putting a capacitor across a fan terminal...

mynci · Post by **mynci** » Sat May 03, 2003 1:28 am

this may belong in the briefing room, however, it occured to me last night that if i were to put a capacitor accross the +ve and -ve wires of my panaflo's they may start reliably at 5v.
My thinking was - at 5v there is not enough to start some of the fans. so the capacitor will charge quickly, when the capacitor charges to a certain level there will be a charge in it to leave as a current along with the current from the supply. this may be enough to start it.
i dont have time to try it today, but if anyone can tell me whats wrong with my thought process it will save me some time

if it would work what sort of value would i need? im thinking i can get a good idea of an ideal size by doing some calulations, however it occures to me that as long as its big enough to work going bigger wont hurt.

cheers
mynci

FrankT · Post by **FrankT** » Sat May 03, 2003 4:04 am

I mentioned this before, but maybe you can try running 2 panaflos with the same specs in series from 12V. They should get 6V each and hopefully start.
As for the capacitor across the fan terminals, I don't think it would work as you think. All it would do is smooth out the voltage from the supply. It will not inject any more current into the fan at start up.

Mr_Smartepants · Post by **Mr_Smartepants** » Sat May 03, 2003 4:09 am

OK, here's the problem.
A capacitor across the + & - wires would actually delay the fan startup, not help it. Here's why.
At initial startup, the capacitor would have zero charge. Once voltage is applied to the fan circuit, the capacitor would hog it all since it's wired in parallel until it reached full charge (the larger the cap, the longer to charge.)
Once the capacitor reached full charge, then the normal voltage would be applied to the fan to try to start it. If the voltage wasn't enough to start the fan in the first place, it still won't start.
Now once the capacitor is charged and the fan is spinning, if you turn off the system the fan will continue to spin with power drawn from the capacitor! The larger the capacitor, the longer the fan will spin without power!

The only reason you would want to put a capacitor in parallel with a fan is if the fan motor brushes were arcing and creating electrical noise(spikes).

mynci · Post by **mynci** » Sat May 03, 2003 4:12 am

cheers, i suspected it wouln't work like that, as otherwise everyone would be doing it.
they do start with a bit of a splutter at 5v so im not too worried, but it would have been nice to have got them working perfectly with a little fiddle.

cheers
mynci

johnoh · Post by **johnoh** » Tue May 06, 2003 12:53 pm

At Frank's suggestion I put two fans in series at 6v and they start fine.

0dB · Post by **0dB** » Tue May 06, 2003 5:39 pm

What if you put an inductor-resistor series in parallel with the fan? it behaves pretty much like the opposite of a capacitor.
At start-up it would let no current thru itself so the fan gets full power (amperage) and as time goes on u pretty much have a fan in parallel with a resistor. Put a pot in place of resistor and u could vary the continous power (current) the fan gets.

aristide1 · Post by **aristide1** » Tue May 06, 2003 8:36 pm

If the fan has a resistor in series with it then you put the capacitor in parallel with the resistor.

mynci · Post by **mynci** » Tue May 06, 2003 11:50 pm

ok i have been thinking, granted the capacitor was a stupid idea, i should have used thought and anot a guess. however, what if you were to put some sort of passive second order filter, perhaps a Chebyshev or Butterworth filter, can remember which one, but i think butterworth. as i recal they have a damped oscilatory gain response that peaks above the final gain level. if you were to set one up to peak at aroung 1.2X gain and then level out at around 5.5v then the fans would get a peak at around 7v to start followed by 5.5v or whatever.
//hang on this would only work with some form of AC power supply, dammit. is their a dc equivalent?

there must be some way of getting a full voltage followed by a drop off to a wanted voltage. i just cant think of one.
there was a whole lot more of this reply but i deleted most of it as i realised it wouldn't work - i left the above just incase it sparks an idea, even though its wrong.

cheers
mynci

Mr_Smartepants · Post by **Mr_Smartepants** » Wed May 07, 2003 12:46 am

Hmmm, that's an interesting idea. I'm going to have to think on this one a bit.
I suppose it's possible to create a charge/discharge colapsing circuit using a transistor switch and capacitors. Feed it a 5V constant DC, charge the capacitors, then discharge the capacitors on the same output line to boost the perceived volts to more than 5V.
It might just work, but it'd look fugly. Probably take up a 3.5" bay by itself. And it'd be a transient voltage spike, probably not lasting more than say half a second to a second. It might be enough to start the fan though.

hmmm. <goes off to think scratching head>

mynci · Post by **mynci** » Wed May 07, 2003 1:08 am

im sure there is a simple way to do it with active components i just thought their must be a passive way to get the voltage off of the fan after a certain time.
i shall be interested to see what you come up with though.

cheers.

Will35 · Post by **Will35** » Wed May 07, 2003 6:19 am

I don't have a clue how all these electrical things work, whenever I need a resistor or something I go see the pocket protector guy at the local electronics store.

There seems to be circutry in most current PSUs that start the fan(s) @ 12v and within seconds drops the voltage to 5v or whatever. Couldn't this be duplicated for case fan use?

Will

Mr_Smartepants · Post by **Mr_Smartepants** » Wed May 07, 2003 6:26 am

Will35 wrote:There seems to be circutry in most current PSUs that start the fan(s) @ 12v and within seconds drops the voltage to 5v or whatever. Couldn't this be duplicated for case fan use?

Yeah, that's the general idea. However, since I don't have access to a PSU that does that (yet) I can't reverse-engineer the circuit so I'm coming up with my own design. I've got a few scribbles on paper but the main sticking point right now is to cut off the supply after the DC V dump to prevent the circuit from re-energizing (think oscillator). Maybe I should run this diagram by Fancontrol to see what he thinks? Hmmm.

IceFire · Post by **IceFire** » Wed May 07, 2003 6:53 am

If you have a power supply that starts at 12V and than drops to 5v for the fan, why can you just tap into that line? Maybe have a circuit that reads the voltage on that line and match it with the out to the fan (power coming from another source)?

Mr_Smartepants · Post by **Mr_Smartepants** » Wed May 07, 2003 7:07 am

IceFire wrote:If you have a power supply that starts at 12V and than drops to 5v for the fan, why can you just tap into that line?

Aw, that would be too easy!

I'm giving my pocket protector a workout now!

The key would be to come up with a more or less universal design so that those people without a PSU that ramps up then down to get the same effect for regular case fans hooked up to the 5V line of a 4-pin molex (or 3.3V from the Aux connector).
The other drawback to my design is that there is an inherent delay of fan startup. When you'd turn on the PC, the fans would not start until the circuit had time to charge/fire. Maybe two seconds? Depends on the input voltage. The fans would then run at about 7-V then back down to the input voltage.

mynci · Post by **mynci** » Wed May 07, 2003 7:12 am

or you could have the device plug into the 12v line and then introduce a resistance after a certain amount of time

i cant quite get the cct in my head tho.

mynci

herosformula · Post by **herosformula** » Wed May 07, 2003 8:40 am

A timer switch using a simple IC, such as a 555, and a couple of passive components could be set up that would take power off of the +12V circuit, then switch to the +5V lines after a specified amount of time.

fancontrol · Post by **fancontrol** » Thu May 08, 2003 3:51 am

This thread talks about the same thing.

Roger Beckett · Post by **Roger Beckett** » Thu May 08, 2003 2:45 pm

Zener diode

mynci · Post by **mynci** » Fri May 09, 2003 2:48 pm

i had a thought, and a real scrappy cct on a piece of napkin, im faily sure that you could build really simple little cct for a few pence perhaps £1, using a few passive components a transistor and a relay.
if you monitor the back emf on the fan, if its over a certain value the transistor switches the relay to 12v supply(implies the fan is not running) if no back emf then it switches the relay reverts to 5v. i may try to plug something into multisim over the next week or so and see if i can come up with owt.

anyone have any view on this, is it a pointless idea?

cheers
mynci

JimK · Post by **JimK** » Fri May 09, 2003 3:32 pm

If you put a cap across a fan that you are controlling with one of those "variable pulse width" devices it would "smooth" the on\off transitions. Might this reduce the clicking some fans dispay with these controllers? Been too many decades for me to be sure about this

loply · Post by **loply** » Sat May 10, 2003 4:08 am

Yeah a Diode will cause about a 0.7v (iirc) drop in voltage and this drop wont kick in for a moment or two, allowing the fan to start before it kicks in...

So run the fans at 7v and use a few diodes in series to bring that down to 5v... should work, no?

I didnt know panaflos didnt start at 5v anyhow! Im sure ive ran them at that before?

mynci · Post by **mynci** » Sat May 10, 2003 4:17 am

a Si diode will cause a drop of 0,7v, but im fairly sure that they do this as soon as they start to conduct current, therefore you would have no current for the fans untill sufficient current had been injected into the depletion region within the diode to overcome its (small)resistance to a forward current flow. the capacitive effects in the juction could cause delays, i guess but that would be orders of magnitude less than any time reasonable to start the fan.
please argue if im wrong on this but i cant see how it would work

most do start at 5v, however you get some in a batch that are a little reluctant. if they do start then they stall very easily. i just thought it would offer me piece of mind and a little less noise if there was a really simple cct that would kick up the supply if the fan stopped/didnt start.

powergyoza · Post by **powergyoza** » Sat May 10, 2003 11:49 am

A zener diode like the one in this circuit http://forums.silentpcreview.com/viewtopic.php?p=6243 will allow for a minimum voltage to always go through, but it won't help you with supplying a full-power jolt on startup. For than you can see what was done here: http://people.freenet.de/s.urfer/fan_control.htm He used a 47uF cap in parallel with the NTC to do what you need.

Mr_Smartepants · Post by **Mr_Smartepants** » Sat May 10, 2003 12:50 pm

I've been toying with a few ideas on paper and I haven't come up with anything productive yet.
Some ideas I've come up with were:
1) A simple circuit using 12V and ground and steadily increase the resistance (adjustable) until the desired end voltage is obtained. A bypass switch is in the circuit so that if you're in LAN party mode, you can run the fans at 12V with the flick of a switch. (end result: Fan would start at 12V and then reduce speed until final set voltage. Bypass available for instant 12V supply)

2) A circuit that uses 12V and an adjustable final volt (5V reduced with variable resistor)and ground. The 12V feed is gradually transitioned to the final voltage by a parallel circuit with a timed buffer in between. Secondary circuit could be fitted with a thermistor as well if needed. I believe this method is used by some PSUs already (end result: Fan would start at 12V and then reduce speed until final set voltage. Added thermistor would increase voltage to fan if needed for heat buildup)

3) A circuit that has an adjustable final input voltage(enough to keep fans spinning) that uses capacitive discharge to temporarily boost output voltage over the input voltage to get the fans to start. To me, this doesn't look like it's going to work. I can't get it through my head to stop the circuit from restarting after discharge (oscillating). I'm thinking a diode fitted somewhere on the output wired back to the base on the transistor to hold it with a feedback voltage. (end result: Fan would delay-start at reduced voltage [~5-7V] then reduce to minimum voltage required to keep fan running [adjustable])

I don't have circuit engineering software so I'm just drawing things on paper and working the math in my head.
We've been swamped here at work so I barely have free time for myself.
Maybe some other kind soul can pick up on my ideas and slap something together.

fmah · Post by **fmah** » Sun May 11, 2003 10:51 pm

cpemma has a diagram of this fuction here, at the bottom. I don't know what the values should be if you want to do some other voltage levels

http://www.cpemma.co.uk/7volt.html

fmah · Post by **fmah** » Tue May 13, 2003 9:33 pm

I tried the above circuit with slightly different components and couldn't get it to work. Maybe I'm doing something wrong. I used a 4004 diode instead of 4001, I had it in the right polarity since turning it around stopped the fan. I used a 2200uF capacitor instead of the 3300uF shown in the picture. I figure that would just mean the 12V startup would be for a shorter period of time. It looked like I only got ~7V though.

Anyone else tried this?

powergyoza · Post by **powergyoza** » Wed May 14, 2003 12:00 pm

fmah wrote:I tried the above circuit with slightly different components and couldn't get it to work. Maybe I'm doing something wrong. I used a 4004 diode instead of 4001, I had it in the right polarity since turning it around stopped the fan. I used a 2200uF capacitor instead of the 3300uF shown in the picture. I figure that would just mean the 12V startup would be for a shorter period of time. It looked like I only got ~7V though.

Anyone else tried this?

I haven't tried it, but maybe you just need to switch the polarity of the cap?

fancontrol · Post by **fancontrol** » Wed May 14, 2003 7:27 pm

fmah wrote:I used a 2200uF capacitor instead of the 3300uF shown in the picture. I figure that would just mean the 12V startup would be for a shorter period of time.

It also depends a lot on the fan you're using, since it is the R in the RC circuit. You can get the equivalent resistor of the fan using ohm's law R = V/I. The time to 5V would then be .54*RC (using seconds, ohms, and Farads).

For a 100mA (12V) fan your circuit would hit steady state in 0.14 seconds, probably faster than you can measure without a scope.

fmah · Post by **fmah** » Sat May 17, 2003 4:17 pm

Okay, then it may have worked, but not long enough to notice, since I was using a higher current fan the time is much lower. That means the cap will need to be quite big, more in the range of 50,000uF and bigger for a 330 mA and 1.5 second start.

energy · Post by **energy** » Mon May 19, 2003 11:32 am

shouldnt be too hard to make a circuit with a resistor, capacitor, transistor and relay to switch from 12v to 7v once the cap is charged. Pretty simple really... but I dont have the scematic to hand...