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Author Topic: How to power 3 servos with one arduino?  (Read 1619 times)
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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.

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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.

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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.
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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?
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EE is about measurements. So the best way to determine the current is to measure it under typical load..
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Make sure you know the startup surge. Power Supply should rate a good bit higher for long life.

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retrolefty says in a number of posts that he allows 1A per servo...  The datasheets are very quiet on the matter.
 
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It's not a good idea to just supply a bit more power than you need. Overestimate then double that and your PS will live longer. Same with a PC, always get extra power supply.

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It's not a good idea to just supply a bit more power than you need. Overestimate then double that and your PS will live longer. Same with a PC, always get extra power supply.
Good advice.
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I once measured the stall current of this very popular 9g servo.

It was 700 mA


http://www.hobbyking.com/hobbyking/store/__662__HXT900_9g_1_6kg_12sec_Micro_Servo.html
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It's not a good idea to just supply a bit more power than you need. Overestimate then double that and your PS will live longer. Same with a PC, always get extra power supply.

But then on the flip side of that, when something goes disasterously wrong having a huge supply of current available, your circuit will turn to bacon long before the power supply trips whatever protection it has or blows open itself. I guess it depends on whether you want to replace the supply or replace your entire circuit. GoForSmoke indeed! smiley-wink

(Just teasing... it is a very good rule of thumb -- but consideration above is real.)
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Design the circuit right and the expensive parts won't fry. Of course design it right and it will take physical damage to cause anything to fry.

On PC's, power supplies dieing has been the cause of many other parts getting replaced as the spikes kill them even before the PS karks it.

But then I had one buddy who didn't believe that grounding really matters. He would go whole months between PC's quitting on him.
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I find it harder to express logic in English than in Code.
Sometimes an example says more than many times as many words.

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