Multiple resistor's in series to control fan speed?

esn · Post by **esn** » Thu Mar 18, 2004 2:04 pm

So, I decided to go with the decidedly low tech method of resistors in series to control the speed of my case fans. One of my machines is a PIII 1Ghz; not a performance machine. It's an HP Vectra and there's a long story as to why it needs a case fan where one was not originally supplied by HP. Anyway, I tested a 100 ohm R on the 80mm Panaflow and found that it wouldn't whereas it would start with a 47 ohm R. So I built and installed a 47 ohm jumper. I then got hold of a 68 ohm R, found that it would allow the fan to start so that is what I'm running now.

So much for background, now for the question: Since I want to slow this fan down as much as possible (and do so simply and cheaply from nearby Radio Shack stock), are there any fundemental electrical principles that would prevent me from using a 47 and 33 ohm in series to achieve 80 ohms?

Oh, FYI they are 1/2 watt resistors.

EDIT
FWIW, second post here. When times permits I'll post my succes story which owes a lot of thanks to this excellent website.

sthayashi · Post by **sthayashi** » Thu Mar 18, 2004 2:12 pm

No. 47Ohm + 33Ohm = 80Ohm, no matter what. Just make sure that they stay in series and not in parallel. And always watch out for shorts.

exeter_acres · Post by **exeter_acres** » Thu Mar 18, 2004 2:19 pm

Umm... I just use bare wires from the fan and push them into an empty molex connector... black to black.. red to red...and poof 5v

seems easier than resistors...
or on my main rig I use a Radio shack $3 rheostat and slow everything down....

cpemma · Post by **cpemma** » Thu Mar 18, 2004 3:31 pm

You could do a better job, probably cheaper as well, with just a zener diode in the fan wiring.

A 4V7 1.3W zener will drop the fan voltage to (12 - 4.7) = 7.3V, or you could go to 6.9V with a 5V1 zener. A 5V1, 1.3W, zener can carry 250mA, equivalent to around 5W of total fan power rating.

Advantage over the 7-volt trick is a speed wire still works.

flyingsherpa · Post by **flyingsherpa** » Thu Mar 18, 2004 4:03 pm

sthayashi's certainly right, that will work fine. but you'll probably find that that's not the right the voltage either and you'll go off looking for another resistor. eventually you'll have several in series and a few in parallel trying to get just the right speed and you'll have spent $10 on resistors and you have a huge ugly mess inside your case. i've been down that road before and it wasn't that pretty and i could never get it just right.

my advice: skip the above step and go right to a Zalman fanmate (or other adjustable fan controller) so you can adjust it just the way you want. i bought mine for about $5 and haven't looked back.

esn · Post by **esn** » Thu Mar 18, 2004 4:27 pm

Hmmm, when I first started looking into this the only data I found on diodes indicated ~.6v drop per diode (different diode no doubt). Hence, I was looking at 6 diodes in series (YUK!). This looks sweet. I might have to give it a try.

...Speed wire...? This computer doesn't have the capacity to monitor the fan beyond a braindead, fixed, hidden, friggin' bois setting that looks for the fan during boot and will only tell the user if it doesn't see the fan and susequently not boot. BUT, my performance machine with a 47ohm R is showing strange fan speeds. I was wondering if the resistor had something to do with that.

Just curious: Being somewhat electrically challenged, how do I calculate the voltage drop across a diode? actually, I should seach this site some more; the answer has to be here somewhere.

Thanks!

cpemma wrote:You could do a better job, probably cheaper as well, with just a zener diode in the fan wiring.

A 4V7 1.3W zener will drop the fan voltage to (12 - 4.7) = 7.3V, or you could go to 6.9V with a 5V1 zener. A 5V1, 1.3W, zener can carry 250mA, equivalent to around 5W of total fan power rating.

Advantage over the 7-volt trick is a speed wire still works.

sthayashi · Post by **sthayashi** » Thu Mar 18, 2004 6:01 pm

Nearly every multimeter I'm aware of can tell you the voltage drop of a diode. Just set the dial to the diode setting (it looks like a schematic drawing of a diode).

A Zener Diode has a much greater voltage drop when put in reverse, I've seen them for 5v and 9v right off the top of my head, and you can probably get them for other voltages as well.

esn · Post by **esn** » Thu Mar 18, 2004 6:11 pm

Or I can re-read the previous message and realize the 4V7 = 4.7 volts and 5V1 = 5.1 volts. DOH!

And, ya, I keep thinking I need to get a multimeter.

sthayashi wrote:Nearly every multimeter I'm aware of can tell you the voltage drop of a diode. Just set the dial to the diode setting (it looks like a schematic drawing of a diode).

A Zener Diode has a much greater voltage drop when put in reverse, I've seen them for 5v and 9v right off the top of my head, and you can probably get them for other voltages as well.

lenny · Post by **lenny** » Thu Mar 18, 2004 7:43 pm

esn wrote:And, ya, I keep thinking I need to get a multimeter.

Aw come on. This is America, where folks drive SUVs instead of compact cars. Overkill is a way of life. You need an oscilloscope. Preferably a digital storage oscilloscope.

Just kidding of course

A multimeter is very useful. Definitely get one. It's an great tool to troubleshoot many basic problems (like - is there any voltage on this wire? Are these two wires connected electrically to each other? etc.)

Edit: Ooh... post #666. Guess it's appropriate in a post that mentions SUVs

Never thought I'd post so much when I started...

Post by **MikeC** » Thu Mar 18, 2004 8:44 pm

exeter_acres wrote:Umm... I just use bare wires from the fan and push them into an empty molex connector... black to black.. red to red...and poof 5v

seems easier than resistors...

Sure does. And it is guranteed to start almost any 12V fan, certainly Panaflos. Mind you, the airflow is pretty dang low.

lenny · Post by **lenny** » Fri Mar 19, 2004 12:15 am

MikeC wrote:
exeter_acres wrote:Umm... I just use bare wires from the fan and push them into an empty molex connector... black to black.. red to red...and poof 5v

seems easier than resistors...
Sure does. And it is guranteed to start almost any 12V fan, certainly Panaflos. Mind you, the airflow is pretty dang low.

Actually I got a bunch of FBA08A12L1BX that will not start at 5V, though they work fine at 6V and above. I haven't tried the couple of L1As though.

All 120mm I've tried (FBK, FBA L and M) start just fine at 5V. In fact, I once tried some FBK at 3V and they start just fine. They're practically useless at 3V, of course. But they spin.

The few 80mm NMB B10s I have start fine at 5V. If you stop them with your fingers, they pause for a while, then spin up again.

cpemma · Post by **cpemma** » Fri Mar 19, 2004 4:35 pm

sthayashi wrote:Nearly every multimeter I'm aware of can tell you the voltage drop of a diode. Just set the dial to the diode setting (it looks like a schematic drawing of a diode).

Result tends to be lower than you get in practical circuits, similarly with leds, as the meter test current is so low. Datasheets often show a graph of forward voltage against current, but for a common 1N4001, my two meters show 0.50 and 0.52v, at around 15mA its a bit under 0.7v, I measure about 0.75v with a 180mA fan in series, and it gets near 0.9v at the rated 1A. Spec usually says 1V max at the rated current.

esn · Post by **esn** » Sun Mar 21, 2004 6:39 pm

Ok, I just tried a 1N4733A diode and it works. It's rated as: 5.1V. 1.0W, 49mA. Note that the amperage is 1/5 what you specified. Is this a problem for an 80mm L1A Panaflo?

cpemma wrote:You could do a better job, probably cheaper as well, with just a zener diode in the fan wiring.

A 4V7 1.3W zener will drop the fan voltage to (12 - 4.7) = 7.3V, or you could go to 6.9V with a 5V1 zener. A 5V1, 1.3W, zener can carry 250mA, equivalent to around 5W of total fan power rating.

Advantage over the 7-volt trick is a speed wire still works.

sthayashi · Post by **sthayashi** » Sun Mar 21, 2004 7:04 pm

esn wrote:Ok, I just tried a 1N4733A diode and it works. It's rated as: 5.1V. 1.0W, 49mA. Note that the amperage is 1/5 what you specified. Is this a problem for an 80mm L1A Panaflo?

You should use 2 in parallel.

The L1A draws about 100mA at 12V officially. The output of my DC power supply says that it's closer to 70mA running. The kicker is that you're not giving it 12V, you're giving it 7V. I don't have access to the power supply right now, but off-hand I'd say that you'll be running it close to the limit. Having 2 in parallel effectively doubles the current limit, making it safer for the diodes in general.

esn · Post by **esn** » Sun Mar 21, 2004 9:05 pm

And Radio Shack just happens to sell these in pairs. Thanks!

sthayashi wrote:
esn wrote:Ok, I just tried a 1N4733A diode and it works. It's rated as: 5.1V. 1.0W, 49mA. Note that the amperage is 1/5 what you specified. Is this a problem for an 80mm L1A Panaflo?
You should use 2 in parallel.

The L1A draws about 100mA at 12V officially. The output of my DC power supply says that it's closer to 70mA running. The kicker is that you're not giving it 12V, you're giving it 7V. I don't have access to the power supply right now, but off-hand I'd say that you'll be running it close to the limit. Having 2 in parallel effectively doubles the current limit, making it safer for the diodes in general.

stuartlee · Post by **stuartlee** » Sat Apr 10, 2004 10:39 am

esn or others:

What's your recommended way of connecting the conductors of the diode with the cables?

The only way I know, other than to use your hands is to use small screw on wire connectors:

I want to use this for the psu fan (I want to see if the stock 2-conductor stock fan undervolted will be as good as the Nexus 80 mm fan undervolted to 7v).

Oh yeah, anyone know if wire tap ins will work?:

http://www.radioshack.com/product.asp?c ... =64%2D3053

tay · Post by **tay** » Sat Apr 10, 2004 11:03 am

solder + electrical tape + heatshrink is what I use. Pretty hard to mess this up imo.

Gooserider · Post by **Gooserider** » Sat Apr 10, 2004 11:19 am

The best way is to solder. Soldering is a fairly easy and very useful skill to aquire, and low budget equipment (Radio Shack) is pretty cheap. Shrink tube is the best way to insulate your soldered joints, but electrical tape will work if you have to use it.

Wire nuts are really only for use with larger wires, especially house hold wiring. The crimp on splices will work maybe, but they are fussy about what size wire you use them with, to small and they won't reliably connect, to large and you damage the conductor. They are also a real headache in any sort of environment that's corrosion prone - Not a problem w/ most PC's but lots of people use them for automotive and motorcycle applications where they cause all sorts of MAJOR problems a few months to a year or more after installation. (I will FIRE any mechanic that I catch using one of those crimps on a vehicle that I own!)

My opinion is that connections in a PC should be either plugs for things that come apart, or solder & shrink tube for things that don't.

Gooserider

Gooserider

esn · Post by **esn** » Sat Apr 10, 2004 12:13 pm

stuartlee wrote:esn or others:

What's your recommended way of connecting the conductors of the diode with the cables?

I bought some fan cable extensions, cut a small portion out of the middle of the + wire and soldered in the diodes. I used shrink tubing to insulate the diodes and marked the connector with the resulting voltage value. I've always found that after a while electical tape makes a gummy mess so I stay away from it. I recently found that Radio Shack sell non-adhesive silicone tape. The stuff is stretchy ands bonds to itself but not to other stuff. I've used it a little bit and like it so far.

sthayashi · Post by **sthayashi** » Sat Apr 10, 2004 5:45 pm

lenny wrote:Actually I got a bunch of FBA08A12L1BX that will not start at 5V, though they work fine at 6V and above. I haven't tried the couple of L1As though.

All 120mm I've tried (FBK, FBA L and M) start just fine at 5V. In fact, I once tried some FBK at 3V and they start just fine. They're practically useless at 3V, of course. But they spin.

Was just rereading this thread, but Lenny, where'd you get the L1BX?

Lone Ranger · Post by **Lone Ranger** » Tue Apr 13, 2004 4:35 am

cpemma wrote:You could do a better job, probably cheaper as well, with just a zener diode in the fan wiring.

A 4V7 1.3W zener will drop the fan voltage to (12 - 4.7) = 7.3V, or you could go to 6.9V with a 5V1 zener. A 5V1, 1.3W, zener can carry 250mA, equivalent to around 5W of total fan power rating.

Advantage over the 7-volt trick is a speed wire still works.

Hellp Cpemma

The other day I opened up my Zalman "Fan Mate 1". See http://www.zalman.co.kr/usa/product/cnpsfanmate.htm for product details. This is an illustration of the device I mean:

I had expected to find a big variable resistor in there but there was no such thing.

I know extremely little about zener diodes but the circuit board in the Fan Mate seemed to have some components wired in a similar the same way to what your diagram of a zener diode shows but with the addition of a tiny variable resistor.

I very much like the way the Fan Mate performs. I had originally intended to use some 10W fixed resistors to make my own inline resistor as a fan speed dropper like one of these

but now that I have used the Fan Mate I would far rather make something like a Fan Mate. Can anyone specify what components I should use to make myself one of these?

(Is such info tucked away somewhere on this website as I can't find it!)

Thanks, guys.

tay · Post by **tay** » Tue Apr 13, 2004 6:45 am

I believe you are looking for this?

http://www.bit-tech.net/article/56/

EDIT: gg silverarg

. cpemmas site has great info!

silvervarg · Post by **silvervarg** » Tue Apr 13, 2004 6:48 am

I very much like the way the Fan Mate performs. I had originally intended to use some 10W fixed resistors to make my own inline resistor as a fan speed dropper like one of these

A 2W resistor is usually enough, and a lot less clumpsy than a 10W.
You might even get away with a 0.5W resistor.
You can use Kostiks fan resistor calculator to figure this out (can be downloaded from SPCR software link page).

but now that I have used the Fan Mate I would far rather make something like a Fan Mate. Can anyone specify what components I should use to make myself one of these?

It will be cheaper for you to buy them if you want nice housing, connectors etc than it will be to build them.
If you still want to build them for some reason Cpemma has great information on his site:
http://www.cpemma.co.uk

For this particular item you want to check out the first one on this page:
http://www.cpemma.co.uk/reg.html

tay · Post by **tay** » Tue Apr 13, 2004 6:57 am

I believe you are looking for this?

http://www.bit-tech.net/article/56/

cpemma · Post by **cpemma** » Mon Apr 19, 2004 11:33 am

esn wrote:Ok, I just tried a 1N4733A diode and it works. It's rated as: 5.1V. 1.0W, 49mA.

That spec doesn't make a deal of sense. W = V x I, so a 1W 5V1 zener will handle 196mA max.

The Fairchild datasheet shows various properties at a 49mA test current, but this is not the max.

The Fanmate construction has been covered in this forum, it's a 7805 1A 5V regulator connected as an adjustable, which gives a built-in minimum of 5V output and is a few pence cheaper than using a 317T.

esn · Post by **esn** » Mon Apr 19, 2004 12:09 pm

cpemma wrote:
esn wrote:Ok, I just tried a 1N4733A diode and it works. It's rated as: 5.1V. 1.0W, 49mA.
That spec doesn't make a deal of sense. W = V x I, so a 1W 5V1 zener will handle 196mA max.

The Fairchild datasheet shows various properties at a 49mA test current, but this is not the max.

The Fanmate construction has been covered in this forum, it's a 7805 1A 5V regulator connected as an adjustable, which gives a built-in minimum of 5V output and is a few pence cheaper than using a 317T.

Well that's what the package said, but I don't remember it specifying neither test nor max and the Radio Shack web site doesn't even specify amperage. It would have been nice to know since I ended up with 2 1N4733A diodes in parallel based on the assumption that 49mA was max. Oh well, no harm, it works nicely.

zak · Post by **zak** » Tue Apr 20, 2004 1:36 pm

Zeners in parallel may o may not share current equally. If they are not identical, the one with the lowest voltage may get most of the current.

It is better two put 2 in series instead (of half the voltag each). Aldo keep in mind that the diodes are supposed to dissipate the heat through the wires - if at maximum, some kind of heat sinking thick wire needs to be attached there.

Finally, in forward conduction, a diode drops around 0.6-0.7 volts. You can use a numberof1N400x diodes in series to drecusce voltage in 0.7 volt steps at 1 amp max current.

Thomas