# 12 volt to 6 volt with constant current

Discussion in 'General Electronics Discussion' started by tarnedoo, Sep 26, 2012.

1. ### tarnedoo

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Sep 19, 2012
hi, I am new to the electronics world and am really interested in the subject
I got a problem while doing a DIY project , the details are under
Project : Converting a manual car jack to automatic
Resources available and things done :
I have been able to get a 6 v DC motor with its gears(found one in a children toy car which has a capacity to pick a load of around 12-18 kg)
I have successfully attached the motor gears with the jack
Requirements and problems being faced :
The toy car operated with a 6 volt 7.2 Ah SLA? battery
I want to get the motor connected with a cigarette lighter socket in my car which operates at 12 volt and there has a fuse labelled with 15 amps max in the fuse box
Basically what i have understood is that i have to convert the 12 volt to 6 volt without decreasing the current because the motor will draw the current what it needs , right ?
I want some directions in making such a circuit
What i found out was that i can use only some resistors to do the job, or use some voltage regulator like LM317 ???

2. ### GreenGiant

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6
Feb 9, 2012
you are going to need a few regulators to get the current to 15 amps, but I highly doubt that the motor will draw anywhere near that, if it typically works on a 7.2Ah battery, Im assuming that the car runs for a while on a full charge
Resistors you are going to need MASSIVE ones, 6V passing 15A is going to be.... 0.5 Ohm rated at 90 Watts, not going to happen

Take a look at the motor and see what it is rated at, if it is rated at 12V then you can just run it as is

3. ### tarnedoo

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Sep 19, 2012
Thanks for the reply, the motor is rated at 6 volt (sorry i forgot to mention earlier) ,the only thing written on the motor is "pm-550-50132 6v/DC"
,so I am a bit unsure that i should run it at 12 volt .....

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Aug 3, 2010
5. ### tarnedoo

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Sep 19, 2012
No i never said that the motor operated on 15 amp, i said 15 amp was the fuse of cigarette lighter in my car ....
yup the motor looks like exactly the one in the picture but the model number differs a bit, mine is PM 550 (6 volt), and the one shown is RS 550..

6. ### rob_croxford

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Aug 3, 2010
ahhh sorry i missunderstood.

Ov well you could use a voltage divider to drop the 12V to 6V . However you mentioned you need to keep the current constant. by that did you mean you wish to supply the motor with enough current to run or you want to keep the motor at a constant current (torque)??

7. ### tarnedoo

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Sep 19, 2012
sorry if i was unable to express myself clearly,
basically i tested the motor on a ac to dc wall charger which was 220 to 6 volt at 500mA , but the motor didnot operate, i again tested it with 12volt DC from cigerrate car lighter , it operated successfully, but i know that on 12 volt the motor wont last for too long caz it is rated for 6 volt. Basically i need to provide it with 6 volt and enough amps from a cigarette lighter socket to keep it operating ..... Hope i was able to express myself more clearly this time , sorry for the last time

8. ### rob_croxford

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Aug 3, 2010
well technically you should be able to drop the voltage out of your ciggerette lighter using a voltage divier to achieve 6V as the output has 15A avaliable. But it would be usefull to know the current rating of the motor. Do you have a data sheet?

A better way to do it is by using a PWM signal to switch a mossfet accross the voltage rails. A PWM signal is simply a square wave with a varying duty cycle (how long it is "high" and how long it is "low"). For this application you could use a 50% duty cycle which would supply 50% of the voltage to your motor. Which would be 6V.

9. ### tarnedoo

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Sep 19, 2012
thanx for the reply but seriously i have got no clue as to what u said in your second paragraph " a better way to do ............ "
I will surely get into details of PWM signal, mossfet etc etc
Love this way of learning

10. ### rob_croxford

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Aug 3, 2010

Look up DC/DC converter - specifically by using PWM. Im sure a lot of sites will tell you to use a Microcontroller but you should be looking up 555 timers and how they function.

This should give you the basis of formulating a design but also touch up on your DC motor theory as that will come in usefull.

Have a look at those and see if you can understand how they operate and draw some conclusions on how you could utilise these ideas in your design. Then come back and ask questions on anything you dont quite understand and we will try our best to help you out

11. ### MrEE

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Apr 13, 2012
Since the motor is not running continuously, efficiency is not an issue. Try a power resistor if you have one handy. Insert one in series with one of the motor terminals.
It won't hurt the motor if you exceed the rating by a few volts. You are only running it for a few seconds at a time. The resistor value should be around a few ohms rated about 5 - 10 watts.

12. ### tarnedoo

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Sep 19, 2012
Thanx a lot I am surely going to need your help in that

13. ### tarnedoo

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Sep 19, 2012
@MrEE ....surely am gonna check it as well, thanx

14. ### BobK

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1,688
Jan 5, 2010
I would take a wild guess that it runs at about 3A and then cacluate the resistor:

V = I R

You need the resistor to drop (12 - 6) = 6V so 6 = 3 R, R = 2 Ohms.

The power is V^2 / R so 6 * 6 / 2 = 18 W.

Find a 2 Ohm at least 20W resitor or, more likely 2 10W 1 Ohm resistors in series then try hooking this up and measure the voltage across the resistor. Adjust the resistance up or down then to get it close to 6V. The resistor will get warm, but as long as you run it within its wattage rating and keep a lot of air around it, it should be fine.

Bob