DC Basics help

[Grizzman]

Could someone please point me in the direction of a page with the basics of DC circuitry design layed out.
I haven't done any since college and my minds a bit rusty on it. Here's what I want to do. I want to have a capacitor charged when a switch is flipped. the when the switch is released, the capicitor discharges to run a fan for about five minutes.
A brief diagram:
_______/\/\/\___
| r |
^ |
Ca f
- |
| |
__|______________|

And here is the type info I'm looking for:
If the Capacitor,Ca, discharge is say 12V, 200mA
and the fan,f, requires 12V, 80mA what size resistor,r, is required to lessen the amperage? Or will it lessen the voltage?
This is what I need a brief bit of refreshment on. I'm not even sure of what words to use in a search engine to find this out.
any direction would be greatly appreciated.
Thanks,

 

[nbucska]

If you want to store the energy in the cap, it will be HUGE
and expensive but if you only want to use the cap to
generate the delay it is much simpler.

I can't understand the drawing or your description.
"Flipping" may mean changing steady state (On to off or off to on)or momentarily (off-on-off or on-off-on).

Releasing the switch has no effect unless the switch
is spring loaded e.g. momentary. I suggest give time diagram e.g. :

SW: _______--------________ - = on, _=off
FAN:_______--------------___

-------------------<====> 5 min



<nbucska@pcperipherals DOT com> subj: eng-tips
read FAQ240-1032

 

[Grizzman]

Thanks for pointing out my poor choice of words. I reread my first post and can see from where the difficulty stems.
By flipped I meant when the switch is moved from position 1 to 2, it charges the capacitor. when it is released, it will flip (or spring) back to position 1 from position 2 to close the circuit containing the capacitor and fan.(and open the circuit that charged the capacitor)
I realize the difficulty of emailed questions and the generality of mine in particular, that's why I was looking more for a web site that might help me refresh my basic understandings.
If a capacitor can't be set up to release a small current relative to its voltage it might not be what I'm looking for. That's just the first thing that came to my mind.
The basic idea is I need a fan to run for about 5 minutes after a button is pressed and reset each time the button is pressed. In fact the voltage and amperage numbers I listed above aren't the actual ones I'll need but something to help illustrate the kind of information I'm looking for.
Being as structures is my field, I'm not all that familiar with electronic terminology.
Thanks again,

 

[nbucska]

To store enough energy in a cap to run the fan for 5 min-s
requires big and expensive capacitor. I suggest use electronics ( e.g. a 555 timer or at least a transistor )

Let-s assume the switch is momentary, and connects
its output to +5V , when pushed:

SW: ___----------_______ the FAN can run
a.) ___----------------__ 5 min. longer than SW pushed OR
b.) _____________------___ 5 min. after SW released
c.) ___-----_____------___ 5 min. after each SW transient

Which one do you want ?


<nbucska@pcperipherals DOT com> subj: eng-tips
read FAQ240-1032

 

[Skogsgurra]

Is it necessary to have the energy stored in a capacitor? I mean, is the fan isolated from any stationary energy source durong operation? Or is all you need some kind of timed control of the fan?

In the latter case you should consider a simple time relay. In the former case a (rechargable) battery and a simple timer would be a better solution than a capacitor with its varying voltage and rather limited energy store.

So, is your solution what sprang to your mind and what you think is needed? Or is it the timing that you need? Or energy store? Or both?

 

[IRstuff]

You need to determine what the minimum voltage droop acceptable to the fan is. The time integrated current divided by the allowable droop will equal the capacitance.

Assume 2V droop and 80mA for 5min --> 12Farad capacitor

TTFN

 

[Grizzman]

skogsgurra: The solution is what sprang to my mind. The fan doesn't have to be seperated from it power source. My degree didn't get much into electrical and we never touched on relays or timers, so I'm just working with the knowledge I already have.
The timing is based on an air change, which is also a factor of fan speed.
I hadn't thought about the gradual slowdown of a capacitor. Sounds like there are a lot of alternatives out there.
nbucska: I need the switch to engage the fan for five minutes. whether the switch remains pressed, or is pressed numerous times after that, I need the fan to run for five minutes. It'd even be ok if pressing the switch a second time during the five minutes after the switch was pressed the first time if the timer was reset to run another 5 minutes. I'm trying to keep the schematic simple so I can make a layout and have a basic understanding of what I need so when I go to radio shack for the parts, I can clearly explain what I'm looking for.
Again, thanks for all the input and patience.

 

[nbucska]

Use a 555 timer. The momentary switch grounds the trigger input which is pulled up to V+ with a 10Kohm.

The output drives the anode of a Solid State Relay with 330
ohm in series, the cathode is connected to GND. The output
is in series with the AC fan.

The 555 is in monostable configuration. Search the
<

http://www.national.com>

for LM555 and get the data sheets.



<nbucska@pcperipherals DOT com> subj: eng-tips
read FAQ240-1032

 

[peebee]

Grizzman -- I think the big point of confusion here is if you plan to actually run the fan off the energy stored in the capactor, or just to use the capactor as a timer to control the fan:

1. Run the fan off the energy stored in the cap -- bad idea as others have pointed out. Caps are really not designed for that, you should use a battery for that instead. The only reason I could think of that you'd ever want to do something like that would be to demonstrate to electrical newbies that capactors store energy.

2. Capacitor as a timer is not a bad idea, but not a great one either. You'd discharge the cap to ground through the resistor, and the rate of discharge would be determined by the time constant RC. You'd need some kind of comparator to throw a switch to stop the fan once the voltage fell to a predetermined level. You could do that with a simple 1 or 2 transistor circuit.

But even #2 is not an optimal solution unless you're trying to demonstrate electronics or something. Much better ideas include the timing relays or special purpose IC's. You'll get higher accuracy timing at lower cost and zero design effort.

Don't get me wrong -- if you're just looking around to mess around with electronics, or trying to get some high-schoolers interested, have fun, just like you would if you were building a solar-powered car or something. But if your looking for cost & time efficient solutions, go buy something off the shelf -- same as you'd go buy your commuter car from a dealer. . . .

 

[sdmays]

Grizzman, do a search for one shot trigger. You mentioned going to Radio Shack. From what I have read above, I believe a 555 timer will work AND you can buy them by the dozen at practically any Radio Shack. Some Radio Shacks might still have the old (OLD) 555 Timer book written by Sims III. The given Radio Shack where you go might not have an LM555, instead it might have a CMOS version of it instead (which is fine, since they wire the same). Take a look at the link below (towards the bottom of the page):

http://ourworld.compuserve.com/homepages/Bill_Bowden/page9.htm

You can download the datasheet (search for LM555) on the internet and learn a lot more about the timer, including how the calculate the resistor and capacitor you need. I don't use the timer, I would prefer a microcontroller, but I think you will get good use out of it. Also, should you have other simple timing projects in the future, you will have a good tool in your belt!

 

[emmgon]

Grizzman, I think it is very impossible to have a capacitor-charging circuit that can provide power to a fan for five minutes. I agree with most of the respondents. Try to learn how to use the 555 timer IC. This chip is very easy to use and is very cheap. You just need to add some capacitors and resistors into this chip...dont forget ofcourse the switch.

--------------------------------
Emmanuel A. Gonzalez, E.C.E.
ECE Department
De La Salle University - Manila