3v voltage regulator

[electronicsfreak]

After searching around I am having a very hard time finding a suitable voltage regulator for a project...it needs to be able to output around 3v and supply at least an amp. The voltage going in would be around 12v.

any ideas on what I should use?

 

[Papabravo]

Lemme see, for a linear regulator

(12V - 3V)*1A = 9 Watts

I would star with a biga heat sink
Look for an adjustable or fixed regulator in a TO-220 package
Don't put your lips on it while it's running

 

[audioguru]

An LM317 can make any voltage from 1.25V to 34V with two resistors and two capacitors. Its max output current is 1.5A so it will be fine if the heatsink is big enough.

 

[electronicsfreak]

hehe, yeah. well, I've been looking at the LM317, but do you think that it would be completely necessary to have protection diodes routed specifically for the regulator? It will be powering a small motor controlled from an output of a PIC, and the motor will already have a protection diode connected with it in parallel.

 

[kchriste]

The smarter thing to do would be to use a 12V motor.

 

[electronicsfreak]

Well, I've already got many, already geared down motors that I'm planning on using on this project that all run on 3v and this same circuit that is described above will have to be multiplied by 13. so, it would be a lot easier if I just lowered the voltage running to the 3v motors then going out and buying 13, 12v motors.

 

[audioguru]

The max allowed output of a PIC is 25mA which is too little current to power a motor.
The PIC can drive a transistor that might power the motor.
The motor needs to have a diode across it, not the voltage regulator. This diode dumps the current that would try to make a voltage spike into the output capacitor and voltage setting resistors of the regulator.

 

[kchriste]

Well, if you are stuck on using 3V motors on a 12v supply, then at least use a switching regulator like the LM2679-3.3 or equiv to cut down on heat and wasted power. Like AudioGuru mentions, you'll need to drive the motor with a FET or transistor. Do you intend to have the motor direction reversible? Speed control? If so, PWM and some H-bridges might be the ticket instead.

 

[electronicsfreak]

**broken link removed**


above is just a sample of one of the channels that this thing will be controlling
I know that the motor would need to be driven by a transistor and then in turn the actual pin that is on the PIC, just like the DPDT relay next to it. (note the resistor values haven't been put in yet because I still need to measure the resistance of both the motor and the relay)

what I was wondering is if it is completely necessary to put in a few protection diodes in line with the voltage regulator as I've run into a few sample diagrams where there are a few. (such as in this datasheet--figure 20 LM317 datasheet pdf datenblatt - ON Semiconductor - 1.5 A Adjustable Output, Positive Voltage Regulator ::: ALLDATASHEET :::)


also, note that where the +3v line is is where the regulators output pin would be attached.

 

[Hero999]

You can power your 3V motor directly from 12V by using PWM. If you limit the maximum duty cycle to 6.25% the RMS voltage across the motor will be 3V.

 

[arhi]

If you limit the maximum duty cycle to 6.25% the RMS voltage across the motor will be 3V.

just for the reference, you can calculate the RMS voltage using the following formula:

Vp = peak voltage
D = duty cycle
Vr = RMS voltage

Vr = Vp * sqrt(D)

in your case, Vp = 12V; Vr = 3V
D = (Vr/Vp)^2 = 0.0625 = 6.25%

 

[electronicsfreak]

Sorry, I probably should have mentioned this before. This circuit will be powered by batteries, meaning that they'll slowly drain until it hits around 11v which is when there won't be enough to throw the relay. The reason why I need to use a voltage regulator for the motor is because I need the voltage that runs to the motor(s) to stay the same all the time. (this is why I can't just use a seperate battery supply for the motors at 3v)

 

[Hero999]

The speed of the motor shouldn't drop much as 11V at 62.5% duty would give 2.75V RMS to the motor, if this is a problem then you could measure the supply voltage and increase the duty cycle as the voltage drops.

 

[arhi]

The speed of the motor shouldn't drop much as 11V at 62.5% duty would give 2.75V RMS to the motor,

I'd say 6.25% not 62.5% but I might be wrong.

Also, with low duty cycle (as 6% is) you will have to tune frequency to avoid buzzing sound and to get smooth motor operation

 

[Hero999]

Sorry I did say 6.25% the first time, I got the decimal point in the wrong place the second time round.

 

[electronicsfreak]

Thankyou for all of your help, but I'll probably just end up using an LM317 because I don't quite fully understand how I could incorporate a Pulse Width Modulation system into the output pin on the PIC. I know it is a very large amount of power loss through heat, but it won't be on for anymore than half a second at a time and at the most would probably only have 3 motors on at a time.

 

[blueroomelectronics]

The linear regulator is wasteful and will simply turn excess voltage & current into heat. Your DC motors won't be very powerful if they're running significantly below their rated voltage. PWM and motors are a much better solution.