re: making a 3v/1.2v charger with only a 3v supply?
 

can this be possible? I have a 3v 700ma supply I use for charging my dual cell speedsters. can I insert some sort of bypass circuit on a toggle switch with a resister in it to bring it down to 1.2v for charging my normal single celled cars?

Or do I need to get an adjustable power supply? I'm cheap so the charger I have was pirated from my (RIP) portable CD player.

If this can be done what resister do I need to find for this application. Sorry i don't know how to draw a schematic plan for what I'm talking about.

I think the IQ on this bulletin board will prevail though :D

look in bit science, charging with voltmeter

Easiest way is to construct a voltage splitting circuit. Basically just two resistors of specific values wired in series with the power supply. Then you will basically tap the positive charger lead into the node with the correct voltage and the negative charger lead onto the negative power supply. I'll see if I can draw up a schematic if I get a chance.

Here is a basic diagram of what you need. The two resistors are labelled R1 and R2. The easiest way is to select two resistors of equal value, this will give you 3v at the node labeled 1, 1.5v at the node labelled 2, and the node labelled 3 will always have 0v.

If you want to be more precise you can mix and match resistors and get 1.2v at the 2nd node, but you have to understand how the equation below works and how it is applied to circuits.

V(voltage) = I(current) * R (resistance)

thanks mpbiv!

I don't fully understand yet but with a little study I will get it. you are saying I need to be able to tap power from two positions from either a 3v charge position, or one that is 'resisted' down to a lower level for lesser voltage.

what resisters would I be after? Couldn't I just set up a toggle on another pathway to the positive charging terminal with just one inline resistor to bring it down to an acceptable 1.2 voltage for single cell charging?

If my charger is 3v and DC output is 700ma-- to size a resistor for lower voltage charge (1.2v) I would write as:

1.2v=.7 * X ohms, and solve for X?

So a 1.71 ohm resistor is needed if i were to just simple set up a toggle between full charger power and a reduced pathway? I know this would change the beautiful diagram you made, but am I on the right track for what I want to accomplish here--just charging dual and single celled cars?

Thanks again for your response!

3v 700mah charging station is basically what comes with every car. 2 aa batterys =3v. most standard AA batterys have anywhere from 1100 - 2600mah the cheapo rayovaac(blue silver) have ratings here.. looks to be somewhere around a 2000 average (http://www.rayovac.com/busoem/oem/specs/bat815.shtml)
to make it work as a full 3v just insert a switch and place it somewhere on the outside of the controller. wire the the batterys so that current has to go through the switch befor it hits the board in the controller for 1 cell. and another set of wires that goes directly to the charger...

or you could get one of those fancy switches is not an on off... i.e. on position 1, and on position 2. that way position 1 could be 1 cell. and position 2 could be 2 cell...