Dc Transformerless Power Supply For A Fan

Discussion in 'Power Electronics' started by Kenny, Sep 16, 2006.

  1. Kenny

    Thread Starter AAC Fanatic!

    Oct 11, 2004
    I have a 6v little fan that works with ‘D’ batteries (4x1.5V). I want to replace the use of batteries or DC adapters, making a power supply that can be hidden into the fan case, in order to plug and use.

    I copied a schematic from http://www.geocities.com/tjacodesign/index.html and I tried to design a 5V DC transformerless power supply applying my scarce knowledge about AC circuit analysis.

    Before trying with the transformerless power supply I needed to know the fan motor resistance.

    As I didn’t know the current or resistance of the fan motor, I put a 10 Ohms shunt resistor in series with the motor and I powered them with a 6V DC power supply. With this I measured Vrshunt=2.5V and Vmotor=3.5V, I calculated that the resistance of the fan motor is 14 Ohms, and its current is 0,25 A.

    Then, I tried to design a DC transformerless power supply that can manage the fan, something like 5V (0,25 A).

    I attached the design and its calculus are:
    Iac=0.948 A, (120V-5.6V)/Z
    Idc=0.474 A

    I used an electronic circuits software simulator, and I saw that the 14 Ohms resistor load makes the Zener DC output to have too much ripple, I tried with various capacitor values but I saw that the problem only disappeared when the load was greater (1K and up), then I guess that ripple is not a problem in the implementation.

    I ask for your help. I need to know if it is correct the analysis and design that I have made.
    Now, I’m going to see what componnent values I have in my benchmarck and if the analysis is right, to make the needed adjustments, i.e, I think to replace a 2 Ohms (2W) resistor with 4 resistors in series of 0.5 Ohms (0.5W each one).
  2. beenthere

    Retired Moderator

    Apr 20, 2004

    The old cheap and dirty transformerless mains supply has a teeny drawback - the very real possibility of lethal voltages being present if there is any falut in the insulation or a component.

    A wall transformer is quite inexpensive, and provides a much safer means of powering your fan. Look at www.hosfelt.com for truly cheapo wall transformers.
  3. hgmjr

    Retired Moderator

    Jan 28, 2005
    I would like to echo beenthere's safety concerns.

  4. thingmaker3

    Retired Moderator

    May 16, 2005
    When your zener fails open, you'll have line voltag across your little fan.

    Bad. Dangerous.

  5. Kenny

    Thread Starter AAC Fanatic!

    Oct 11, 2004
    OK, I understand what you say.
    In this specific case if some day the zener fails and the little fan receives AC that destroy it, no matter because when it happens this little appliance already have acomplished its work.

    OTOH, Is there any way to avoid the risk of a failure that draws excesive
    current from the wall that makes the wall circuit breaker jump.

    Really I want to power that little fan without a bulky transformer. I
    ask you: is it right the calculus and design that I have made?
    Thanks in advance.
  6. Søren

    Senior Member

    Sep 2, 2006

    I wouldn't worry too much about that aspect, since you're not stupid enough to build such a circuit without a fuse in front of it, right ?
    And the typical failure mode for zeners is short circuit anyway, but you're missing the point... Zener or not...
    The whole enchilada will be mains carrying !!!

    Sure, the afforementioned fuse (with a rating suitable for the load), which you, of course, allready know, since only morons consider building mains stuff without including a fuse.

    R1 has to be larger. It's purpose is to limit inrush current to the cap. 100 Ohm is about the smallest you can get safely away with.

    Your capacitor should be a non-electrolytic, non-polarized cap with a working voltage of at least 250V !!!

    The value of the cap could be halved if a full wave circuit (think Graetz bridge) was used, but still non-polarized, 250V abs. minimum etc.

    If you think a (tiny) 5V, 1.25VA transformer is bulky, you ain't gonna like the size (or price for that matter) of a 22µF cap with the specs you need.