Black Lives Matter - Action and Equality. ... Adafruit is open and shipping.
0

Power Supply
Moderators: adafruit_support_bill, adafruit

Please be positive and constructive with your questions and comments.

Power Supply

by dze on Wed Sep 01, 2010 3:06 am

Hi there,

I found (maybe) the right motor for my needs. The DC motor is running at 6 volts but max 12 amps. This kind of power supply is really hard to find. What I've found so far is 12 volts 10 amps. I guess i can run the motor that i'm going to use at 10 amps but how do i lower the 12 volts to 6 volts. I understand that there's a way to voltage divide like adding a resistor in parallel, is this the way around it?

Bascially i would need a total of 6 of these motors to realise my project. Can anyone give me some advice on how i can meet the requirement of the power supply? What i have found so far are power supply for leds giving 12 volts 10 amps and even up to 25 amps. How could i use these?

any advice would be greatly appreciated.

thanks
best
dze

dze
 
Posts: 47
Joined: Sun May 02, 2010 10:12 pm

Re: Power Supply

by richms on Wed Sep 01, 2010 4:53 am

Try 5V massive amps from an old PC power supply and see how it works on that.

Otherwise, look around for an open frame 5V power supply at the current you need with an adjustment - they can normally get them up to at least 6v by turning the adjustment.

Forget dropping 12v to 6v with resistors. I wouldnt even think about seriesing the motors unless you can guarentee that they will have the same load on them at all times.

richms
 
Posts: 558
Joined: Tue Jan 20, 2009 3:05 am
Location: New Zealand

Re: Power Supply

by zener on Wed Sep 01, 2010 1:27 pm

PC power supplies are pretty unreliable in my experience. They are made very cheap. They will fail a lot especially if you load them hard. You might consider finding a 12V motor with less current draw. In any case you should probably use a big battery to handle surges. As was just said, definitely do not use dropping resistors. I would go the 12V route if at all possible.

zener
 
Posts: 4567
Joined: Sat Feb 21, 2009 2:38 am

Re: Power Supply

by uoip on Wed Sep 01, 2010 2:28 pm

Meanwell offers a line of power supplies where you can pretty much select what you want. The LPS-100-7.5 will give 6V at 13.3A (it's nominally 7.5V, but can be adjusted for anything from 6 to 9V). The SE-600-5 will give up to 5.5V at 100A, which is a wee bit low on the voltage, but enough current to easily supply all six motors at once. The SE-450-5 will do 5.5V at 75A, still probably enough for all the motors. BTW, Meanwell isn't the only such company.

Motors usually have a high startup current. Was the 12A at startup? Or steady state?

But design is often all about parts selection and thinking holistically about the entire design. You may find that a different motor is easier to power -- for example, 12V supplies are ubiquitous, and you'll need less current at 12V to get the same output power. So maybe a 12V motor is appropriate? I don't know your project well enough to say. But if you find yourself fighting too hard to find a supply for your chosen motor, consider choosing a motor that's easier to power with widely available supplies.

Don't try to drive a high current 6V load with a 12V supply. Dropping with resistors is a terrible idea for many reasons, varying load being one, and waste heat being another. Dropping with a linear regulator is a little better, but you'll still have way too much waste heat to get rid of. Dropping with a buck converter isn't so bad (and at 6V, you'll get about 15-18A out of that 12V 10A supply), but a buck converter for that amount of current will be expensive, big, and a hassle. Better to avoid the detour through 12V, and just get a supply designed to output the right voltage to begin with.
uoip
 
Posts: 127
Joined: Wed Sep 23, 2009 5:48 pm

Re: Power Supply

by zener on Wed Sep 01, 2010 7:15 pm

www.mpja.com has some cheap supplies

zener
 
Posts: 4567
Joined: Sat Feb 21, 2009 2:38 am

Re: Power Supply

by dze on Thu Sep 02, 2010 7:52 am

hi there!

firstly thanks for all who gave valuable advice!

Zener wrote:PC power supplies are pretty unreliable in my experience. They are made very cheap. They will fail a lot especially if you load them hard. You might consider finding a 12V motor with less current draw. In any case you should probably use a big battery to handle surges. As was just said, definitely do not use dropping resistors. I would go the 12V route if at all possible.


uoip wrote: Motors usually have a high startup current. Was the 12A at startup? Or steady state?


it was Current at P Max 12,0 and Idling Current 0,77.

I'm using dc motors for airplanes propellers and need the speed/nr of motor turns as it's the main element for my project. I've tried the 12v motors but they didn't give me enough nr. of turns as this and this is the next faster motor.

The SE-450-5 will do 5.5V at 75A, could be the trick. But first I'll try it on an old pc supply to get the first prototype working.

Another question;

so far I'm using the motorshield to drive 12v motors. In my code i've test the difference in changing the speed of the motor. but anything below setSpeed(100) creates noises in the motor while spinning. Is this right and why is this so?

What would then be a better means to modulate the speed of the motor? i.e slow spinning down of the motor to fast spinning up?

dze
 
Posts: 47
Joined: Sun May 02, 2010 10:12 pm

Re: Power Supply

by adafruit_support_bill on Thu Sep 02, 2010 8:12 am

anything below setSpeed(100) creates noises in the motor while spinning. Is this right and why is this so?


It could be noise from the PWM you are hearing. Also, some DC motors just don't run well at lower speeds. You can experiment with different PWM frequencies to see which works best with your motor.

Another thing to consider - especially for lower voltage motors - is that there is a voltage drop (usually about 1.4v) through the H-bridge. If you are using a 6v supply, you are only getting about 75-80% of the rated voltage to the motor. You will get better results with a power supply in the range of 7.5v. A slight (10-15%) over-voltage should not be a problem if you are not at full throttle all the time.

adafruit_support_bill
 
Posts: 77911
Joined: Sat Feb 07, 2009 10:11 am

Please be positive and constructive with your questions and comments.