Capacitor backup system

[Jeff7]

I am wondering about making a capacitor-based fan battery system. To elaborate: I am planning to equip my computer with a capacitor system that will hopefully keep the fans running for a minute or so after the computer is off. The reason? Example - turn on a stove burner for a few mintues. Now turn it off, and touch it a few seconds later. If you do, you will get burned, even though the power is off. Same thing happens with a computer - power is off, and the cooling shuts down too, but the components remain hot. But anyway, how would I go about hooking up capacitors to the computer? I am thinking that installing a diode between the capacitors and the rest of the computer's power supply should prevent any leakage to the motherboard, etc, after the power is off. Can anyone suggest any dangers of doing this?

[OuTpaTienT]

Instead of messing with the power supply and motherboard, why not just make a capacitor circuit thats placed in between the power supply and just a couple of the more important fans (like the CPU fan & video card fan).

Then you wouldn't have to worry about damaging the sensitive electronics of the motherboard.

Actually, I think it's an excellent idea you have. Most cars that use electric fans for cooling the radiator/engine do the same thing, they run the fans for a couple minutes after you turn off the engine.

Keep us updated on how the project comes along.

[Jeff7]

That's what I was thinking - the route would be this: Starting at the power supply, then a drive connector, then diode, capacitor bank, and then the fans. I can make a diagram of it if you'd like. Heck, here is a quick drawing, assuming I can get the link to work right:
http://www.myfreeoffice.com/jeffelec/auction/power.jpg

[This message has been edited by Jeff7 (edited 08-29-2000).]

[This message has been edited by Jeff7 (edited 08-29-2000).]

[garybeafl]

I think you could buy a time delay relay for your fan. Like the ones used on an electric furnace. It keeps the fan running to cool down the heat strip.

[This message has been edited by garybeafl (edited 08-29-2000).]

[Jeff7]

Wouldn't work though - if the power supply isn't putting out any power, a time-delay relay wouldn't do too much.

[Graham]

You would need some *&^%:dy big capacitors to get it to work for a minute, far better to use some small nicads (rechargeable cells). trickle charge them from the psu when on they can drive the fans when off.

Diode (if psu draws current when off)

+12v__>|___________________________ other bits
| |
/ -
resist \ ^ Diode (1N4001 or sim.)
~4k / |
| |
+----+
|
-
___ NiCads 1.2v/cell
|
0v _________|_________________________


this is a bit simplistic but would work. really you should have a proper charging cct. for the nicads. You could use three of the small pcb mounting nicads to save space, providing you don't have too many fans.

If you cant understand this crappy diag, let me know I can email a better one.

G

Edit.....
Sorry the formatting got totally screwed, will post again later.
G

[This message has been edited by Graham (edited 08-29-2000).]

[Jeff7]

Oh, I've got some good capacitors - how does 12 1.4Farad caps sound? http://sysopt.earthweb.com/forum/wink.gif

[Graham]

Here it is for real ???

http://www.btinternet.com/~graham/battback.jpg

G

[JRcomputers2]

Please keep in mind that the speed in witch the board cools is very important. Explanation and contraction can cause circuit breaks. Let that board change Temp. slowly or you may be in for short life.

[BBA]

Jeff...Id like one of those 1.4Fd caps ( for my car amplifier http://sysopt.earthweb.com/forum/smile.gif )

As for the nicad circuit...not good, it would have to have a tdr to disconnect it after a minute anyway.

Hooking a cap to the fan that is big enough to run the fan will kill your PS the next time you turn the PC on ( because the current to charge a capacitor that big will almost short out any reasonable size computer PS. )

Just let it air cool.

[Richard_Cranium72]

Capacitors are used for instantaneous bursts of power, as in heavy loads in Motor starting circuits or power surges in electronics.

The total duration of the power output is ms or ns.

Try a UPS instead, much simpler.

DrVette

[Jeff7]

Well, I have already done some 'preliminary' testing with power bricks, and three capacitors will keep a 80mm case fan going for about 20 seconds or so. (the capacitors are only 5.5v each, they need to be hooked in series)

I know I can't use Nicads - they can't recharge with a direct blast of 10 amps or so - it'd kill them quickly.
I don't think that a 1.4F capacitor will damage the power supply - the power brick I was using was only a half-amp, and it did fine. The computer PSU is rated to put out 8 amps at 12v.

[tonym]

Dumb question time!

Why does it bother you that the components in the computer are still hot after you turn the power off?

1.) This increased temperature won't adversely affect the reliability of the components.
2.) The moment power is removed, the components start to cool...albeit at a slow rate due to the thermal time constant of the components in the case.
3.) You really don't want to put large (multi-farad) capacitors in-line with the +12V unless you use a current limiting circuit or resistor in series with them.
4.) A typical fan draws 2.5 Watts. This means that is requires approximately 0.2A (200mA) of current from a nominal 12V source. This means that dt = the time you will have power for the fan from a capacitor until Vfan=9V (the minimum input) = C/I * dV, where dV = 12V-9V =3V, C = let's say 2F and I =0.2A. This means that dt = 2/.2 (3) = 30 seconds.

Now 2F is pretty sizeable physically, even at the 16V capacitor rating, so you'll be wasting a lot of space for an approximate 30 second fan run-time.

Also, remember that this 2F capacitor will draw a huge slug of current from the +12V supply at turn-on, even if the power supply is soft-started. And it will take a long time to charge to +12V if you put *reasonable* current-limiting in series with the supply.

And one final thing...these current slugs are much larger in magnitude and duration than the power supply would ever see without the added caps. It COULD shorten the life of the +12V output by stressing this output to the max (to current limit) at each turn-on of the power supply.

My advice...let the computer convection cool when you turn it off...but if you're going to do this and can't be persuaded otherwise, and you/want need some advice, send me some e-mail and I'll send you some ideas for a SAFE battery supply for the fans...


Tony

[Jeff7]

To answer your first question: ever been to Virtual Hideout (http://www.virtualhideout.net/)? Look over some of the stuff there, and you'll know just how crazy some people can be. I am among the lesser crazy people, but I would still be considered among VH's following.
And the capacitors won't really take up too much space - they are only 1" high by 1" diamater (2.5cm). Plus, I had an idea to cut holes in the top of the case for a strange, almost futuristic look to it. (No, I'm not completely crazy...)

Tonym - feel free to provide info for making this rig safe if I do decide to go ahead with it. I just want good prolonged cooling - not a hige safety hazard.

[Graham]

BBA, I envisisaged letting the nicads run down each time, I realise it is not an ideal solution.
Jeff7, why would the nicads get a blast of 10 amps??, the cct shown would only allow charging of a few milliamps !!!

G

[This message has been edited by Graham (edited 08-30-2000).]

[Jeff7]

Ok, yeah, you're right - my mind sort of blanked out the resistor in that circuit. That way could work too I guess. Well, I have a spare power supply and a 386 motherboard; after some more tests with the power brick, I'll give the computer a try. (while I'm at it, I'm gonna overclock that 33MHz thing all the way to 40!!! wow.)

[OuTpaTienT]

You could just never turn the computer off. That's what I do.

[Jeff7]

Windows wouldn't allow that - it would insist on locking up. Plus, how would you upgrade then? http://sysopt.earthweb.com/forum/smile.gif

[howste]

OK, just an observation. If your heatsink/fan is doing its job, then when you turn off the power, the CPU should already be cool enough anyway.

I'm sure you're probably just doing this for fun and to see if it can be done anyway. Don't let mine or anyone else's comments spoil your "gee whiz" fun project.

Steve

[BBA]

Windows locks up if you leave the computer on?

You must have some faulty hardware or you have crappy software loaded that kills it.

My PC's stay on indefinitely, my Q3 server usually runs for weeks on end without even stopping the game server..much less shutting down.

Trust me, it is not Windows messing up...it's something you have or did.

[Jeff7]

Sorry, that was a sort of crappy joke - Windows, locking up...
Yes, it's not Windows alone that causes it - it's definitely something I am doing to the computer - I am using it. http://sysopt.earthweb.com/forum/smile.gif

Internet Explorer freezes, the TV tuner causes lockups sometimes, AIM can cause it; just your typical Windows 95 crashes, that's all.

Yes, I suppose this is a sort of fun, "gee whiz" project. But without them, where would we be?

[Richard_Cranium72]

WELL ??

What was the resolution, did the capacitors work as expected in the long run or what ??

DrVette