Author Topic: Taking 2 1/2 watt resistors and wiring them together to make a 1w r  (Read 26581 times)

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Ian

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Taking 2 1/2 watt resistors and wiring them together to make a 1w r
« on: May 05, 2011, 01:44:22 am »
Hi
So I only have 1/2watt resistors and I need 1watt resistor, can I just stick them together and it will make like a 1watt resistor

sacherjj

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Re: Taking 2 1/2 watt resistors and wiring them together to make a 1w r
« Reply #1 on: May 05, 2011, 01:54:45 am »
Hi
So I only have 1/2watt resistors and I need 1watt resistor, can I just stick them together and it will make like a 1watt resistor

Yes.

If you need them in series (2 times the value of 1), then the current is the same, but voltage is halved.  So each has half power.

If you need them in parallel (1/2 of the value of 1), then the voltage is the same, but the current is halved.  Again each has half the power.
« Last Edit: May 05, 2011, 01:57:05 am by sacherjj »

Jimmy

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Re: Taking 2 1/2 watt resistors and wiring them together to make a 1w r
« Reply #2 on: May 05, 2011, 02:04:01 am »
If adding in parallel you need to put it under 1

eg

1            1
-    +      -          = 50 ohm
100         100

1            1        1
-    +      -        -     = 33.333 ohm
100         100    100

Ian

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Re: Taking 2 1/2 watt resistors and wiring them together to make a 1w r
« Reply #3 on: May 05, 2011, 02:14:43 am »
Your not making sense to me. If input them in parallel then the wattage doubles and then I do the equation to solve for the resistance right?

Tony R

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Re: Taking 2 1/2 watt resistors and wiring them together to make a 1w r
« Reply #4 on: May 05, 2011, 02:36:16 am »

Current is divided in two equal parts when the resistor values are equal, following the formula P=IV, If I is half that that it was before for that particular resistor the power dissipated by that resistor is half. However this comes with a price, The resistance will go down. so if you need 100 ohm resistance at 1 watt putting two 1/2 100 ohm resistors in parallel will  give you the wattage you need but your resistance will drop in half to 50 ohms

basic rule is that resistance follows Req=(1/R1+1/R2+...)^-1 when hooked in parallel also if you are only talking about two resistors it follows the "product over sum" rule (R1*R2)/(R1+R2) which is the same as previously

Tony R.
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Focus: Embedded Assembly Programming, Realtime Systems,  IEEE Student Member

Mechatrommer

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Re: Taking 2 1/2 watt resistors and wiring them together to make a 1w r
« Reply #5 on: May 05, 2011, 02:49:30 am »
if you have 1/2w 1ohm resistors. paralleling them will obtain 1w 1/2ohm resistor, so if you want the resistance back 1ohm, you need another 2 parallelled resistors and put them in series, so in order to get 1w 1ohm resistor, you need 4 of 1/2w 1ohm resistors.

--\/\/\/-- = 1/2w, 1 ohm

|--\/\/\/--|
-|            |- = 1w, 1/2 ohm
|--\/\/\/--|

|--\/\/\/--.--\/\/\/--|
-|                           |- = 1w, 1 ohm
|--\/\/\/--.--\/\/\/--|
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ElektroQuark

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Re: Taking 2 1/2 watt resistors and wiring them together to make a 1w r
« Reply #6 on: May 05, 2011, 07:27:11 am »

Current is divided in two equal parts when the resistor values are equal, following the formula P=IV, If I is half that that it was before for that particular resistor the power dissipated by that resistor is half.

The I is half of the total I per resistor, but is not the half of the initial I. If you parallel two resistors each one feeds the same current that one alone will. So individual wattage is the same.

ElektroQuark

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Re: Taking 2 1/2 watt resistors and wiring them together to make a 1w r
« Reply #7 on: May 05, 2011, 07:32:00 am »
Hi
So I only have 1/2watt resistors and I need 1watt resistor, can I just stick them together and it will make like a 1watt resistor

Best solution is two resistors in series. Each one with a value equal to the half of the desired resistance.

Mechatrommer

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Re: Taking 2 1/2 watt resistors and wiring them together to make a 1w r
« Reply #8 on: May 05, 2011, 07:48:42 am »
So I only have 1/2watt resistors and I need 1watt resistor, can I just stick them together and it will make like a 1watt resistor
Best solution is two resistors in series. Each one with a value equal to the half of the desired resistance.
you still have 1/2w resistor by putting it in series... to the OP question, the answer is yes, if you stick them in parallel. but u'll get halved resistance if using only 2 resistors.
series... wattage same, R doubled
parallel... wattage doubled, R halved.
« Last Edit: May 05, 2011, 07:51:16 am by Mechatrommer »
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ElektroQuark

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Re: Taking 2 1/2 watt resistors and wiring them together to make a 1w r
« Reply #9 on: May 05, 2011, 08:37:27 am »
In series the voltage is the half for each resistor, so the wattage supported is the half too.

mikeselectricstuff

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Re: Taking 2 1/2 watt resistors and wiring them together to make a 1w r
« Reply #10 on: May 05, 2011, 08:42:54 am »
Bear in mind that resistor power ratings are pretty optimistic, and running one at full rated power will usually get extremely hot unless there is some airflow.
Also bear in mind that you will only get a full combined power rating from two if they are far enough apart that they don't heat each other up.

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EEVblog

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Re: Taking 2 1/2 watt resistors and wiring them together to make a 1w r
« Reply #11 on: May 05, 2011, 08:52:47 am »
The maximum continuous wattage rating of resistor is given into free air at a specified temperature.
So although your 1W resistor might be capable of handling 1W for an indefinite period, it will likely do so at a very HOT temperature.
That will get even hotter if it's inside a box without airflow, and may damage or effect nearby components. Beware.

Dave.

Ian

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Re: Taking 2 1/2 watt resistors and wiring them together to make a 1w r
« Reply #12 on: May 05, 2011, 10:26:48 am »
Thanks all you guys where a bIg help!!!!!!

Mechatrommer

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Re: Taking 2 1/2 watt resistors and wiring them together to make a 1w r
« Reply #13 on: May 05, 2011, 01:19:42 pm »
In series the voltage is the half for each resistor, so the wattage supported is the half too.
o ok. you are correct then. from P=VI.
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bilko

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Re: Taking 2 1/2 watt resistors and wiring them together to make a 1w r
« Reply #14 on: May 05, 2011, 02:36:25 pm »
If you connect to identical resistors in series power per resistor is 1/4 that of a single resistor
P=I2R
or P=VI where V=V/2 and I=I/2

ElektroQuark

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Re: Taking 2 1/2 watt resistors and wiring them together to make a 1w r
« Reply #15 on: May 05, 2011, 03:47:15 pm »
No, I is the same. It is not I/2.

bilko

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Re: Taking 2 1/2 watt resistors and wiring them together to make a 1w r
« Reply #16 on: May 05, 2011, 03:57:08 pm »
Only if you double the voltage

Zero999

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Re: Taking 2 1/2 watt resistors and wiring them together to make a 1w r
« Reply #17 on: May 05, 2011, 05:00:06 pm »
In my view, connecting in series is better because it doubles the voltage rating.

Note that this all assumes the resistors have the same value, if they're different you need to work out the power dissipation in each resistor using Ohm's law. If the resistors are in parallel, it's easier to use P = V2/R, if they're in series use P = I2R.

jimmc

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Re: Taking 2 1/2 watt resistors and wiring them together to make a 1w r
« Reply #18 on: May 05, 2011, 05:23:33 pm »
If you connect to identical resistors in series power per resistor is 1/4 that of a single resistor
P=I2R
or P=VI where V=V/2 and I=I/2

True but that doesn't answer the original question.

If you want to make a 1W rated resistor of value R from 0.5W resistors then you need two resistors of R/2 in series
(or two of 2*R in parallel).

Jim

bilko

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Re: Taking 2 1/2 watt resistors and wiring them together to make a 1w r
« Reply #19 on: May 05, 2011, 05:37:03 pm »
If you connect to identical resistors in series power per resistor is 1/4 that of a single resistor
P=I2R
or P=VI where V=V/2 and I=I/2

True but that doesn't answer the original question.

If you want to make a 1W rated resistor of value R from 0.5W resistors then you need two resistors of R/2 in series
(or two of 2*R in parallel).

Jim

To make a 1 Watt resistor out of two 0.5w resistors you would connect them in parallel, but you would halve the resistance
« Last Edit: May 05, 2011, 05:38:50 pm by yachtronics »

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