How to determine size of small electric generator

[sstrik]

I am designing a small timing circuit and need a power source of around 4 volts and producing around 15 to 20 mA for around 5 to 10 seconds.

I am thinking of using a simple generator (iron core, copper coil) and need to get an idea of how large this needs to be. Can anyone help me with some calculations that I can use to get an estimate on how many coils, size of magnet rotational speed required etc.

Thanks


Steve

 

[nbucska]

15 mA seems to be awefully high current for a simple timer.
Can you give some spec's? couldn't you use a solar cell for
power ? <nbucska@pcperipherals.com>

 

[pebe]

Wouldn't it be easier and cheaper to use 3 x AAA cells?

 

[sstrik]

Solar cells are not practical for this application (object may need to be used at night). this is only a concept at this stage and I can't give out specs.

Yes, AAA cells would be easier and cheaper, but they don't meet my other requirements as they are far too large for our application, but a single 1/3N cell is possible, if I can overcome the shelf life and temperature limitations that batteries impose.

The timing circuit is only part of it. The power source is also charging a capacitor with sufficient energy to function a device at the end of the delay.

At this stage I am just trying to see if the electric generator idea is feasible with the requirements that I have, but I don't know how large this device needs to be. If I can get some ballpark calculations on required field strength from the magnet and number of coils of the copper and required RPM, I should be able to decide if the idea is worth pursuing further,

Thanks


Steve

 

[nbucska]

How much energy does the switched device need ? Can you reduce it ?How much life time do you need ? What starts the timer ? ( manual/mechanical switch, power-on, microcomputer etc.)

Can you tell what this device is ?
<nbucska@pcperipherals.com>

 

[sstrik]

I'd rather not at this stage. The switched device requires 1 amp and 1 watt to operate. The timer is started manually by the user. The power source only needs operate between 3 and 10 seconds maximum and only once. The time requirement is determined by the customer.

thanks

Steve

 

[nbucska]

How long do you need the 1 V x 1A ? The timer is negligeable
besides this... Do you want to supply the DEVICE, too, with
the generator ? <nbucska@pcperipherals.com>

 

[sstrik]

The device needs to be fully supplied complete to the customer with no further assembly or preparation required prior to use. We are using a capacitor to provide the 1A, 1W and the output from the timing circuit shorts the capacitor. I can't explain more as my knowledge of electronic circuits is minimal. I think (suspect) that the required supply length of time is in the milliseconds range.

 

[nbucska]

If you need only 1 Watt for 1 mSec and plus the need for
the timer for a few sec's, this is less than 2 mW -- easy
to generate by a coil/magnet.
<nbucska@pcperipherals.com>

 

[sstrik]

How do I determine the physical characteristics of the coil and magnet? eg size of magnet, proximity to coils, number of coils, diameter of wire?

Thanks

Steve

 

[nbucska]

The approach depends on quantity : if this is a test only
-- single piece -- overdesign it by any acceptable factor.

If massproduct: Start with selecting the cheapest magnet
-- and experiment.

The coil can be the thinnest if the R is low enough -
the duty cycle is so low that it wont overheat.

You may need to build several ones, to get the cheapest
and easiest to manufacture design.

E-mail me as much data as U can.



<nbucska@pcperipherals.com>