6 Resistors in series, why ?!!!

Discussion in 'General Electronics Chat' started by AymanKhuzundar, Jul 25, 2019.

  1. AymanKhuzundar

    Thread Starter Member

    Apr 2, 2017
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    Hi everyone,
    I'm studying a product , when I opened it I found a set of 6 SMD resistors connected in series and want to know why to add 6 instead of 1 or 2.
    the circuit is a delay-on timer, it uses 220VAC mains voltage, the circuit uses a capacitive divider to decrease the voltage to 20V, the circuit contains a relay; so to avoid the increasing of the voltage they put 6 resistors as a load when the relay is off so the same current flow and the same voltage will still.
    when dividing the 20V by (6*270ohm) resistors we get 3.33V across each resistor and the dissipation in each resistor is about 0.04W, but here they uses 0.125W SMD resistors.
    1- Sorry because I haven't the full schematic of the circuit.
    2- why do they use 6 resistors instead of 1 or 2 equivalent ? (I know when dealing with high voltages they prefer to connect more than one resistor to avoid humidity of any thing that may destroy the circuit, but why 6 !!!)
    3- why the resistors size is 3 times the required size? (SMD resistor with 3.2mm length, 1.6mm width).

    Thanks.
     
  2. MrAl

    AAC Fanatic!

    Jun 17, 2014
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    Could be for less heating.
     
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  3. AymanKhuzundar

    Thread Starter Member

    Apr 2, 2017
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    why they don't use higher wattage resistor ? I guess it would be cheaper than using 6 !
     
  4. iimagine

    Senior Member

    Dec 20, 2010
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    They have an abundance of them in stock and want to get rid of?, or got a great discount maybe
     
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  5. Danko

    Active Member

    Nov 22, 2017
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    When relay is off, energy, stored in coil as magnetic field, produced at coil leads high voltage pulse.
    Energy of this pulse dissipates on resistors and heats them.
    For lovering temperature of resistors, energy spreads between 6 resistors.
    For same purpose resistors have bigger size (more area of dissipating surface - lower temperature).
     
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  6. AymanKhuzundar

    Thread Starter Member

    Apr 2, 2017
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    :D:D:D:D:D don't think so, its a great company with a great stock
     
  7. AymanKhuzundar

    Thread Starter Member

    Apr 2, 2017
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    the relay has its own fast-switching diode connected to its coil, I don't think so
     
  8. MrAl

    AAC Fanatic!

    Jun 17, 2014
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    Hi,

    Well sometimes they just dont want to have to stock more parts than they have to. It's not a matter of being a big company or not.
    Stocking one kind of part to be used for multiple purposes is common. We used to do that too when I worked in the power control industry.
    When you think of all the parts that goes into products if you have to keep adding new parts eventually it gets to be a hassle. Keeping the parts in the stock room to a reasonable number helps with that. Also reduces paperwork.

    There are also other reasons of course. One is that the smaller part is cheaper and ordering multiple quantities brings the price of each one down quite a bit. Go on a site that sells parts and see how much the price decreases per part as the quantity goes up. Some times it is very significant. If you order 1000 of one part vs 10 of another part, you get a huge discount.

    But why 6 instead of just 3. That could be because they want to keep the resistance from changing as much as 3 would cause as they heat up.
    It could also be that the resistors act as jumpers as well and reduces the number of layers the board needs without having to actually install jumpers or 0 ohm resistors.
    It could also be a design error. Maybe they overestimated the power requirements.

    So there could be several reasons why we'd have to know what they were thinking.
     
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  9. AymanKhuzundar

    Thread Starter Member

    Apr 2, 2017
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    ahaaaa, that was very informative ! thanks alot
     
  10. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    Usually smd machines are limited in component pick and place spaces, so they choose the minimum amount of components to do the job.

    So if they are already using a 270 ohm resistor for something else, its easier to use it again in Multiples.
     
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  11. Janis59

    Active Member

    Aug 21, 2017
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    Two reasons - first, the maximum voltage on smd may happen 150V or slightly more depending on size. So, if 220V have a V(p-p) =311V, there already must stay minimum 3 resistors.
    Other reason is power, if say 22 kOhm dissipates about 2...3 W then smd mainly have 0,0625W or 0,125W. If there were small voltage, one may have a choice betw parallel and serial connection, but in this case there are no choice, and seems that 6 pieces are satisfacting as voltage as power demands simultaneusly.
     
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  12. BR-549

    AAC Fanatic!

    Sep 22, 2013
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    Because of area. Larger area can dissipate at a lower temp. The total heat dissipated has a much larger overall environmental area. It's more efficient at a lower temperature difference. Probably cheaper too.
     
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  13. Wolframore

    Well-Known Member

    Jan 21, 2019
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    It's an evil plot by the resistor company to sell more resistors... resistance is futile!
     
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  14. Alec_t

    Expert

    Sep 17, 2013
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    Perhaps the multiple resistors enable tolerances to be 'averaged out', if a very specific resistance is required, rather than employ a more expensive close tolerance resistor.
     
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  15. crutschow

    Expert

    Mar 14, 2008
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    That should work well if the resistors have a normal (Gaussian) distribution of values around the nominal.
     
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  16. WBahn

    Moderator

    Mar 31, 2012
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    Except that six resistors pulled from a tube in a pick and place machine are unlikely to be randomly distributed about the mean. Whatever is causing the first resistor to be off is likely causing all of the nearby resistors to be off by a similar amount.

    Plus, the application wouldn't seem to warrant the effort -- they are just trying to keep the load fairly constant so that the capacitive divider operates in the same region. There's going to be enough variation in the coil resistance of the relay to make the effort of trying to average out the variations in the resistors pointless.
     
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  17. Alec_t

    Expert

    Sep 17, 2013
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    True.
     
  18. MrAl

    AAC Fanatic!

    Jun 17, 2014
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    Anybody feel like measuring the resistance of a handfull of resistors?

    If the tolerance was +/- 1% over 0 to 100C and the temperature was 20C, then they might aim to keep the 20C resistance lower because thats 80C from max and only 20C from min. With 0C being -1% and 100C being +1%, that would make the ideal resistance at 20C about 0.6% low.
    Course if it runs at 50C it would be very close :)
     
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  19. Tonyr1084

    Distinguished Member

    Sep 24, 2015
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    Honestly I don't know the answer to why they used six resistors. My thought is that it could be they wanted a very specific resistance. Since resistors typically come in preset values, getting a resistance of a very specific value may require some series resistors, or series / parallel resistors or even just parallel resistors.

    I will say this: I'd like to see a picture of the board. Better yet, a schematic. There could be inner layer connections that may be using the resistors as a divider network. But for the moment it's all guesswork without further information.

    Six resistors, each at 270Ω equates to 1.62KΩ. That's not one of the common values available. There may be several ways to achieve that value. Since the total voltage across all six resistors is 20 volts, 3.1/3 volts drops at each point. So you would have 3.3 volts, 6.6 volts, 9.9 volts, 13.2 volts 16.5 volts and 19.8 volts. Given, my numbers are based on 3.3 volts per divider, not the 3.333 • • • volts, which when added together they equal 20 volts.
     
    Last edited: Jul 25, 2019
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  20. atferrari

    AAC Fanatic!

    Jan 6, 2004
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    You could find such a practice in circuits where they apply mains directly to the resistors.

    If I recall right, there is a very old application note from Microchip where they use that modality. I recall another circuit (Italian origin) using a string of resistors where mains is applied directly to them.
     
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