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Reducing Voltage to Reduce Current - is it a proportional reduction?

Discussion in 'General Electronics Discussion' started by JunkRoom, Dec 15, 2015.

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  1. JunkRoom


    Jul 11, 2015

    Okay, I've run another few searches on this and it seems that "7-volting" (described below) is a four pin molex trick and not something I can use for the application I have in mind.

    But I'm guessing using a resistor is a potential alternative so I guess that's the answer?

    Since resistors reduce current rather than voltage, I guess it renders the question moot, but I guess it should be easier. Although it does raise another few questions about resistors...

    ...for instance, can you use a resistor with a voltage controlled fan where the voltage might change?

    Reading about simple series resistors it seems they might not be suitable for use with a fan on a voltage controlled fan-header...

    Oh well, I guess I've got some reading to do :0/


    Another "out of left field" question...

    Not sure if this is a stupid/easy (or just completely baffling) question or not, but here it is...

    The question In short:

    If I cut the voltage available to a 12V 32A fan by 58.33% will it cut the current it draws by 58.33%? (i.e. does the reduction to the current it will draw scale proportionally to the reduction in voltage available to the fan?)

    And the long version of the same question:

    If I reduce the voltage available to a device, will it reduce the current which that device draws proportionally to the amount of the voltage reduction?

    i.e. If I reduce the voltage available to a device by 25% will it reduce the current it draws by 25% ?

    and likewise, if I reduce the voltage available to it by 50% will it reduce the current it draws by 50% ?

    and so on, so that if I reduce the voltage available to it by 75% will it reduce the current it draws by 75% ?

    Basically what I need to know is this: is the reduction proportional (rather than, for example, logarithmic, exponential, or potentially dependent on some other factor I'm not aware of) ?

    The reason I ask is this:

    I have a 12V 0.32A fan that I want to reduce to about 21A by dropping the voltage.

    I can drop the voltage available to the fan through a (relatively) common process called "7-volting" (i.e. altering the feed so that there are only seven volts available to the 12-volt 0.32A fan).

    If 7 volts is 58.33% of 12 volts, and 58.33% of 0.32A is 0.18A, then, if the reduction scales proportionally, it'll bring the current drawn by the fan to within the 21A limit I'm restricted to.

    So basically, is it correct for me to assume that if I cut the voltage available to the 12V 32A fan by 58.33% then it'll cut the current it draws by 58.33%?

    Basically I imagine it should be a proportional reduction but I know these things are often counter-intuitive and I just wanted to check.

    Thanks in advance to whoever might have the answer (and kudos for making it this far). I guess it's a long-shot but you never know. :0P

    Last edited: Dec 15, 2015
  2. dorke


    Jun 20, 2015
    Let's start with the simple question:
    Are you talking about a PC cooling fan?(32A or 0.32A).

    What you call "7-volting" is merely using the diffrence between the +12V and +5V power supply voltages found in a PC .
    Will it work?
    When you measure the voltage difference between the pins of the Molex connector you will get the expected 7V but ...that is not enough.
    For it to work the 5V power supply needs to be able to "sink current" and most probably it can't :rolleyes:,if this is indeed the case ,no soop for you little fan.


    As for the "reduction" of current being linear:
    If the load is "purely passive resistive" the answer is yes.
    In other case the general answer is no:
    1.A fan is a motor and thus not a linear device.
    2.Some active devices exhibit "negative resistance"...
    here is something for you to digest :confused:
    Last edited: Dec 15, 2015
    JunkRoom likes this.
  3. duke37


    Jan 9, 2011
    A fan normally has a torque proportional to the square of the speed so reducing the voltage by say 20% will reduce the speed by less than 20% since the fan turns easier, you can work the maths for a motor which has speed proportional to voltage but the speed will also depend on the torque.
    Some motors have speed regulation so will not behave as expected.

    Some devices need a minimum voltage to work at all. It all depends on the circuit.

    You can not drop the current of an 0.32A fan to 21A by dropping the voltage.:p

    Stuffing current into the output of a voltage regulator is asking for trouble.
    JunkRoom likes this.
  4. JunkRoom


    Jul 11, 2015
    Thanks :0)

    Also, yes sorry I screwed up the formatting, it should of read 0.32A. I'm not sure I can change it at this point :0/

    But that's really interesting. "7-volting" and "5-volting" is a common practice in the world of P.C. building/modifying. You can buy adapted sockets for the purpose on eBay (it's even described here: I've been hearing about it for such a long time that I don't think I ever really stopped to question it until now.

    Also, there's another term you'll hear a lot in that context when dealing with fans and cooling: "voltage regulated". It describes a fan that (as I understand it (take that for what it's worth ;) )) has its speed regulated by the power being fed to it (via a 3-pin fan header) rather than PWM (pulse width modulation) which needs a fourth pin to transmit the PWM signal. I guess that's where I got the idea that dropping the voltage was a good idea for dropping fan speed.

    But thanks for the help you guys, at this point I'm just going to keep it simple and probably just fit the most efficient fan I can find that'll come in under the 0.21A limit I'm working within.

    From what you've written it seems this is definitely an area I'm going to need a better theoretical understanding of to properly or effectively understand and retain the info that could potentially be given.

    But thanks so much. Those are some really interesting insights, it's very appreciated.
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