# 12v to 3v

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Can somebody please give me assistance with a circuit to power from 12v a
variable 1.5 to 3v circuit.  Presently the small fan is powered by 2 x D
cells and runs for around 6 hours.  I would like to have a potentiometer
that goes from 1.5 to 3.0v.
The fan will be finally powered from a 12v SLA battery charged from a solar
panel.

Thanking you in anticipation for assistance.

Re: 12v to 3v

solar

How much current does the fan require?
What is its specified operating voltage?

Re: 12v to 3v

http://icc.skku.ac.kr/~won/electro/psu.html will give direction and get
back to us if you are confused ( low voltage section)

Re: 12v to 3v

Yes, I am confused as the one that looked like it could apply the link was

What I would like is a list of a few components and send me on my way to
Jaycar and then to solder it together.

"atec77" <"atec 77 at hotmail dot com"> wrote in message

Re: 12v to 3v

I don't know how much current the fan takes.  I wonder what the capacity of
D size batteries is?  Googling at the Ah capacity of D size carbon zinc
batteries, and the period the batteries currently last, I would assume the
fan takes 1/2 amp, but lets work on 1.0A for safety.
The specified operating voltage is 3.0v and normally powers from 2 x D
cells.
As this will be powered from a sealed lead acid battery with a small solar
panel, the voltage supply will reach 14.3v.

Re: 12v to 3v

put finger to keyboard and composed:

If the fan current is less than 500mA, then you should be able to
modify just about any mobile phone car adapter/charger. In fact I've
done something like this for a fan inside a DVD player. Many of these
adapters use a Motorola PWM IC, MC34063, which can regulate down to
1.25V. You'll just need to modify one resistor.

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

Re: 12v to 3v

500mA is a little tooooo minimalist for my liking.  I would rather spend the
couple of extra dollars and make it more bullet proof and go for 1.0A.
These need to be reliable.  Have the space of the 2 x D cells to fit the
electronics in, and the cover for the batteries will place the
potentiometer.
I also need it to be variable 1.5v to 3.0v

Re: 12v to 3v

put finger to keyboard and composed:

As suggested elsewhere, you could add a cheap transistor to the
circuit to handle the extra current.

See the application circuits in the datasheet:
http://www.onsemi.com/pub/Collateral/MC34063A-D.PDF
http://www.onsemi.com/pub/Collateral/AN920-D.PDF

A pot in place of, or in addition to, a resistor can give you the 1.5V
to 3.0V range. Having said that, if your fan is a brushless type, then
it may not run reliably at reduced voltages, nor will it tolerate
direct PWM control as suggested elsewhere in this thread.

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

Re: 12v to 3v

You could try this circuit:
http://www.cpemma.co.uk/555pwm.html

It's a basic PWM speed controller. As shown, the output will vary from 5% to
95% of the 12v supply. You want 10% to 25% - read the circuit explanation
where it talks about the potentiometer - by working out the ratios, you can
reduce the size of the the potentiometer and add a fixed resistor to each
side of the pot to limit its operating range.

Re: 12v to 3v

Poxy looking for something a little simpler.  Something like using one of
those three pin voltage regulator chips with a few extra components.  I am
familiar with PWM and is really over kill for this situation.

Re: 12v to 3v

Normal three-terminal regulators will be very inefficient in dropping a nominal
12V supply to 1.5-3V, and givne that the 12V battery is solar-charged I'd expect
you cant afford the 25% efficiency.

You really DO need to go switchmode for any sort of decent efficiency, or PWM.

My personal choice would be an MC34063 and an external transistor if required
for the load current.

Re: 12v to 3v

25% efficiency would be worse than using a resistor - yes/no?

Could I use something like a 2N3055 with a few bits and pieces?

Re: 12v to 3v

25% efficiency IS what you get with a linear reg or a resistor, or any other
non-switching method of losing the excess volts.  They all do the same.

YOU get to decide if 25% is something you can afford, THEN the decison as to
switching-vs-linear can be made very simply.

Re: 12v to 3v

Max sounds like a lost cause - for every good bit of advice he's been given,
he's come back and either rejected it, called it mickey mouse, or passed over it.

Argusy

Re: 12v to 3v

Troll ?

Re: 12v to 3v

Max is far from a lost cause, and most appreciative for the replies.  You
see a little knowledge is dangerous, and was under the impression that if
you used a 3 pin regulator or transistor you did not get the amount of
wasted power like you do using a resistor.

I did investigate the options in kits at Jaycar and DSE this morning, but
nothing really suitable.

I am going to get one of the units this afternoon and see if we can adapt a
12v 80mm Jaycar fan to fit in place of the 3v fan.

Sincerely thanks all who replied.

Re: 12v to 3v

a

You get the easiest build by stripping a mobile cell charger as was earlier
suggested.

Re: 12v to 3v

It is what's needed , if you scrub off 9 vots dc at one amp thats a lot
of wasted energy the solar unit has to make , a simple switch unit save
most of the wasted energy and so less light/energy will keep things
running .Use the circuit or somthing like it .

Re: 12v to 3v

use PWM, it's easier and will give the same effect.

OTOH, 12V fans are easily available, some even have speed control
built in.  I've got one from an old PC powersupply with a temperature
sensor attached, I could replace that with a variable resistor to have
a speed control.

Bye.
Jasen

Re: 12v to 3v

A potentiometer by itself won't be much good, but if you tie it up with an
LM 317 regulator circuit it will be fine.

Or there are more sophisticated motor speed circuits that use pulse width
modulation -- check DSE or Jaycar for kits.