View Full Version : Dropping 12V to 6V or 8V

14-06-03, 02:10 PM
I need to know the simplest circuit I can make up to drop 12V down to 6V or 8V without using something like a $50DC 'model train speed controller'. Its for an electric water pump and I don't have the funds to buy the electronic (automatic) controller right now, so plan to run it on a 8v/12v switch or something like that. Any advice will be greatly appreciated :)

A mate also mentioned a 'thermistor' kit or something that basically acts as an electronic resistor based on temperature (thats how the electronic controllers work, but they're $150!). Anyone used or setup one of these? Cheers,

14-06-03, 04:23 PM
Look up voltage regulators at jaycar.com.au or equivalent.

Something like a LM338K +3-35V 5A Voltage Regulator will do the job if 5A is enough, if you aonly need 1A or so there are cheaper ones.

Look up the data sheet using the model number on the internet and they will have sample circuits to set the regulators output voltage to what you require. It will only be a matter of a fer resistors and capictors, very easy.

You will also need a heat sink, buy a nice big one without going too crazy as the voltage drop is turned into heat.

Thermistor will vary its resitance according to temp. I suppose you could use this in conjunction with a simple transistor switch circuit which will turn on when the resistance allows enough current to switch the transistor.

16-06-03, 08:23 AM
Depending on the current requirments and whether you need a solution thats is efficent power wise there are several options.

Ajustable voltage regulator as mentioned good for up to 5 amps.

PWM contoller based off two 555 timers (or a single 556) the current handling of which is dependant upon the switching transistors used.

A dropping resistor, the simplest chepest easiest method provided suitable resistors are selected.

16-06-03, 12:26 PM
In the back of a dick smith catelog is quite a few simple circuit diagrams for voltage regulators and the like, also all the data on the regulator ics you'd be lookin at using. Coupla bucks for the mag at any of their stores. What current do you need to provide???

16-06-03, 01:50 PM
Thanks for the replies :) have been really appreciated. I think the max current is around 7.5amps @ 12 - 14v, but I can pull that straight from the battery. This lower voltage would be on a switch to lower the speed of the pump. I think 5amp would be fine at 6 or 8v - but i'm no electrician :D

I've got a dick smith catalogue here so I'll have a look through it. Eventually i'll be running the pump from the ECU i'm going to buy which has configurable outputs/inputs (ie, vary voltage depending on water temp - just as the expensive standalone controller does).

16-06-03, 05:43 PM
What is the water pump for a water air intercooler?

16-06-03, 08:44 PM
No, coolant water pump... its a davies craig unit (flows up to 80L/min @ 12v)

18-06-03, 07:21 PM
use ohms law to figure out the draw at 8v
should be a MAX of 5A neat while 6v is 3.75A
if that helps u at all

19-06-03, 12:01 AM
Cheers buddy... i'm a moron for not thinking of that one :D

19-06-03, 02:34 PM
Dont use a regulator... its not needed.
Just use a resistor (or a few to handle the heat).

Try using a ignition ballast resistor... itll drop 12V to 6V at around 8A, so, it might be around the correct value for you.

If not, also look at early 80s injector current limiting resistors.

If you still dot get the correct voltage with those, buy a few 10W resistors from Jaycar, etc and play with those.

tandy ass
19-06-03, 05:26 PM
Dont use ONLY a resistor, you'll be better off with a regulator. MWP, if you forgot about electric motor startup current, you're forgiven :)

If you simply use a resistor to limit the voltage, its VERY possible that the motor could overheat from not starting as startup current is easily 5 times more than operating current.

On the other hand, you could place something like a 10,000uF/16V capacitor across the resistor to give a huge kick in current when the motor is started. Negative of the capacitor goes on the motor side of the resistor when spliced in with the positive line to it. If you splice the negative line, the negative of the capacitor goes to ground, or away from the motor.

19-06-03, 05:38 PM
And anyways, for the extra $5 and 5 minutes soldering involved for the reg, you will have a rock solid voltage of your choice relatively independant of the battery/alternator voltage and the input resitance/current draw of the device your running.

25-06-03, 04:43 PM
GREAT NEWS (for me) hehe.... Jaycar have a new DC voltage controller kit that will handle 10A (i need to handle 7.5A max)

10A 12VDC Motor Speed Controller Kit $23-$24 :)

25-06-03, 05:49 PM
wheres the fun in that :p

25-06-03, 06:47 PM
lol, well because its reliable & I can trust it & get it working ASAP :)