I'm trying to make a dispenser for the kitchen that will have 3 containers, all of which that will open and close via servos.
Now if i'm understanding correctly, according to my servos stall torque rating (1.2kg/(4.8V); 1.6 kg /(6.0V)), one servo will carry about 1.3kg using the 5volts that the arduino provides. Since i want 3 servos, the voltage will be divided, and each one will only be given about 1.5v, in which they are inoperable?
How can i power 3 servos using one arduino? Would I have to use an external power source and control them with transistors or something?
edit: or are the servos running in parallel and the voltage not being divided, in which case all 3 are operable?
htimmis:
The current required to supply a servo is quite a bit (to much for the Arduino most times) because of this it can cause problems with other processes.
How can i determine how much external power I should use?
it really depends on the amount of work the servos will perform I have powered 3 servos with 4 AA batteries 900mah and that seems to work pretty well for light to medium work. such as moving an ultrasonic sensor.
Could you explain why an external is better? and also, regardless if it is better or not, would using the 5V supply still be possible?
The arduino internal power supply is not designed to power motors, which could draw several amps, and possibly damage the arduino voltage regulator. Also, servos operate better at 6v than 5v.
The servos would all be in parallel with the supply. So the voltage would not be divided.
Even one servo running off the Arduino supply could cause problems. It may run fine most of the time, but eventually you will have problems. And you will most likely have a lot of problems if you try to operate all three at once. For one, the current required could be high enough to overheat the regulator on the Arduino. It could also cause the voltage on the regulator to dip to the point that it resets the Arduino. If you are powering the Arduino itself from an external supply of 800mA or more, you might be ok with a servo or two. Over USB, you have only got 100mA possible.
Obviously the final application will be installed in the kitchen so you are likely going to be using a battery or a wall wart. Your servos can run off 7V just fine and the arduino can run off 7V just fine. So use the same supply to power both, but that means powering the servos directly from the supply, not from the Arduino 5V pin. If you use two separate supplies, make sure the grounds are connected together. Your supply should be rated for 800mA plus whatever the current of all three servos is. In reality, though, your arduino is unlikely to actually draw 800mA. So I think anything at least 1A would be fine.
Thanks for the help gentlemen i really appreciate it. Just one more question. If the servo I am buying does not have a spec on current draw, could i just measure the resistance of the servo, and use Ohm's law? Is it applicable here or is the resistance constantly changing?
could i just measure the resistance of the servo, and use Ohm's law?
No, you can't. The actual current servo needs to actuate is difficult to calculate. There is a peak when servo starts, and it depends on the mechanical forces as well.. You must measure the current under actual load. Or make a qualified judgment
Arduino --- from any I/O pin try to keep 20mA or less. Total draw = 200mA. See why less? Less adds less to the total.
So you connect resistors and transistors to pins, or you connect drivers or opto-isolators and run the main power through that.
But I look at what you got and if there's a house current version of your valves then look into 5V control power relays that switch 120/240VAC direct. They might handle a good deal of DC too. Then you won't have to write motor control, the relay does it.
remedemic:
Thanks for the help gentlemen i really appreciate it. Just one more question. If the servo I am buying does not have a spec on current draw, could i just measure the resistance of the servo, and use Ohm's law? Is it applicable here or is the resistance constantly changing?
That will tell you the stall current of the motor so that is the absolute maximum it will take. In normal operation it will take much less, maybe 8 or 10 times less.
or is the resistance constantly changing?
The resistance is constant but the current through the inductor depends on the energy coupling between the magnetic field and the load you put it under.
So all in all, how should I determine how much current a servo will draw if I cannot find a spec? Will it not matter since it will most likely be in the range of a battery's current output capability?
