How do voltage and resistance work for two 5v servos when using an external 12v power supply?

I have a 12v/1.5 amp barrel plug power supply on my Arduino Uno. I have two servos that say they are rated for 4.8v to 6v, but no amps are specified. When my circuit is configured as it is in this image, it appears to work fine. (In my test sketch, I'm constantly and simultaneously rotating them from 0 - 180 degrees and then in reverse.)

But I don't understand why it works. I assumed the 5v pin can only supply 5v of total power to the whole configuration. Is it instead supplying a regulated 5v at as needed between both servos from a total pool of 12v (presumably up to 10v at once)? Do I need voltage regulation? Shouldn't I have to know the amps for the servo(s) and use some sort of resistance?

Thanks for any information and advice you can share on learning about the concepts underlying a situation like this.

Update: I ultimately modified my approach based on the feedback received here by wiring a separate 5v/2a power supply to the breadboard for the two servos, and wiring the rest of the circuit appropriately (using a common ground, etc.).

• Do not do this. Although the "total voltage" type of concern you raise is based in misunderstanding, there are real problems. A servo encountering mechanical load can easily draw more power than the arduino's regulator can supply, causing brown out and misoperation (or if you are lucky, merely reset) of your program. Even when not causing brown out, servos are notorious for putting "grunge" noise onto power supplies. Give your servos their own regulator, preferably a switching one. Commented Oct 17, 2014 at 1:26
• Ugh. I've got a lot to learn! Thanks for mentioning this. You mean something like this (two: one for each servo)? sparkfun.com/products/107 Commented Oct 17, 2014 at 3:13

Because the supplies of both servos are connected in parallel, they both see the same voltage. Hence there is no "pool", both are being supplied the same 5V.

There is a regulator on the Arduino which reduces the 12V input down to 5V. It does so by burning off the other [7V times however much current is being used] as heat. Since you are supplying it with 1.5A, the most it can supply is in turn 1.5A.

• Hmm, I understand less about this than I realized! I don't get how you can send 5v of electricity to two devices that both need 5v at the same time and they are able to work simultaneously. And I reckon I get that 1.5A is technically available to the servos, but I don't understand why I don't have to find out if the servos need a lower amount of current and put in resistance down to that amount, if necessary. (Or put another way, did I just get lucky that they can handle receiving 1.5A?) Commented Oct 16, 2014 at 4:34
• "Volts" isn't a quantity, it's a characteristic of the electricity itself. It's the amount of energy imparted to each unit of charge. As for the current, most devices will only draw what they require regardless of how much is being supplied. The exception to this is P-N semiconductor junctions, since they absorb energy as charge flows across them ("reduce the voltage") and are generally not affected by current beyond consumption of power. Commented Oct 16, 2014 at 4:40

Addition to Ignacio. Explanation from real life.

Gregir, look at your home devices like TV, micro wave, fridge and so on. They all need 220-230 V AC and you can plug all them to one socket. As Ignacio said voltage doesn't plus. The current does plus, for ex. you have 2 servos connected in parralel, both 5V, no load current 0.1A. So to run them with no load you will have to supply 5V and 0.1+0.1=0.2A.

I tried to explain at very basic level ;)

• Ah, of course! That makes good sense. I am still a little confused about the servos and their amperage. How do you get the 0.1A? (I can't seem to find it anywhere on the data sheets for the servos or on the manufacturer's site.) Commented Oct 16, 2014 at 14:46
• Gregir that's for example :) Different motors consumes different amount of current. Bigger motor = more power = more current. First mini servo in google search rctoys.com/pdf/hitec-servos/HIT-HS55.pdf .You can google information about servos like this interesting pdf bpesolutions.com/bpemanuals/Servo.Info.pdf Commented Oct 16, 2014 at 15:23
• Ah, OK. I see so few examples of people using resistors with servos so far, and hadn't found much info on my particular servos. Easily confused at this stage. Thanks for setting me straight. Commented Oct 16, 2014 at 16:06
• Be careful though as a servo that stalls due to load will easily draw up to 1A so if they both stall you might draw more than can be supplied. Commented Oct 23, 2015 at 6:38