I am looking for a way to keep my fan running for a few minutes after I unplug it. I intended for my PSOne PJ to be fairly portable, so having to keep it plugged in for a bit after use is a drag. Also, I don't want to damage my PJ if the power goes out. I could use some form of battery backup, but I didn't think that batteries charged quickly enough. This is why I think that capacitors would work great. The only issue would be if the capacitors needed were high micro-farad values because this would make them large and expensive.

I will be running this fan, (120 mA, 1.44 W) at 7.5 volts. (yes I know that it says 8-14v, but it will run at 7.5v because my current fan runs at this voltage, having the same voltage rating on its package) My current fan is rather loud, thus is the reason why I will be replacing it with a SilenX fan, when it comes in. There must be an easy(ish) formula to calculate the drain of a capacitor with a certain current draw. (I might have just forgotten it since my last electronics class)

Please correct me if this isn't possible (or practical). If it isn't, could you offer another solution to this problem?

Well if the fan had a constant resistance (or current draw) it would be simple. But since the fans current draw will be a function of the voltage, which will decrease with time after the power source is removed, it would be a differential equation (not fun).

I would say but the biggest capacitor(s) you are willing to spend the money on, and see how long it lasts and go from there.

I could use a step-up voltage regulator (switching, I think it's called) to keep the voltage, and thus the current, the same. And no, I haven't gotten to DiffEQ yet. That's college. I'm in high school.


Not a bad suggestion either. I will have to try that tomorrow with some capacitors I have sitting around.

Sounds like a plan.

Remember if you have multiple capacitors wire them in paralell with the battery and fan, and if you have multiple capacitors wire them all in paralell.

If you have an array of capacitors you could make a graph of Farads vs Fan run time, and extropolate the curve to see how many Farads it would take for a desired fan run time.

Capacitors are a bad idea. Even a large bank will only run a fan for 5 seconds or so unless you buy $100 worth. If you only want around 5 seconds then it might be OK.

Charging caps is a little tricky too. A transformer is fine but a solid state PS will have to deal with a “short” at start up.

I'm open for a debate on whether it is better to let a lamp cool slowly or fast with a fan.

When you turn out the lamp it stops making heat too.

Use a thermal switch. You owe me two cents. What do you see as the downside(s) of active cooling after powering down? You thinking thermal stress or something else?

I would, except you didn't read the part about "keep my fan running after I unplug it."


edit: This is from HowStuffWorks.com
Now, that means that I could get about 5 seconds of power from a capacitor the size of a tuna can, if the fan only took one volt.

This confirms what arizonavideo said (I will still try it tonight, just because I can ). Is there another way to accomplish the same idea? How about those 15-minute charge batteries? The only issue that I could see with those is that they might overheat in my PJ case, but I could always put them near my lens (away from the hot bulbs). These should work, right?

Well, I did read it but chose to ignore it. These aren't really plug&play units IMHO and I just can't see the sense in going for a capacitive storage device to accomplish your goal. What's the big deal in leaving something plugged in for 5 minutes to achieve cooling? But that's simply my opinion.

That said I'd strongly lean towards either dry cells or rechargeables. Your "on" time and the low self-discharge rate of NiCd cells will allow for on-board charging during use. You'd still need the thermal switch anyway to shut 'em off.

Stay away from NiMH due to them being less robust chargewise, self-dischargewise, and heat tolerantwise than NiCd cells. Again, my opinions.

Good luck!

Simple.

Click to view attachment

To charge the cap bank just use a 1 watt 200 ohm resistor to ease the pull on the power supply.
Big caps are everywhere just take old broken things apart and try them.
!!!!!!!!!!!!! CAPS HOLD A CHARGE FOR A VERY LONG TIME SO BE CAREFUL!!!!!!!!!!!!!!!!!!!

Yes, that would be the problem, the "big" part - there's not much room in my case (remember I made it to be portable).

I agree (after some quick research on the benefits and drawbacks of NICD and NIMH batteries) that NICD batteries are better suited for my application. Are there any 15-minute NICD battery chargers? They don't have to be made for AA's; they could be made for, say, a cordless drill or RC car.

In the plog for your PSOne projector you say that the outlet air from your fan is 96.3 degrees fahrenheit.
Is this fan backup for a different projector? If its for the PSOne projector using the fluorescent bulbs I don't think that you need to keep the fans running after the lamps are turned off. It's cool enough already.

Tgreenwood

Oh. Well, in that case, never mind. It still would be nice for others who may be reading this to finish the concept though.

It is an interesting concept and I'm sorry if I burst your bubble. Didn't mean to.

Tgreenwood

That's OK, it just makes my projector lighter without batteries and a charger. I could still test it, but I won't spend much money or time on it. If someone else is interested in the concept, by all means give it a try.

edit: I tried the capacitors, and it's definitely not worth it. If you need your fan to stay on after your projector's unplugged, use one of the 9-volt rechargeable batteries illustrated above.