Using a transistor as a power on switch

I will give as much background info as I can. Bear with me.

I am in an alternative energy class and we are working a project using a computer fan to generate a small amount of power to do something. We got it working but realized the small amount of power generated was not going to do much other than lighting a few LEDs which personally I am tired of making LEDs blink. It doesn't take much brains to do that and serves no practical purpose.

My idea, and please let me know if I am headed down the wrong road, was to use the power from the fan, after it has been rectified using a half-wave (we chose a half wave because it gave use a little more voltage), to turn on a transistor that would allow the flow of current from a 9V battery into the Arduino Uno.

I am confusing myself on the amount of current I need into the base and what size resistor to use. I have a 2N3904 but have some 2222 also. Right now I am using a 3V battery to substitute for the fan as it is at school.

Any pointers or admonitions would be greatly appreciated!

-Paul

So you really want a P-channel MOSFET between 9V battery and barrel jack or VIN of the arduino.
When you fan runs, it puts current into base of NPN transistor, turns it on, and collector pulls gate of P-channel low to turn it on and allow current to flow from 9V battery.

So something like this:

Wow. Thanks. That will at least send me down another road. I never used the VIN. That will make things a little easier. I will most likely be back this afternoon with a success/failure report! Thanks again.

-Paul

The problem is your energy is coming from the battery and your fan is simply generating a signal.

You are basically demonstrating how difficult it is to get a useful amount of energy from wind (or solar). :wink:

If you can charge a rechargeable battery, you can possibly "capture" and store energy over a 24-hour period and then do something more useful for a few minutes or an hour or so while draining the battery.

...was to use the power from the fan, after it has been rectified using a half-wave (we chose a half wave because it gave use a little more voltage),

I'm not sure... You might be getting less energy/power... As you may know, power is calculated as Voltage x Current and Energy is Power x Time.

If you are getting more voltage with the same load, or the LEDs are brighter with the half-wave rectifier, then YES, you are getting more power/energy with the half-wave setup (under these particular conditions).

(we chose a half wave because it gave use a little more voltage)

No you don't you get less. With half wave rectification you are throwing half the power away.

Grumpy_Mike:
No you don't you get less. With half wave rectification you are throwing half the power away.

Just because you are using only half the cycle does not mean you are getting half the power. The situation is more complicated. There is an optimum load that will give you optimum power output. Using the full cycle may slow the fan which will lower the peak voltage. If your load is high enough to stall the fan it may be possible to increase power output by reducing the percentage of the cycle which conducts.

Just because you are using only half the cycle does not mean you are getting half the power.

Yes it does.

If your load is high enough to stall the fan it may be possible to increase power output by reducing the percentage of the cycle which conducts.

Silly.

Just think about it for a second, you are extracting power from the turning of the fan. Any power extracted increases the mechanical load on the fan and slows it down. Half wave rectification means you are only drawing power from half the cycle. This is exactly the same as reducing the load.

Grumpy_Mike:
Just think about it for a second, you are extracting power from the turning of the fan. Any power extracted increases the mechanical load on the fan and slows it down. Half wave rectification means you are only drawing power from half the cycle. This is exactly the same as reducing the load.

You argue exactly my point. You can't look at it as "...throwing half the power away" when you use half wave rectification. You are only throwing half the cycle away, not half the power.

You are only throwing half the cycle away, not half the power.

I can see us two are not getting on here.

Do you like confusing beginners and incorrectly nit picking?

Grumpy_Mike:
I can see us two are not getting on here.

I thought we were getting along. I am sorry if I offended you. I thought your post needed clarification.