using potentiometer in circuit

[phlox]

hi all, im new here and i have a question which im not too sure about.

im trying to build a simple circuit with a couple of old laptop battery cells,which are roughly 2.8v each, im putting them in parallel to achieve most amperes because im going to be using it on my model airplane and the glow plug needs quite alot of amps, anyway i used 6 rechargeable 1.2v batteries and they work fine but i want to use the laptop cells so that i can se the others batteries in camera or so, but my question is this, ive put a potentiometer in the circuit and it drops volts to what is required, 1.2 -1.5 volts for glowplug, but then the amperes isnt enough, so sdoes the pot throw away the amps also? or how would i go about building the setup without losing amps but still be able to bring down voltage ?

any help would be appreciated

 

[BobK]

If you are measuring 1.5V across the glow plug, then it is drawing all the current it will draw at 1.5V. There is no such thing a limiting the current without changing the voltage.

Bob

 

[phlox]

so basically ,say the batteries put out 10 amps @ 2v ,then @ 1.5 v they would say put out 6amps? so as i understand it the amperes also drop as the voltage drops?, what was interesting as well was @1.5v i measured it without the glowplug and it gave 1.5v but with the glowplug connected there was no voltage? > and the glowplug didnt light up

any suggestions?
thanx

 

[BobK]

A battery that can supply 2V at 10A can also supply 1.5V at 10A. You would put a resistor in series that would drop the 0.5V based on Ohms law:

V = I R

0.5 = 10 R
0.05 = R

So a 0.05 Ohm resistor in series with a circuit that draws 10A at 1.5V would let it operate off 2V.

I think I know now what you were trying to do. You placed a potentiometer across the battery and adjusted it until the middle lug read 1.5V then connected this to the glow plug. That will not work. The resistance of the glow plug is probably much less than the resistance of the POT so, it changes everything when you connect it.

Bob

 

[phlox]

Thanx very much, the only reason i put in the pot was to maybe utilize the full 2.8v of the batteries for another project maybe, but its okay,mill build a new battery pack, anyway, so i just remove the pot and replacevtht with the resistor in seies on the positive terminal and all should be well..

Ill see if i can find a resistor tommorow and put it in and se what happens .

 

[phlox]

Oh and another question, if the resistor in series works, how about charging? , i will probably need another resistor for a 12v charger which is the only one i have at the moment

 

[BobK]

First of all, you MUST measure the current draw from a stiff 1.5V supply before calculating the resistor. Do you have any test equipment?

Second, If you use a resistor to drop the voltage from 2.8V to 1.5V you will have to calculate the power in the resistor and choose an approriate rated one. Once you know the current draw at 1.5V you can calculate the power as I**2 R. For example, if it is a 1 Ohm resistor carring 2 Amps the resistor will need to be at least a 2 Watt resistor and will get very hot if you don't move up to a 5 Watt one.

Finally, do not try to charge the 2.8V batteries from a 12V battery charger. Chargers need to montor the voltage and current they are charging at and depend on the kind of battery they are charging. They are not interchangeable.

Bob

 

[phlox]

I cant tet a charge for the cells i currently have, and therers no store in town whrer i can buy resistors and so on,but i found a place on internet where i can buy cheap rechargeable 1,2v 2000mah batteries for next to nothing, so im just gonna buy about 8 or so, they sell in packs of four,and solder them in parallel and use a normal batery AA battery charger to charge them, uhm ive read somewhere that batteries in parallel charge from the one with the lowest voltage first and then go on until all are charged fully? Dont know if its correct but ima try it anyway.

I have used six 1,2v 2000mah batteries in parallel on the glow plug and that works great but i use the batteries in my camera and so on as well, so im gonna buy those cheap ones anyway and just use them for the model airplane. Maybe later on ill build something better but for now that is the easiest way and it will work.