watt - circuit. You specified 1/2watt resistors to start with. Yes, the total rating on the 4 1/2 w resistors is 2 watts.
My super fancy drawing in post #4 -- what's the wattage there?
Originally Posted by alchymist
With all that said, why not figure out the max current draw at the max applied voltage, calculate the power rating required, and just buy a wirewound 470 in the desired rating?
Again, see my post, #4, last line, I state:
Originally Posted by jtharvey
Ideally, I'd like to have a single 470 - 500 Ohm, 2 Watt, wire wound resistor that wouldn't be as fragile, given they sit right in the valley of these monsters.
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From: Durham, NC
Originally Posted by jtharvey
That was my original request. You stated to run them in series parallel, as I indicated in my drawing. Now I'm quite confused. My super fancy drawing in post #4 -- what's the wattage there?
Again, see my post, #4, last line, I state:
That's what I'm going to end up doing, 1 wirewound resistor. I was just trying to get it done last night, and was being impatient. Now, it's just a matter of trying to figure out what is correct and me learning some new information.
My super fancy drawing in post #4 -- what's the wattage there? Again, see my post, #4, last line, I state: That's what I'm going to end up doing, 1 wirewound resistor. I was just trying to get it done last night, and was being impatient. Now, it's just a matter of trying to figure out what is correct and me learning some new information.
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Lead Driver
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From: Columbia, MO
Originally Posted by Bob Ayers
I'm in total agreement with alchymist, why not use a single 470 ohm resistor with the proper power rating?????