Question is in the title, im trying to control 2 4.5v DC motors with a 12v battery (i need the 12v for other purposes)
What is the best way to do this?
A solution being using a voltage regulator like L7906 but this would imply a lot of dissipation heat regulation etc...
Is it possible to power the motor directly with 12v trhough the motorshield without killing smthg? and if yes how do i proceed?
Thank you in advance
Nate
dk electronic motor shield ? Which one is that ? Can you copy a link to it in your post ?
Can you estimate the load current for the motors in operation ?
If so , you can add a dropping resistor for each motor, similar to what your car uses for the different blower speeds.
Let ILoad =0.500 A (500 mA)
the Rdropping= (VIN-VLoad)/iLoad= (12V -4.5V)/0.500 A = 15 ohms
Power rating for RDropping
PR= ILoad * VRDropping = 0.500 A * 7.5 V = 3.75 Watts
RDropping = 15 ohms, 4W (5W ok)
One of these for EACH motor IN SERIES with the motor will drop the voltage to 7.5V and limit the current to 0.5A (500 mA)
We've reasoned together on this "dropping resistor" proposal once before as I recall.
What's the effect of the "dropping resistor" when the motor has little work to do and, by implication, draws less current?
If you have another idea , by all means let's hear it.
This is an alternative:
http://www.ebay.com/itm/LM2596-Step-Down-Module-DC-DC-Buck-Converter-Power-Supply-Output-1-23V-30V-New-/251066005460
If it was a couple of volts then I'd say, "definitely pad it with diodes", but that might be pushing the envelope in this case.
But then again, that's better than resistors.
So, yes, 7 or 8 resistors diodes, and both motors could feed off that.
Be that as it may or may not: nothing is known of the "dk electronic motorshield", so we're just shooting into the dark.
That's the context for deciding what to do.
PWM with a maximum duty cycle of 40%. Do not waste your time or energy with dropping resistors.
Why do so many cars use them for the 5-speed A/C Blower motor ?
Than you for your answers, the motorshield is the adafruit motorshield V1 (Given by a friend so i wasnt sure about the reference) : Adafruit Motor/Stepper/Servo Shield for Arduino kit [v1.2] : ID 81 : $19.50 : Adafruit Industries, Unique & fun DIY electronics and kits
Motors are DAGU gearmotors : http://www.robotshop.com/en/dagu-gear-motor-pair-90-degree-shaft.html
12V pb battery is there to power both motors and 12v fan (thats why i kept it)
Regards
Nate
Those guys, with their darlington outputs, lose 3-4V (it's load related) anyway.
Maybe there's nothing to worry about, maybe you'll be OK with the limited-range PWM idea, maybe lots of things.
raschemmel:
Why do so many cars use them for the 5-speed A/C Blower motor ?
Assuming that's so, I suppose it's because their loads don't change so much as a motor running wheels (uphill, downhill, faster, slower, start and slow down) on the floor or whatever.
Does that seem plausible?
Well, I see that Runaway Pancake is willing to take some risk, even using PWM to allow the 4.5V motors run with 12V power supply.
You can use PWM by using the analogWrite() function with a value of 0...128, instead of 0...255. So the motors get a maximum of 50%.
I would rather use a DC-DC converter. One of about 2A and with adjustable output.
With 12V supply, to motor voltage will probably be around 9V [it's that darlington thing]
PWM notwithstanding, the motors still get full amplitude (whatever the output is.)
2A DC-DC converters aren't cheap. At that rate, why not just get a 12V motor and do away with the complication?
It's not that I'm "willing to take some risk", there are no easy answers here, something simple, inexpensive, one-size-fits-all UNLESS we go back to the diodes.
So, if "we" have 12V and we lose 3 in the motorshield output, that leaves "us" with 9V. 5V from 9V leaves us with another 4V to lose, approximately 4 diode drops. [Or get out your wallet. That's simple.]
Dwg attached.
1 dollar and 7 cents $)
For 7 cents less, you get 50 diodes 
However, I agree, why not just get 12V motors.
Ok that means that in any case i have to go down to 5V. Im thinking to finally use the L7806, since Ill loose power anyway. I saw exemples on the datasheet that mention capacitors in // wiith the component, are these mandatory, and what values to use? And I guess ill need heat regulation too...
Again thank you for your help
Natural
If you are thinking about the 7806, then a maximum of 50% pwm has suddenly become a good option.
Using the 7806 with a heatsink is the worst option, but it will work.
Can we talk you into one of the better options ? (max 50% pwm, 12V motors, dc-dc converter, diodes).
A capacitor for the power for the motors is good, but perhaps not needed. For example a value of 470uF or 1000uF, but anything between 220uF to 4700uF is okay. The motor shield probably has a few capacitors, so you shouldn't need to add extra capacitors.
There is one combination that doesn't work: the 'diodes'-option with a big capacitor after the diodes. In that case use the capacitor at the 12V and not at the lowered voltage after the diodes, or the capacitors could be slowly charged with a higher voltage and the motors will get too much voltage when turned on.
Peter_n:
For 7 cents less, you get 50 diodes
You're making something of a "straw man argument."
You don't need 100, or even 50, just 8.
I have a plenitude of diodes around here.
Natural:
Ok that means that in any case i have to go down to 5V.
Well, it doesn't.
You can lead a horse to water but you cannot make him drink.
It seems you're hell-bent on going to a lot of effort to make the problem fit the solution.