Resistor in series to incrase power rating? legit? - Page 1
Forum section: electronics / projects

author: sean87
author_rank: Regular Contributor
timestamp: April 29, 2013, 08:09:44 pm
content: Hi all!I am repairing an amplifier, so far I have found a 1k, 1/2 watt resistor that my multimeter reads its value like 600 Ohm only. So I guess it has drifted a lot.I do have some spare 1/4 watt 500 Ohm resistors. Can I use 2 of them in series so that 1/2 watt rating gets satisfied? Or I should say, does putting resistor in series helps achieving higher rating at all?

author: kfitch42
author_rank: Frequent Contributor
timestamp: April 29, 2013, 08:22:00 pm
content: I believe that series does not increase power rating, parallel does. You could do 2 parallel strings of 4 resistors each. Or 3 strings of 6 ... might give a bit more headroom.Then again if you found 1 bad component you might find more... perhaps you should be putting an order into digikey (or RS, or whatever makes sense over there in Europe) for the right resistor and a bunch of caps... and anything else you find that is fishy.EDIT: Yikes! After reading the rest of the replies I realized I responded to this WAY toooo quick. I actually 'thought' it through "P=I^2 * R"... and said to myself: "in series the current is the same and the total resistance is the same ... ". Missed the boat there by a pretty wide margin

author: mariush
author_rank: Super Contributor
timestamp: April 29, 2013, 08:24:23 pm
content: You put resistors in parallel to increase the power dissipation capability , so you'd have to use 2 x 2k 1/4w to get a 1k resistor 1/2w

author: Excavatoree
author_rank: Frequent Contributor
timestamp: April 29, 2013, 08:38:58 pm
content: Two resistors in parallel or series (if identical) will share the power equally, regardless of whether they are in parallel or series.That is:1K ohm (1/2 watt) = 2 (1/4 watt) 500 ohm in series, or 2 2K ohm (1/4 watt) in parallel.In the parallel case, voltage is the same, but current is halved for each.In the series case, current is the same, but voltage is halved for each.It's a simple ohms law calculation.

author: Unknown
author_rank: Unknown
timestamp: April 29, 2013, 08:46:25 pm
content: Putting the resistors in series to share load is a fine solution. Carbon film resistors have a negative tempco, so voltage sharing should work fine in the series configuration.

author: ddavidebor
author_rank: Super Contributor
timestamp: April 29, 2013, 09:30:23 pm
content: You can put all the resistor in series or parallel that you want.You need to do math only if the value of the resistors are different.

author: sean87
author_rank: Regular Contributor
timestamp: April 29, 2013, 10:12:28 pm
content: Quote from: Excavatoree on April 29, 2013, 08:38:58 pmHmmm I got a feeling that the first resistor in series case, will be taxed more...I don't know why! maybe because its the first resistor and has the first voltage drop? hmmm I am confused.

author: ejeffrey
author_rank: Super Contributor
timestamp: April 29, 2013, 10:16:36 pm
content: Quote from: sean87 on April 29, 2013, 10:12:28 pmNope. They don't care which is first. The current flow through both is the same, so P=I^2*R.

author: Psi
author_rank: Super Contributor
timestamp: April 29, 2013, 10:20:37 pm
content: The power rating is about the heat it can handle.So both series and parallel work to increase the power rating since they both provide more area to dissipate heat.However, as said above, if you use resistors of different values then the voltage across each (series) or the current through each (parallel) is different and therefor so is the distribution of heat between them.eg, Here's an extreme case.. 1ohm and 1megIn series the 1M will dissipate practically all of the heatIn parallel the 1R will dissipate practically all of the heat.

author: Hardcorefs
author_rank: Regular Contributor
timestamp: April 30, 2013, 02:39:56 am
content: Quote from: sean87 on April 29, 2013, 10:12:28 pmUnlikely, they are not semiconductors.

author: IanB
author_rank: Super Contributor
timestamp: April 30, 2013, 02:56:12 am
content: Quote from: Psi on April 29, 2013, 10:20:37 pmI think that may depend on the resistor. For small resistors some of the heat dissipation may be assumed to be along the lead and into the circuit board. If two resistors are wired in series end to end, one of the conduction paths for heat removal may be missing. So in that case, one plus one may not equal two.

author: lowimpedance
author_rank: Super Contributor
timestamp: April 30, 2013, 03:08:27 am
content: Quote from: sean87 on April 29, 2013, 08:09:44 pmCurious, in my experience resistors drift high in value not 400 odd ohms low. Still in cct. then?. Lift one leg and measure again.

author: c4757p
author_rank: Super Contributor
timestamp: April 30, 2013, 04:23:19 am
content: Quote from: Hardcorefs on April 30, 2013, 02:39:56 amHuh? Even a semiconductor doesn't know if it's first! Or do we need to dig up Kirchhoff and have a word with him?

author: jebcom
author_rank: Regular Contributor
timestamp: April 30, 2013, 05:02:26 am
content: WOW!It MUST be true; I read it on the Internet!QuoteQuoteQuoteQuoteListen to sanity:QuoteQuoteQuote

author: IanB
author_rank: Super Contributor
timestamp: April 30, 2013, 06:05:06 am
content: Quote from: jebcom on April 30, 2013, 05:02:26 amNote that the power rating of a resistor is based on how much heat it can reasonably dissipate and still remain below its safe operating temperature. It is not at all to do with how much heat is generated inside the resistor due to current flow. Ohm's Law doesn't come into it at all.The expected rate of heat dissipation is an estimate based on typical installed conditions and includes heat conduction axially along the leads (or through the solder joints if it is an SMT device) and convection through local air currents.If you remove or reduce some of the expected heat removal paths the resistor will have to be de-rated.

author: codeboy2k
author_rank: Super Contributor
timestamp: April 30, 2013, 09:26:25 am
content: Quote from: c4757p on April 30, 2013, 04:23:19 amLOL..

author: BravoV
author_rank: Super Contributor
timestamp: April 30, 2013, 09:40:03 am
content: LOL ... if there is a vote for the most hilarious thread of the month in this forum, I'd vote for this one.

author: ddavidebor
author_rank: Super Contributor
timestamp: April 30, 2013, 09:43:54 am
content: i had stop myself... because i want to say: "what fuc**** hills of bullshit are saying?"

author: amyk
author_rank: Super Contributor
timestamp: April 30, 2013, 10:22:06 am
content: Quote from: sean87 on April 29, 2013, 10:12:28 pmClassic misunderstanding of voltage... yes both ends of the first resistor will be at higher potentialrelative to groundbut that's not what matters here.

author: sean87
author_rank: Regular Contributor
timestamp: April 30, 2013, 03:30:58 pm
content: haha thanks for your replies guys!