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

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
series?

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?

Thanks

 

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

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

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 ) .

Graham

 

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
| |
'---------o
|
.-----.
| 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.

--
Regards,
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)

 

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

[snip...snip...]

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.

 

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!

 

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$
Notepad):

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

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
easiest.

Good luck
Chris

 

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.

--
Regards,
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)

 

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

Cheers
Chris

 

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.

Mark

 

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.

Ed

 

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.

Yannick