+ Post New Thread
Page 1 of 2 1 2 LastLast
Results 1 to 20 of 21
  1. #1
    Member level 2
    Points: 321, Level: 3

    Join Date
    Feb 2019
    Posts
    42
    Helped
    0 / 0
    Points
    321
    Level
    3

    Wiring of DC fans in Series

    Hi I have an enclosure with a 30vdc power supply inside.
    I would like to add some fans to keep things cooler inside however its much simpler to buy a 15vdc fan than a 30vdc fan.

    So I figured if I connect 2 in series I would split the voltage and bob's-your-uncle as they say however a friend of mind pointed out that it might not be good since a fan is inductive not resistive...
    How does (if) this affect the fans in series?

    Thanks :)

    •   AltAdvertisement

        
       

  2. #2
    Super Moderator
    Points: 81,772, Level: 69
    Achievements:
    7 years registered
    Awards:
    2nd Helpful Member
    betwixt's Avatar
    Join Date
    Jul 2009
    Location
    Aberdyfi, West Wales, UK
    Posts
    13,411
    Helped
    4472 / 4472
    Points
    81,772
    Level
    69

    Re: Wiring of DC fans in Series

    Generally not a good idea but it depends on the actual fan design. Most low voltage DC fans do not draw a precise current and it quite likely goes up and down as the fan rotates. As a consequence, if you wire them in series there is a high likely hood that the voltage across each fan will vary considerably and will probably go out of specification. Also consider what would happen if one fan stalled or was jammed, it would try to take more current and the voltage across it would drop, that means more voltage across the remaining fan.

    Brian.
    PLEASE - no friends requests or private emails, I simply don't have time to reply to them all.
    It's better to share your questions and answers on Edaboard so we can all benefit from each others experiences.



  3. #3
    Member level 2
    Points: 321, Level: 3

    Join Date
    Feb 2019
    Posts
    42
    Helped
    0 / 0
    Points
    321
    Level
    3

    Re: Wiring of DC fans in Series

    Nothing is simple :D 7815 perhaps might solve the issue!



  4. #4
    Super Moderator
    Points: 81,772, Level: 69
    Achievements:
    7 years registered
    Awards:
    2nd Helpful Member
    betwixt's Avatar
    Join Date
    Jul 2009
    Location
    Aberdyfi, West Wales, UK
    Posts
    13,411
    Helped
    4472 / 4472
    Points
    81,772
    Level
    69

    Re: Wiring of DC fans in Series

    Yes, you can use two 7815 regulators to drop the 30V down to two 15V supplies but the drawback is they will almost certainly run very hot. They have to dissipate (voltage dropped * fan current) Watts so check what current the fans draw and do the calculation. If the result is more than about 0.25W you will need heat sinks on the regulators.

    A 'dirty' way to do it is to use two 15V Zener diodes, one across each fan then connect them in series. If the fans draw identical currents the diodes will do nothing but they will help to balance the load by ensuring neither fan can have more the 15V across it. It isn't an ideal solution but it goes some way to helping.

    Brian.
    PLEASE - no friends requests or private emails, I simply don't have time to reply to them all.
    It's better to share your questions and answers on Edaboard so we can all benefit from each others experiences.


    1 members found this post helpful.

  5. #5
    Advanced Member level 5
    Points: 15,049, Level: 29
    schmitt trigger's Avatar
    Join Date
    Apr 2013
    Posts
    2,501
    Helped
    808 / 808
    Points
    15,049
    Level
    29

    Re: Wiring of DC fans in Series

    The current draw of small DC fans do indeed change with rotation. Additionally, you cannot expect the two devices to draw the exact same current, even if they are the same brand and model.

    Bryan's zener recommendation is a cheap and simple way to prevent overvoltage conditions on the fans.
    For safety margin, assume that worst case the zeners will have to carry at least 1/2 the fan current.

    For instance, if the fan's are rated for 150 mA, assume that worst case the zener will cary 75 mA. Multiply it by 15 and and it will give you 1.125 watt. Use a 2 watt zener. Better safe than sorry.
    My batteries are recharged by "Helpful Post" ratings.
    If you feel that I've helped you, please indicate it as a Helpful Post


    1 members found this post helpful.

  6. #6
    Super Moderator
    Points: 78,524, Level: 68
    Achievements:
    7 years registered
    Awards:
    Most Frequent Poster 3rd Helpful Member

    Join Date
    Apr 2014
    Posts
    15,939
    Helped
    3613 / 3613
    Points
    78,524
    Level
    68

    Re: Wiring of DC fans in Series

    Hi,

    A power voltage divider could be made with two equal resistors, an NPN and a PNP...
    Not very precise, but good enough for fans

    Klaus
    Please donīt contact me via PM, because there is no time to respond to them. No friend requests. Thank you.


    1 members found this post helpful.

    •   AltAdvertisement

        
       

  7. #7
    Advanced Member level 5
    Points: 11,635, Level: 25

    Join Date
    Aug 2015
    Location
    Melbourne
    Posts
    1,896
    Helped
    698 / 698
    Points
    11,635
    Level
    25

    Re: Wiring of DC fans in Series

    a 5W 16V zener across each fan will be the way to go, else a buck converter to 15V to run both of them in //.


    1 members found this post helpful.

    •   AltAdvertisement

        
       

  8. #8
    Member level 2
    Points: 321, Level: 3

    Join Date
    Feb 2019
    Posts
    42
    Helped
    0 / 0
    Points
    321
    Level
    3

    Re: Wiring of DC fans in Series

    I really like all the solutions with zeners and potential divider. I was thinking of using 15v fans because its half my supply however now its a different story. I have a bunch of fans I could use a voltage divider

    R1 R2 V
    10K (0.5w) 39K (2w) 23.88V @ 60mA
    4k7 (2w) 3k3 (1w) 12.38V @ 70mA
    10K (5w) 2K2 (2w) 5V @ 180mA
    Last edited by Xenobius; 16th October 2019 at 06:28.



  9. #9
    Advanced Member level 5
    Points: 11,635, Level: 25

    Join Date
    Aug 2015
    Location
    Melbourne
    Posts
    1,896
    Helped
    698 / 698
    Points
    11,635
    Level
    25

    Re: Wiring of DC fans in Series

    R's are fine - but extra power draw / loss and no guarantee of sharing at power up where fans can and do draw more than 5x the run current - and its pretty pulsed too...!

    - - - Updated - - -

    don't forget the fans will pull current and distort your calculations - esp at start ...



  10. #10
    Super Moderator
    Points: 78,524, Level: 68
    Achievements:
    7 years registered
    Awards:
    Most Frequent Poster 3rd Helpful Member

    Join Date
    Apr 2014
    Posts
    15,939
    Helped
    3613 / 3613
    Points
    78,524
    Level
    68

    Re: Wiring of DC fans in Series

    Hi

    Try this:
    * NPN, PNP, 2x R = 4k7

    Wire it this way:
    * 30V --> NPN_C
    * NPN_E to PNP_E = 15V output
    * PNP_C to GND
    * resistors in series, one end to 30V, the other end to GND.
    * resistors_center to NPN_B and PNP_B

    Klaus
    Please donīt contact me via PM, because there is no time to respond to them. No friend requests. Thank you.



  11. #11
    Member level 2
    Points: 321, Level: 3

    Join Date
    Feb 2019
    Posts
    42
    Helped
    0 / 0
    Points
    321
    Level
    3

    Re: Wiring of DC fans in Series

    Something like so? So the resistors are just a 15v potential divider that are turning on the transistors but what limits it to 15v?

    Click image for larger version. 

Name:	Fan.PNG 
Views:	5 
Size:	53.0 KB 
ID:	156070



  12. #12
    Super Moderator
    Points: 81,772, Level: 69
    Achievements:
    7 years registered
    Awards:
    2nd Helpful Member
    betwixt's Avatar
    Join Date
    Jul 2009
    Location
    Aberdyfi, West Wales, UK
    Posts
    13,411
    Helped
    4472 / 4472
    Points
    81,772
    Level
    69

    Re: Wiring of DC fans in Series

    You are missing the fan connections and 100R is far too low in value, the resistors alone would dissipate 2.25W each!

    What Klaus is suggesting is basically two emitter follower circuits, they have current gain but not voltage gain so whatever voltage you put on their bases from the potential divider will appear on their emitter pins and hence the fans. For it to work you still need the two near identical 15V fans in series as well. This and the Zener diode idea will work but they both have the same potential problem of one fan stalling and causing increased current through the other half of the circuit. To be safe, consider the situation where one fan goes completely short circuit and look what the power dissipation does in the other circuit across the other fan.

    I'm not sure what the resistor values in post #8 are for, they don't make any sense to me. If you are thinking of simply using a series resistor to drop the fan voltage you should be careful to note the start-up current from stopped condition is usually far higher than the running current so the fan may start very slowly or not at all.

    Brian.
    PLEASE - no friends requests or private emails, I simply don't have time to reply to them all.
    It's better to share your questions and answers on Edaboard so we can all benefit from each others experiences.



  13. #13
    Advanced Member level 5
    Points: 11,635, Level: 25

    Join Date
    Aug 2015
    Location
    Melbourne
    Posts
    1,896
    Helped
    698 / 698
    Points
    11,635
    Level
    25

    Re: Wiring of DC fans in Series

    Q4 is not needed - Q3 will dissipate, Ifan * 15V so be careful there ...



  14. #14
    Super Moderator
    Points: 81,772, Level: 69
    Achievements:
    7 years registered
    Awards:
    2nd Helpful Member
    betwixt's Avatar
    Join Date
    Jul 2009
    Location
    Aberdyfi, West Wales, UK
    Posts
    13,411
    Helped
    4472 / 4472
    Points
    81,772
    Level
    69

    Re: Wiring of DC fans in Series

    Q4 is needed if there are two fans in series. The circuit will try to maintain the center at ~15V but the dissipation would be a problem, especially under the circumstances I described in post #12.

    Brian.
    PLEASE - no friends requests or private emails, I simply don't have time to reply to them all.
    It's better to share your questions and answers on Edaboard so we can all benefit from each others experiences.



  15. #15
    Advanced Member level 5
    Points: 17,555, Level: 32
    Achievements:
    7 years registered

    Join Date
    Nov 2012
    Posts
    3,206
    Helped
    795 / 795
    Points
    17,555
    Level
    32

    Re: Wiring of DC fans in Series

    ... and bob's-your-uncle as they say however a friend of mind pointed out that it might not be good since a fan is inductive not resistive...
    The small DC fans are more resistive than inductive. I do not see any problem with your idea.

    But look carefully: some of the modern cooling fans used in small cabinets are brushless DC and they have an electronic driver built in. So you may be wise to have a 10K2W resistor connected across each so that voltage is shared more or less equally.

    The two zener approach is also better (see post #4): they will also act as spike busters. But if you are using two fans, it may be useful to look at the possible air flow pattern within the cabinet.



  16. #16
    Member level 2
    Points: 321, Level: 3

    Join Date
    Feb 2019
    Posts
    42
    Helped
    0 / 0
    Points
    321
    Level
    3

    Re: Wiring of DC fans in Series

    Well since you mentioned that, I might actually need more then 2 fans, but I never mentioned this because I thought to keep them in pairs. A solution for 2 fans will work for 4 or 6 (at most).
    As for the fans, you're right I find most of them are brushless. I might need something like this #1 blower as I dont have lots of head room

    and perhaps some of #2 these on the sides too. I gave no respect to the voltage in my links, I just wanted to show that they are brushless (and cheap lol)



  17. #17
    Super Moderator
    Points: 78,524, Level: 68
    Achievements:
    7 years registered
    Awards:
    Most Frequent Poster 3rd Helpful Member

    Join Date
    Apr 2014
    Posts
    15,939
    Helped
    3613 / 3613
    Points
    78,524
    Level
    68

    Re: Wiring of DC fans in Series

    Hi,

    Something like so? So the resistors are just a 15v potential divider that are turning on the transistors but what limits it to 15v?
    The two resistors are no "15V" generators. They just divide the input voltage by 2.

    --> If your supply voltage is 10V then the voltage at the divider will be 5V.
    Depending on unsymmetry current... the ouput voltage should be kept within VSupply/2 +/-1V

    Now with the BJTs:
    If an unsymmetric load tries to pull the "emitters" to higher voltage than VSupply/2 then the lower BJT gets conductive and thus compensates against the unsymmetry.
    If an unsymmetric load tries to pull the "emitters" to lower voltage than VSupply/2 then the upper BJT gets conductive and thus compensates against the unsymmetry.

    It maintains about VSupply/2 at the node ... almost independent of load unsymmetry.
    For sure this means the difference current needs to be supplied by one BJT ... causing heat. P_tot = | I_upper_fan - I_lower_fan | * VSupply / 2
    But it will be one of the most power efficient linear circuits. --> no unsymmetric current = no heat in the BJTs.

    ...and it will be no problem when supply voltage fluctuates. It will work in a wide range of VSupply.

    Klaus
    Please donīt contact me via PM, because there is no time to respond to them. No friend requests. Thank you.



  18. #18
    Advanced Member level 5
    Points: 11,635, Level: 25

    Join Date
    Aug 2015
    Location
    Melbourne
    Posts
    1,896
    Helped
    698 / 698
    Points
    11,635
    Level
    25

    Re: Wiring of DC fans in Series

    a cheap buck converter to give to 12V for the fans is looking better and better ...



    •   AltAdvertisement

        
       

  19. #19
    Super Moderator
    Points: 78,524, Level: 68
    Achievements:
    7 years registered
    Awards:
    Most Frequent Poster 3rd Helpful Member

    Join Date
    Apr 2014
    Posts
    15,939
    Helped
    3613 / 3613
    Points
    78,524
    Level
    68

    Re: Wiring of DC fans in Series

    Hi,

    I agree.

    But the OP asks about series connection.

    Klaus
    Please donīt contact me via PM, because there is no time to respond to them. No friend requests. Thank you.



  20. #20
    Member level 2
    Points: 321, Level: 3

    Join Date
    Feb 2019
    Posts
    42
    Helped
    0 / 0
    Points
    321
    Level
    3

    Re: Wiring of DC fans in Series

    True that I asked for a series connection however I might change my design to make it simpler and just use a buck converter. I am grateful for the responses guys and the dedication. This is part of another post that I asked about wiring up 140 leds and will eventually show you what I am doing when I get to a better stage. I believe that for now I have everything for the fans. Thanks a million



--[[ ]]--