resistors in parallel ?

Collapse
X
 
  • Time
  • Show
Clear All
new posts
  • littlejohn
    Member
    • Mar 2022
    • 11
    • Canada

    #1

    resistors in parallel ?

    I need a 1.5 k 2watt resistor , can I use 2 1 watt resistors in parallel for testing purposes while waiting for a 2 watt to arrive?
  • sam_sam_sam
    Badcaps Legend
    • Jul 2011
    • 6027
    • USA

    #2
    Re: resistors in parallel ?

    You can use two 3k ohm resistor @ 1 watt in parallel to get 1.5k @ 2 watt that you are looking for

    Comment

    • eccerr0r
      Solder Sloth
      • Nov 2012
      • 8680
      • USA

      #3
      Re: resistors in parallel ?

      And two 750Ω 1W resistors in series would also result in a 1.5KΩ 2W resistance.

      Comment

      • sam_sam_sam
        Badcaps Legend
        • Jul 2011
        • 6027
        • USA

        #4
        Re: resistors in parallel ?

        Originally posted by eccerr0r
        And two 750Ω 1W resistors in series would also result in a 1.5KΩ 2W resistance.
        Are you sure about this because I thought that if one was 1 watt total because it would just add more resistance but I could be wrong

        Comment

        • eccerr0r
          Solder Sloth
          • Nov 2012
          • 8680
          • USA

          #5
          Re: resistors in parallel ?

          Do the math.

          BTW, should always do the math. You can even do funny things with nonmatching resistors but yeah it gets complicated when they don't match as the power dissipated will be different if resistances are different.
          Last edited by eccerr0r; 06-14-2022, 07:21 AM.

          Comment

          • redwire
            Badcaps Legend
            • Dec 2010
            • 3900
            • Canada

            #6
            Re: resistors in parallel ?

            The heat to be dissipated would be shared over two parts, each one dissipates a ratio of the value a single part would. Regardless of whether the two are in series or parallel.
            50-50 split so 1W each using two 750Ω series or 3kΩ parts parallel.
            If you used a 2.2kΩ and 4.7kΩ in parallel, each see about 1.36W and 0.64W - the same ratio as their resistances.

            Comment

            • eccerr0r
              Solder Sloth
              • Nov 2012
              • 8680
              • USA

              #7
              Re: resistors in parallel ?

              Then you'll need a 1.5W 2k2 resistor which rounds up to 2W, and why not buy a 3k 2W resistor to begin with
              With dissimilar resistors, probably only for last resort or if you have other values to burn... I was trying to do this with a 10R 2W and a 40R 10W in series for a 50R load, now need to calculate the system max sustained wattage... Then realize that inductance in the pair was too high for VHF frequencies...

              Comment

              • EasyGoing1
                Shock Therapist
                • Sep 2016
                • 977
                • USA

                #8
                Re: resistors in parallel ?

                Originally posted by eccerr0r
                Do the math.

                BTW, should always do the math. You can even do funny things with nonmatching resistors but yeah it gets complicated when they don't match as the power dissipated will be different if resistances are different.
                Here's the math if anyone is interested or ever will be interested ...



                What I find particularly interesting about this thought exercise, is that like sam, my knee-jerk reaction to the statement was to pause and question it. And I think that is because we are used to thinking about net values - where resistors are concerned anyways - differently depending on whether the resistors are in series or parallel ... so I think naturally the brain wants to continue that pattern of thought even if the value in question is wattage dissipation. Which as it turns out, does not care if the resistors are in series or parallel, wattage dissipation adds up linearly.
                Last edited by EasyGoing1; 07-13-2022, 05:26 AM.
                sigpic

                Comment

                • eccerr0r
                  Solder Sloth
                  • Nov 2012
                  • 8680
                  • USA

                  #9
                  Re: resistors in parallel ?

                  I think the main reason to answer the way sam did is he was thinking about batteries and considering amps and not watts.

                  two 12V 1Ah batteries in series will give you 24V but still only 1Ah.

                  Now the rhetorical question: if you change the question to two 12V 1Wh batteries in series, how many watt hours do you get out of the system?

                  Comment

                  • sam_sam_sam
                    Badcaps Legend
                    • Jul 2011
                    • 6027
                    • USA

                    #10
                    Re: resistors in parallel ?

                    Originally posted by eccerr0r
                    I think the main reason to answer the way sam did is he was thinking about batteries and considering amps and not watts.

                    two 12V 1Ah batteries in series will give you 24V but still only 1Ah
                    This is what I was thinking about but after I saw the math that EasyGoing did it cleared up the confusion about this thanks to both of you

                    Comment

                    • sam_sam_sam
                      Badcaps Legend
                      • Jul 2011
                      • 6027
                      • USA

                      #11
                      Re: resistors in parallel ?

                      Originally posted by eccerr0r
                      Now the rhetorical question: if you change the question to two 12V 1Wh batteries in series, how many watt hours do you get out of the system?
                      I will run this test and give you a definite answer to this question over the weekend by doing an actual test I think I know the answer to this question but I want to know for sure that I am right because I have a way to do this test

                      Comment

                      • redwire
                        Badcaps Legend
                        • Dec 2010
                        • 3900
                        • Canada

                        #12
                        Re: resistors in parallel ?

                        It's the difference between power and energy. Resistors are only concerned with power they dissipate, regardless of the time, how long they've been doing it.
                        Batteries are limited - if each battery has X watt-hours of energy, then two batteries gives you... well 2X. The energy available just adds up.

                        Comment

                        • petehall347
                          Badcaps Legend
                          • Jan 2015
                          • 4425
                          • United Kingdom

                          #13
                          Re: resistors in parallel ?

                          Originally posted by redwire
                          Batteries are limited - if each battery has X watt-hours of energy, then two batteries gives you... well 2X. The energy available just adds up.
                          only if in parallel . otherwise its the same as the lowest value .
                          resistors in series are the wattage of the lowest wattage one .
                          example .. 2 watts and 1 watts in series = 1 watt .
                          Last edited by petehall347; 07-14-2022, 03:11 PM.

                          Comment

                          • redwire
                            Badcaps Legend
                            • Dec 2010
                            • 3900
                            • Canada

                            #14
                            Re: resistors in parallel ?

                            In parallel you get double the Ah, in series you get double the voltage. So two batteries will always give the energy of... well... two batteries. Not one.
                            Where is the confusion on this I wonder.

                            Comment

                            • eccerr0r
                              Solder Sloth
                              • Nov 2012
                              • 8680
                              • USA

                              #15
                              Re: resistors in parallel ?

                              in this case power and energy don't really matter, just take a fixed amount of time and the math works out.

                              This confusion is because some people equate amps (or amp hours) with, or as a proxy for watts (or watt hours), and then extrapolate to resistors, leading to improper conclusions.

                              Comment

                              • EasyGoing1
                                Shock Therapist
                                • Sep 2016
                                • 977
                                • USA

                                #16
                                Re: resistors in parallel ?

                                Originally posted by petehall347
                                only if in parallel . otherwise its the same as the lowest value .
                                resistors in series are the wattage of the lowest wattage one .
                                example .. 2 watts and 1 watts in series = 1 watt .
                                I can't help but notice that this is similar to capacitors ... parallel ADDS values and series become some ratio of the total ... though this makes sense since a battery is just a pre-charged capacitor I suppose ... mathematically speaking ... or at least in some loose way they are.
                                sigpic

                                Comment

                                • EasyGoing1
                                  Shock Therapist
                                  • Sep 2016
                                  • 977
                                  • USA

                                  #17
                                  Re: resistors in parallel ?

                                  Originally posted by eccerr0r
                                  in this case power and energy don't really matter, just take a fixed amount of time and the math works out.

                                  This confusion is because some people equate amps (or amp hours) with, or as a proxy for watts (or watt hours), and then extrapolate to resistors, leading to improper conclusions.
                                  I struggle with these MAh ratings on these batteries ... so for example, a battery is rated at being capable of sustaining 3,000 MAh ... which says to me that it can continuously provide 3 amps for an hour ... but that never seems to be the observed reality ... in any specific case from what I remember back when I was trying to make sense of all that stuff.

                                  I wanted to make an arduino based LiIon charger at one point and I specifically remember struggling with the actual HOW in calculating how many MAh were going into the battery as it charged.
                                  sigpic

                                  Comment

                                  • brownharris561
                                    New Member
                                    • Sep 2022
                                    • 5
                                    • United States

                                    #18
                                    Re: resistors in parallel ?

                                    Originally posted by EasyGoing1
                                    Here's the math if anyone is interested or ever will be interested ...



                                    What I find particularly interesting about this thought exercise, is that like sam, my knee-jerk reaction to the statement was to pause and question it. And I think that is because we are used to thinking about net values - where resistors are concerned anyways - differently depending on whether the resistors are in series or parallel ... so I think naturally the brain wants to continue that pattern of thought even if the value in question is wattage dissipation. Which as it turns out, does not care if the resistors are in series or parallel, wattage dissipation adds up linearly.
                                    Nice Explained!

                                    Comment

                                    • eccerr0r
                                      Solder Sloth
                                      • Nov 2012
                                      • 8680
                                      • USA

                                      #19
                                      Re: resistors in parallel ?

                                      Originally posted by petehall347
                                      only if in parallel . otherwise its the same as the lowest value .
                                      resistors in series are the wattage of the lowest wattage one .
                                      example .. 2 watts and 1 watts in series = 1 watt .
                                      Originally posted by EasyGoing1
                                      I can't help but notice that this is similar to capacitors ... parallel ADDS values and series become some ratio of the total ... though this makes sense since a battery is just a pre-charged capacitor I suppose ... mathematically speaking ... or at least in some loose way they are.
                                      Not sure why or perhaps I forgot to, but to make it crystal clear, these conclusions are using the watt and amp confusion alluded to earlier, leading to unexpected and improper solutions.

                                      While as rule of thumb using "lowest wattage" = "system wattage" works to be safe, it's more nuanced and potentially significantly pessimistic. You have to calculate dissipation individually to know system dissipation. For that example of the 50 ohm dummy load that I created with those 40 ohm 10W and 10 ohm 2W resistors. What is the system power limit? In this case it would actually be 10 watts if my math is right.

                                      Now for batteries this is a problem because we have to deal with dead batteries. Dead batteries cause problems when in series, so yes, if you have a 120Wh battery in series with a 12Wh battery, you end up with some unknown watt hour system capacity that can't be computed without also knowing each battery's voltage and their characteristics, acceptable or not, when dead. If the battery voltages were both 12V then system power capability would be at least 24Wh; more if you can deal with consequences.

                                      So yes there is a kind of geometric mean when dealing with series devices. Calculating it requires some care as well as understanding the consequences of such. This also includes the series capacitor issue.

                                      Originally posted by EasyGoing1
                                      I struggle with these MAh ratings on these batteries ... so for example, a battery is rated at being capable of sustaining 3,000 MAh ... which says to me that it can continuously provide 3 amps for an hour ... but that never seems to be the observed reality ... in any specific case from what I remember back when I was trying to make sense of all that stuff.

                                      I wanted to make an arduino based LiIon charger at one point and I specifically remember struggling with the actual HOW in calculating how many MAh were going into the battery as it charged.
                                      Most of the times the mAh ratings are due to "marketing exaggeration" if not for internal resistance issues. What you found is what you found (and correct for your situation), and the number printed is simply ... wrong.
                                      Last edited by eccerr0r; 09-08-2022, 07:12 AM.

                                      Comment

                                      Related Topics

                                      Collapse

                                      • chth96
                                        Is it okay to replace 1/2 watt resistor with 1 watt resistor in AC/DC adapter?
                                        by chth96
                                        I have purchased an 100-240v AC/5v 2a DC adaptor from aliexpress,and a month ago It smelled burning when I plugin it powerstrip.
                                        Since then, This AC/DC adaptor have been out of order.
                                        But A few days ago,I replace 0.5 watt resistor with 1 watt resistor(1ohm) in this adapter.
                                        After replacing resistor,It succeed to recharge all of android phone.And When I measure voltage on the two pins of USB port,It shows 5.5v
                                        Is it okay to replace 1/2 watt resistor with 1 watt resistor in AC/DC adapter?
                                        And Is there any possibility that it will suddenly explode as well as to be out...
                                        12-03-2023, 04:40 AM
                                      • momaka
                                        KingWin ABT-650MM 650 Watt ATX PSU [PCB: SW8835REV:A]
                                        by momaka
                                        For today's thread, I have a KingWin ABT-650MM ATX PSU. Official web page with the PSU specs is still up and can be found here:
                                        http://www.rmac.kingwin.com/products.../abt_650MM.asp

                                        I just can't help it when I see a broken / for parts PSU on eBay that's listed for cheap and looks at least somewhat interesting. Well, OK, I won't lie – on the outside, this PSU looks anything but interesting. It's a plain gray box with colorful “spaghetti cables”.




                                        What mostly drew me into buying it is its 2x PCI-E power connectors...
                                        03-29-2022, 11:34 PM
                                      • Crystaleyes
                                        Calculating resistor values in parallel with Caps
                                        by Crystaleyes
                                        Hi all.

                                        I recently picked up a dead car amplifier from the flea market, with the intention of learning some more in the area of audio repair.

                                        It has a stereo output with an added mono woofer output. The stereo channels are on one board, whilst the woofer section is on a separate board. There are also three large (ish) transformers.

                                        Apart from the fact that it was seriously dirty inside, it seems to be essentially complete with the only clearly damaged component being a toasted 1/4 watt resistor on the woofer board and it is this which I am wondering...
                                        05-21-2023, 07:35 PM
                                      • mikey5791
                                        Need help to replace burned resistor in 350w atx psu
                                        by mikey5791
                                        Hi to all forum members,

                                        Got this faulty 350 watt atx psu dirt cheap from a local online seller as i am keen to use it as a learning tool to repair faulty power supply.
                                        The faulty part is quite obvious from the burn area near the 16pin KA7500 BD pwm. The fuse is still intact with good continuity.

                                        Upon power on ac, there is 5vdc standby power on cathode of diode D14 and about 10vdc on cathode of diode D13.(This should be the aux power to generate the 12v and 5v rail.)
                                        Initially i replace the two burned resistors near my marked red arrow to 200 ohm resistor...
                                        12-26-2024, 04:55 AM
                                      • TinkerTron
                                        Schematic vs. Reality: Where Are the Resistors on My Sharp Clock Radio? Model FY-70CH
                                        by TinkerTron
                                        Hey everyone,

                                        I'm currently working on a mid-70s Sharp Electronic Digital Clock Radio, Model FY-70CH. I found the service manual online, printed it out, and have been studying it for a while. My main reasons for tackling this are: (1) I want to learn electronics and figured I'd start with something I had on hand, and (2) I want to fix its issues. Right now, the radio sounds awful, has about 5V DC at the speaker terminals, and the alarm time setting doesn't work. But I'm not asking how to fix these problems.

                                        Instead, here's what's been bugging me so far:
                                        The schematic...
                                        03-06-2025, 02:03 PM
                                      • Loading...
                                      • No more items.
                                      Working...