How to Undervolt a Fan Inside a Small Device?

  • Thread starter Thread starter jaykchan
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*>>
*>> --
*>> "If you want to post a followup via groups.google.com, don't
use
*>> the broken "Reply" link at the bottom of the article. Click
on
*>> "show options" at the top of the article, then click on the
*>> "Reply" at the bottom of the article headers." - Keith
Thompson
How does it look now? Does it show up the right way?
Click to expand...
*>
*> Jay Chan

That looks fine. Very good. Now you just need to learn to snip
the useless portions that are not germane to the reply. In this
case it would be all the lines preceded by * above.
 
If I understand this correctly, you are say these:

- We could have gone down to 56 ohm for maximum reduction of fan speed
and (hopefully) noise. But we may run the risk of the fan fails to
start spinning.
Click to expand...

No, it varies based on what fan is used. Per the same fan,
the higher resistance would reduce speed more and have
higher chance of failure to spin up. Per a different fan
that starts out (with no resistor) having higher RPM,
current, a lower valued resistor would be used than for the
(unmodified) slower fan.

- In order to reduce the chance of fan failure to start spinning while
still cutting down noise, we may want to choose a 68 ohm resistor
instread of a 56 ohm version.

This makes sense to me. Thanks.
Click to expand...

No, the chance of fan failure (to spin up) would be higher
with 68 Ohm, BUT as I mentioned previously, the value
(range) applicable to a particular fan is different than for
a (different) fan. It cannot be assumed that 68 or 56 Ohm
will have same result with one fan as another. For your fan
the threshold might be much higher, like around 160 Ohm. I
cannot predict this as I don't have your fan. Similar (but
only slightly faster) fans I've had can use over 120 Ohm,
but there was not much point beyond 120 Ohm as the fan was
already inaudible.

I presume Zalman chose the 56 Ohm value because the most
typical, common fan has amperage spec around .2A, rather
than your .05A (IIRC you wrote .6W ?). The majority of
people seeking fan noise reduction started out with a much
louder fan that "usually, typically" needs a lower resistor
value than you should, but again I don't have that specific
model of fan so I can only speculate based on experience
with other fans.
 
That looks fine. Very good. Now you just need to learn to snip
the useless portions that are not germane to the reply. In this
case it would be all the lines preceded by * above.
Click to expand...

OK, I will reply through the Show-Options button from now on.

I am still wondering what happened to my previous reply that doesn't
show up.

Jay Chan
 
Thanks for the correction that you and Noozer have pointed out. I got
it backward. The higher ohm rating should mean higher resistance and
the fan will run slower and supposedly quieter.
No, it varies based on what fan is used. Per the same fan,
the higher resistance would reduce speed more and have
higher chance of failure to spin up. Per a different fan
that starts out (with no resistor) having higher RPM,
current, a lower valued resistor would be used than for the
(unmodified) slower fan.
Click to expand...

Seem like this is one more reason to get a range of resistors instead
of just one. Talking about getting a range of resistors. I cannot find
any suitable resistors from RadioShack. Their selection in term of watt
is very limited (1/2, 1, 5, 10 watts). I will have to order online and
pay shipping. Oh well...

Jay Chan
 
.... snip ...

I am still wondering what happened to my previous reply that doesn't
show up.
Click to expand...

Usenet is a fundamentally unreliable system. An article is sent to
some list of entities, which in turn send it on to other lists they
know about. A glitch in transmisssion, or a mistake in a list,
means the article stops promulgating. Google is just one of many
entities receiving usenet articles, in something like 50,000 or
more newsgroups.

Once your article gets to one of those many servers, some user can
come along and query the server about what it has available. These
are usually classified by author, date, subject, newsgroup, and
thread. There are standards defining these things generally known
as RFCs. Your article may never get read, or it may only be read
at a small fraction of the servers receiving it, or it may be
exceedingly popular. Thus it is important to trim it down to the
essentials, and to follow the established conventions.
 
Thanks for the correction that you and Noozer have pointed out. I got
it backward. The higher ohm rating should mean higher resistance and
the fan will run slower and supposedly quieter.


Seem like this is one more reason to get a range of resistors instead
of just one. Talking about getting a range of resistors. I cannot find
any suitable resistors from RadioShack. Their selection in term of watt
is very limited (1/2, 1, 5, 10 watts). I will have to order online and
pay shipping. Oh well...
Click to expand...

If you're going to order, might as well get a 100 ohm potentiometer, and
adjust for the noise/cooling of your liking. Let it go full speed when
you are out of the room, and much slower when you are around it.
 
If you're going to order, might as well get a 100 ohm potentiometer,
and
adjust for the noise/cooling of your liking. Let it go full speed when
you are out of the room, and much slower when you are around it.
Click to expand...

Too late for that, I have already ordered an assorted number of
resistors from eBay. And I don't understand potentionmeter enough to
know what to do with it. Thanks anyway. I hope someone else who reads
this message may find this useful in keeping the noise of his PC down.

I was quite surprised that I could only find one place (eBay) that has
more than one type of 2-watt resistors that are in the range that I am
looking for. Many places don't sell 2-watt any resistor. Those place
that sell 2-watt resistors only sell 2-watt resistors in one ohm rating
(but I need resistors in a range of ohm ratings).

Jay Chan
 
I didn't see any on Radio Shack's website, maybe I
overlooked them but I dug up another source, might as well
get a good deal on 'em?
Click to expand...
http://www.goldmine-elec-products.com/prodinfo.asp?number=G14511&variation=&aitem=2&mitem=11

Yesterday, I browsed around to look for 2-watt resistors, and I was
surpised that I could only find one vendor (in eBay) that offered
resistors in a range of ohm ratings. Most places don't sell any 2-watt
resistors. Now, I _really_ appreciate the fact that you had spent the
time in digging up a link for 2-watt resistors. That had to be quite
hard.

Jay Chan
 
Too late for that, I have already ordered an assorted number of
resistors from eBay. And I don't understand potentionmeter enough to
know what to do with it. Thanks anyway. I hope someone else who reads
this message may find this useful in keeping the noise of his PC down.

I was quite surprised that I could only find one place (eBay) that has
more than one type of 2-watt resistors that are in the range that I am
looking for. Many places don't sell 2-watt any resistor. Those place
that sell 2-watt resistors only sell 2-watt resistors in one ohm rating
(but I need resistors in a range of ohm ratings).

Jay Chan
Click to expand...

Generally the parts would be found at electronics supply
houses, online at places like Digikey, Mouser, Newark,
Allied Electronics, to name a few. The link I previously
gave for the 200 pieces was just one of myriad hit-or-miss
surplus type companies that don't always have what you need,
but when they do it's often cheaper. Main problem with many
suppliers is minimum order requirements like $25. Digikey
comes to mind as one that doesn't have the minimum order
requirement.

Previously a 100 Ohm POT had been mentioned. POTs generally
do not have sufficient current capability for a fan, or
rather, when they do they're called rheostats rather than
POTs. A POT would be better for fine-tuning something like
a LM317 based fan control, but then probably not 100 Ohm
range. More elaborate fan controllers have their place with
those desiring more frequent changing of speed but higher
cost, time to construct, size, etc, make them less viable
for this kind of simple mod.
 
http://www.goldmine-elec-products.com/prodinfo.asp?number=G14511&variation=&aitem=2&mitem=11

Yesterday, I browsed around to look for 2-watt resistors, and I was
surpised that I could only find one vendor (in eBay) that offered
resistors in a range of ohm ratings. Most places don't sell any 2-watt
resistors. Now, I _really_ appreciate the fact that you had spent the
time in digging up a link for 2-watt resistors. That had to be quite
hard.

Jay Chan
Click to expand...


Well I've bought stuff from them before... plus I've heard
of others calling, asking them to send very small items in
an envelope for cheapest shipping/total-cost. In another
post I listed a few more full-featured electronics companies
with online stores... generally such parts would come from
electronics-oriented companies rather than computer focused.
 
I was quite surprised that I could only find one place (eBay) that has
more than one type of 2-watt resistors that are in the range that I am
looking for. Many places don't sell 2-watt any resistor. Those place
that sell 2-watt resistors only sell 2-watt resistors in one ohm rating
(but I need resistors in a range of ohm ratings).
Click to expand...

Why 2-watt? Even at a 5 volt drop, current would have to be 0.4a to
equal 2 watts. At 7v (a 5 volt drop), most 12v fans draw in the 0.05-0.1a
range. I've used 1w carbon resistors (much smaller) and they hardly get
warm.
 
Why 2-watt? Even at a 5 volt drop, current would have to be 0.4a to
equal 2 watts. At 7v (a 5 volt drop), most 12v fans draw in the 0.05-0.1a
range. I've used 1w carbon resistors (much smaller) and they hardly get
warm.
Click to expand...


True, either 1 or 2W could work for this specific
application, but the 2W seems potentially less problematic.
At the time I didn't realize OP would have problems finding
them or go to ebay for these.

Why not 2W? Typically one does not try to match up watt per
watt on power resistors, rather using what the space and
budget allow, overengineering them. It's not a PCB space
issue and cost is maybe a few cents, or maybe even cheaper
per piece since this is spot-pricing on items not often
(percentagewise) sold 1 at a time. Yes, 1W would usually
work, but IMO, 2W is as good or better choice if someone has
nothing (yet).

Also, "barely" warm is a relative term. They can and do get
warm, even 2W, and some components in a PC/router/etc
shouldn't be placed in contact with a "warm" power resistor.
Granted in this one instance it shouldn't get as warm as
others. The installer could isolate it, but then has to
consider that method and typical nylon wire-ties aren't so
great if the resistor did get more than warm. The router
itself will already be warm inside, hence the need for the
fan. Metal, well, sure if challenged we could all think up
some way to do it but ultimately choosing a larger resistor
also results in lower thermal density, it's not going to be
as hot against (whatever it's touching if it were touching
anything).
 
kony said:
True, either 1 or 2W could work for this specific
application, but the 2W seems potentially less problematic.
At the time I didn't realize OP would have problems finding
them or go to ebay for these.

Why not 2W? Typically one does not try to match up watt per
watt on power resistors, rather using what the space and
budget allow, overengineering them. It's not a PCB space
issue and cost is maybe a few cents, or maybe even cheaper
per piece since this is spot-pricing on items not often
(percentagewise) sold 1 at a time. Yes, 1W would usually
work, but IMO, 2W is as good or better choice if someone has
nothing (yet).
Click to expand...

Only because I found a good assortment of 1W carbon resistors at
my local Radio Shack. 2W? No such luck. 1W is plenty.
 
Only because I found a good assortment of 1W carbon resistors at
my local Radio Shack. 2W? No such luck. 1W is plenty.
Click to expand...

Do they have one of those packs with several assorted values
inside or is one forced to buy the individual smaller packs?
I suppose it could depend on the store too, those I've been
to in the past couple years seem to have scaled back their
stock of these types of components. The individual packs
from Radio Shack always seem wildly overpriced to me, though
it could be a good idea for only a few values.
 
kony said:
Do they have one of those packs with several assorted values
inside or is one forced to buy the individual smaller packs?
I suppose it could depend on the store too, those I've been
to in the past couple years seem to have scaled back their
stock of these types of components. The individual packs
from Radio Shack always seem wildly overpriced to me, though
it could be a good idea for only a few values.
Click to expand...

Man, this was years ago. Before I started using potentiometers. As I
remember, each package had about 5 resistors of the same value.
That store isn't even there any more. Progress!
 
Well I've bought stuff from them before... plus I've heard
of others calling, asking them to send very small items in
an envelope for cheapest shipping/total-cost. In another
post I listed a few more full-featured electronics companies
with online stores... generally such parts would come from
electronics-oriented companies rather than computer focused.
Click to expand...

I finally use the following power resistor from eBay to reduce the
speed to the point that the noisy gigabit switch is now very quiet:

100 ohm 2W 5% carbon composite

I have a feeling that I still have more air flow from the new 60mm fan
with reduced speed than what the old noisy 40mm fan can produce in full
speed. This means I could have reduced the speed further. Obviusly, I
am not going to mess with something that is working so well (plus the
fact that this is so difficult to find that type of power resistors).
But just for the purpose of curiosity, which rating of power resistor
would you recommend if someone in my situation wanted to further reduce
the fan speed slightly.

Thanks for your help and any other info that you may provide in the
future.

Jay Chan
 
I finally use the following power resistor from eBay to reduce the
speed to the point that the noisy gigabit switch is now very quiet:

100 ohm 2W 5% carbon composite

I have a feeling that I still have more air flow from the new 60mm fan
with reduced speed than what the old noisy 40mm fan can produce in full
speed. This means I could have reduced the speed further. Obviusly, I
am not going to mess with something that is working so well (plus the
fact that this is so difficult to find that type of power resistors).
But just for the purpose of curiosity, which rating of power resistor
would you recommend if someone in my situation wanted to further reduce
the fan speed slightly.

Thanks for your help and any other info that you may provide in the
future.

Jay Chan
Click to expand...

I dont' know how much further that particular fan will
tolerate a voltage drop so I can't help much. Past a
certain point a larger resistor will prevent it from
spinning up which obviously should be avoided. I doubt you
could use 220 Ohm but slightly higher than 100 Ohm might be
possible. If would be easiest to use a rheostat (variable
power resistor) to determine the maximum resistance value
then keep the (more expensive and large) rheostat for some
future use (like more fan resistor sizing?) and put in the
appropriate value of resistor in it's place.
 
kony said:
I dont' know how much further that particular fan will
tolerate a voltage drop so I can't help much. Past a
certain point a larger resistor will prevent it from
spinning up which obviously should be avoided.
Click to expand...

Placing a cap in parallel with the resistor seems to cure startup
problems.
 
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