Taking 2 1/2 watt resistors and wiring them together to make a 1w r - Page 1
Forum section: electronics / beginners

author: Ian
author_rank: Contributor
timestamp: May 05, 2011, 01:44:22 am
content: HiSo 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

author: sacherjj
author_rank: Frequent Contributor
timestamp: May 05, 2011, 01:54:45 am
content: Quote from: Ian on May 05, 2011, 01:44:22 amYes.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.

author: Jimmy
author_rank: Regular Contributor
timestamp: May 05, 2011, 02:04:01 am
content: If adding in parallel you need to put it under 1eg1 1- + - = 50 ohm100 1001 1 1- + - - = 33.333 ohm100 100 100

author: Ian
author_rank: Contributor
timestamp: May 05, 2011, 02:14:43 am
content: 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?

author: Tony R
author_rank: Regular Contributor
timestamp: May 05, 2011, 02:36:16 am
content: yes wattage adds in parallelCurrent 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 ohmsbasic 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 previouslywhen in series resistance adds.

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 05, 2011, 02:49:30 am
content: 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|--\/\/\/--.--\/\/\/--|

author: ElektroQuark
author_rank: Unknown
timestamp: May 05, 2011, 07:27:11 am
content: Quote from: Tony R on May 05, 2011, 02:36:16 amThe 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.

author: ElektroQuark
author_rank: Unknown
timestamp: May 05, 2011, 07:32:00 am
content: Quote from: Ian on May 05, 2011, 01:44:22 amBest solution is two resistors in series. Each one with a value equal to the half of the desired resistance.

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 05, 2011, 07:48:42 am
content: Quote from: EdoNork on May 05, 2011, 07:32:00 amyou 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 doubledparallel... wattage doubled, R halved.

author: ElektroQuark
author_rank: Unknown
timestamp: May 05, 2011, 08:37:27 am
content: In series the voltage is the half for each resistor, so the wattage supported is the half too.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: May 05, 2011, 08:42:54 am
content: 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.

author: EEVblog
author_rank: Unknown
timestamp: May 05, 2011, 08:52:47 am
content: 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.

author: Ian
author_rank: Contributor
timestamp: May 05, 2011, 10:26:48 am
content: Thanks all you guys where a bIg help!!!!!!

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 05, 2011, 01:19:42 pm
content: Quote from: EdoNork on May 05, 2011, 08:37:27 amo ok. you are correct then. from P=VI.

author: bilko
author_rank: Frequent Contributor
timestamp: May 05, 2011, 02:36:25 pm
content: If you connect to identical resistors in series power per resistor is 1/4 that of a single resistorP=I2Ror P=VI where V=V/2 and I=I/2

author: ElektroQuark
author_rank: Unknown
timestamp: May 05, 2011, 03:47:15 pm
content: No, I is the same. It is not I/2.

author: bilko
author_rank: Frequent Contributor
timestamp: May 05, 2011, 03:57:08 pm
content: Only if you double the voltage

author: Zero999
author_rank: Super Contributor
timestamp: May 05, 2011, 05:00:06 pm
content: 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.

author: jimmc
author_rank: Frequent Contributor
timestamp: May 05, 2011, 05:23:33 pm
content: Quote from: yachtronics on May 05, 2011, 02:36:25 pmTrue 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

author: bilko
author_rank: Frequent Contributor
timestamp: May 05, 2011, 05:37:03 pm
content: Quote from: jimmc on May 05, 2011, 05:23:33 pmTo make a 1 Watt resistor out of two 0.5w resistors you would connect them in parallel, but you would halve the resistance