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zener diode doesn't give my PC fan enough juice to start spinning??

Discussion in 'Electronic Basics' started by [email protected], Feb 25, 2006.

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  1. Guest

    I really am a beginner to electronics, so any help would be
    appreciated. I can limp through with enough instructions, but really
    don't know much on my own.

    I recently bought a CRT projector for my home theatre. It has 5 fans
    that are quite loud, due to air circulation and not from the spinning
    of the fans themselves. On another forum related to CRT projectors,
    I've read that many owners have added resistors or diodes or regulators
    to slow down the fan speed. One common diode that I've read works for
    my specific projector is a zener diode 5.1V 1A or 5A, which they
    install between the red wire with the banded end away from the fan.
    The fans by the way at Matsushita SF80 12vdc 160mA DC brushless.

    I bought a set of these diodes and installed them on two of the fans so
    far. Each had the same outcome, which is that the fans 95% of the time
    do not start when you first turn on the projector. When I first turn
    on the projector, I see that the blade moves about 1/8 inch but does
    not get enough juice to pass the magnet or whatever and then just goes
    back to start positiona and never starts. I did the 2nd fan since I
    wanted to confirm the first fan wasn't faulty or that I did a poor
    install on the first fan. But the 2nd fan did the same thing. I found
    that 1 out of 20 times the fans may start. I also found that if I help
    the fans start with my finger, they work fine afterwards - and they do
    turn slower as intended.

    I have a few questions:

    1) could the problem be that I need a higher volt zener diode (mine are
    5.1v) and if so what volt?

    2) could the problem be that I need a higher amp zener diode (mine are
    1A) and if so what amp?

    3) would it help if I connected two of my 5.1v 1A zener in parralel or

    4) can I use a resistor to achieve the same goal and if so what type?

    5) is there a way to give the fan full juice at start-up and then
    restrict the juice to the same speed that the 5.1v 1A zener puts out.
    I like the speed of the fans that these zener made them spin (once they
    actually start), but it seems like they need more power to start
    turning. I believe regulators do exactly that - is that true and if so
    what type do I need?

    6) are there any other electrical options to slow down my fans?

  2. Pooh Bear

    Pooh Bear Guest

    No - you want a *lower* voltage zener. e.g 3.9V

    The zener voltage is the amount the fan voltage *drops* compared to normal

    It's not *amps* you need to consider it's th *watts* that the zener
    dissipates in this application.

    Zener diodes are rated by wattage not amps.

    As long as the diodes you currently have aren't overheating then it's fine.

    Sort of but the calculations will get more complicated and will depend on
    the fan motor rating in mA.

    A zener makes a lot of sense.
    You can't do this very easily honestly, although it's a good idea. A
    parallel capacitor across the zener *might* work but it'll be large and
    tricky to fit.

    See the stuff sold to quieten PCs.

    Btw you should be aware that the fan cooling in your projector may actually
    be needed to run at full speed to avoid internal parts overheating and
    failing ( early ) .

  3. Bob Monsen

    Bob Monsen Guest

    You aren't getting enough voltage across your fans to start them.

    Instead of using fixed zener diodes, use this circuit for each fan:

    Pos -----o---------.
    | |
    \ |
    470R / c
    Trim \<-------b TIP31C
    Pot / e
    | |
    | FAN |
    Neg ---------------'

    Adjust the trimmer so it just fails to start, and then adjust it back a
    bit. This circuit will allow you to adjust the voltage at the + side of
    the fan from 6V to 10V.

    You can use pretty much any power NPN transistor (not a tiny signal one,
    but one of the bigger TO-220 cases. Radio shack has them), but a TIP31 is
    a good choice, and probably available locally. It might get a bit hot, but
    not terribly so.

    One problem with doing this is that the expensive electronics or lights
    may fail without adequate cooling. You can add baffling on the fans to cut
    the noise.

    Note that there is a whole industry that has risen up around PC noise.
    There are countless sites giving advice on the matter. They may have
    better ideas for you as well, since it is pretty much the same problem.

    Bob Monsen

    The grand aim of all science is to cover the greatest number of
    empirical facts by logical deduction from the smallest number of
    hypotheses or axioms.
    Albert Einstein (1879 - 1955)
  4. Rich Webb

    Rich Webb Guest

    On 24 Feb 2006 19:36:17 -0800, wrote:

    There are "pulse-width modulated" (PWM) regulation schemes that can
    start with a 100% duty-cycle for some period of time to kickstart the
    fans, which then revert to lower duty cycles that are proportional to
    the temperature at some sensing point.

    The easiest approach to this (for me) would be a little 8-pin
    microcontroller with an internal oscillator and a thermistor (or two)
    hanging off of an A/D port. That also makes it easy to enforce a minimum
    fan speed, to close the loop and monitor fan speed, as well as to take
    action (sound an alarm, shutdown the device, etc.) if the temperature is
    too high or if the fans fail.
    Understand that, to a first approximation, the expected time to failure
    of the projector is directly proportional to the fan speed; the slower
    the fans run, the hotter the projector will get, and the sooner it will
    fail. If you're lucky, it will just be the bulb...

    You *may* be able to achieve both cool and quiet operation if you open
    the case and use a larger but slower fan to move an equivalent amount of
    air over the works. This has the down side of potentially exposing
    you/kids/pets to mains voltage. Or, put the whole thing in a sound-proof
    baffle box and use the large-but-slow fans to push/pull room air through
    the box. It may be enough to use a "half box" (fully open on one or two
    sides) lined with "egg crate foam" to muffle the fan noise without
    needing to add another fan.
  5. Art

    Art Guest

    Generally the fans are included in the design to cool the internal
    electronics, there-by allowing them to function. Changing the function,
    speed, or airflow of the various fans may at least void the manufacturer's
    implied warranty and also may eventually render the equipment inoperable.
    They are installed for a specific reason, designed to operate at a specific
    speed for proper cooling or just dust dispersion, probably by an engineer
    with a bit more knowledge that you ?? IMHO!
  6. Chris

    Chris Guest

    You've gotten quite a bit of good advice here. If you've got a
    well-stocked junkbox, with some old 1000uF and up filter caps (just
    about any voltage will do), you might want to try putting that cap in
    parallel with your zener (make sure the + end of the cap is on the
    right side). A 1000+uF cap will give your fan a bump to start at 12V,
    and as the cap charges up, eventually all the fan current will go
    through the zener. A 1000uF cap will give you about a 50ms bump, which
    should be enough to get the fan going. Once that happens, it can run
    on the reduced voltage without problem. View in fixed font or M$

    | 1000uF
    | +||
    | .---------||--------.
    | | || |
    | | |
    | | |
    | | / |
    | o---------|<------- o
    | | / |
    | +| Vz=5.1V |
    | --- |
    | 12VDC - / \
    | | ( M )
    | | \|/
    | | |
    | '-------------------'
    (created by AACircuit v1.28.5 beta 02/06/05

    Of course, you can use a larger cap without problem.

    Again, this is a "brute force" solution, and really isn't economical.
    However, if you've got a well-stocked junkbox, it's definitely the

    Good luck
  7. Bob Monsen

    Bob Monsen Guest

    I was going to recommend this, but just to be safe, I tried this with an
    80mA 12V brushless computer fan (smaller than the OP's fan), and it didn't
    work. The motor appears to require far too much current.

    My experiment was to adjust my supply so that the fan just stoped
    'starting' when I powered it (like the OP's fan). Then, I put a 1000uF cap
    across the zener, and powered it up. To my surprise, the fan was never
    able to start with the cap when it couldn't start without the cap.

    Now, obviously, the additional current obtained in this way will depend on
    how quickly the supply starts up. Faster supply = more current, so it may
    work for him.

    Bob Monsen

    My life is a simple thing that would interest no one. It is a known
    fact that I was born and that is all that is necessary.
    Albert Einstein (1879 - 1955)
  8. Chris

    Chris Guest

    Hi, Mr. Monsen. I tried the small fan again (like yours, a 12V 80mA)
    by turning on the power supply instead of just connecting the +12V as
    you suggested, and it didn't start with the cap in parallel with the
    zener this time. Probably PS rise time, as you said. More than likely
    won't work for the OP, either.

    The TIP and the pot is obviously a better solution.

    Thanks for the spot, and

  9. redbelly

    redbelly Guest

    The problem could be motor inductance limiting the current during the
    time the capacitor is supposed to be "boosting" it. A big enough cap
    should **theoretically** work, but you're already at 1000 uF so this
    would require something really huge.

    But, I agree with others that the fan speed should not be messed around
    with. Other options:

    1. Get another fan that is quieter but provides the same CFM (or
    greater), if one exists.

    2. Add sound-dampening material inside the projector.

  10. ehsjr

    ehsjr Guest

    Like others, I'd be concerned with lowering the speed
    without knowing for sure that it would not hurt the thing.

    That said, perhaps you do know. Here's a circuit that
    will do it for you.

    +12 -----+------+---------+----+------------+
    | | | |k |
    [Zd] >| Ry1 [RY1] [D] |
    | | N/C | | [33K]
    +-----+ +----+ |
    | \ 3V |
    [Motor] NPN |---[Zd]---+------+
    | /e | |
    | | [100uF] [33K]
    | | | |
    Gnd ----+----------------------+-----------+------+

    This will apply full power to the motor for a few seconds
    when you turn the projector on. It uses the normally closed
    point of RY1 to bypass the Zener so that the motor can get
    up to speed. The 100uF capacitor charges through the 33K
    resistor. The second Zener (3V) sets a threshold below
    which the transistor won't turn on. When the charge rises
    high enough (in 3 - 4 seconds), the transistor turns on
    and energizes the relay, opening the RY1 contact and
    allowing the motor Zener to function.

  11. Yannick

    Yannick Guest

    When using zener diodes be carefull that the load resistor is not too
    high because this would limit the current and because of the non ideal
    zener characteristic this would lower the zener voltage significantly ,
    yes indeed you can alter the zener voltage with a pot in a certain
    small range.

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