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

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Offline Ian

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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
 

Offline sacherjj

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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 »
 

Offline Jimmy

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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               

                     
 

Offline Ian

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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?
 

Offline Tony R

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yes wattage adds in parallel

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

when in series resistance adds.


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Offline Mechatrommer

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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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Offline ElektroQuark

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yes wattage adds in parallel

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.

Offline ElektroQuark

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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.

Offline Mechatrommer

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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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Offline ElektroQuark

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In series the voltage is the half for each resistor, so the wattage supported is the half too.

Offline 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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Offline 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 »
I second Mike's comments.
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.
 

Offline 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!!!!!!
 

Offline 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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Offline 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

 

Offline 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.

Offline 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
 

Offline 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.
 

Offline 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
 

Offline 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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