### Author Topic: current supply for servos  (Read 696 times)

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#### dv.phaneeswar

• Jr. Member
• Posts: 18
##### current supply for servos
« on: January 29, 2013, 10:02:37 AM »
sir,
firstly i want to declare that i am very poor in electronics. k if i come to the point i am constructing a  hexapod robot which consists of 14 servos (Futaba S3003) controlled by axon. i am using 6v 2300 mah niMH bateries but they are getting exhausted with in 3-4 minutes so i have decided to buy an 6v adapter. so can any one help me about how many amperes adaptor is needed for my project.

there are 6v 350ma adapters and 5.2v 1000ma adapters available for me in the market so which one you would prefer for this project.

#### jkerns

• Robot Overlord
• Posts: 270
##### Re: current supply for servos
« Reply #1 on: January 29, 2013, 10:29:30 AM »
Based on your numbers (2300 amp-hour, 4 minutes) I would think you would need AT LEAST 4 amps (4000ma) from a power supply - but more likely about twice that.

OOPS - I slipped a decimal place somewhere - the numbers posted by jwatte are better.
« Last Edit: January 29, 2013, 06:07:44 PM by jkerns »
I get paid to play with robots - can't beat that with a stick.

http://www.ltu.edu/engineering/mechanical/bachelor-science-robotics-engineering.asp

#### jwatte

• Supreme Robot
• Posts: 1,343
##### Re: current supply for servos
« Reply #2 on: January 29, 2013, 01:02:46 PM »
2300 mAh, 4 minutes, means your project draws at least 30 amps, which is 30000 mA. The math is simple:
2300 mA * hour / 4 min * 60 min / hour = 34500 mA == 34.5 amps. The "4 minutes" time and the "2300 mAh" capacity is rather full of variation, though.
If you average 1A per servo, and have 14 servos, you need more than 14 amps in your power supply.

I suggest that you get a benchtop power supply with at least a 30A capacity. You'll find those things in industrial supply stores and the like.

#### dv.phaneeswar

• Jr. Member
• Posts: 18
##### Re: current supply for servos
« Reply #3 on: January 29, 2013, 01:28:31 PM »

I suggest that you get a benchtop power supply with at least a 30A capacity.

i am having a doubt i dont know wheather it is correct or not but correct me if i am wrong.
when the servos are in ideal condition then all the 30 amps current will be send to microcontroller or when only one servo is working then all the 30 amps will be send to only one servo will damage the microcontroller or servo.
if i select a high current adapter will it damage the servos?

#### jkerns

• Robot Overlord
• Posts: 270
##### Re: current supply for servos
« Reply #4 on: January 29, 2013, 06:10:52 PM »
The actually current flow will depend on how many servos are drawing power. If we assume they draw about 2 amps each when moving, then if one servo is moving, the power supply will give you 2 amps. If 10 are moving they will draw 20 amps. etc.

The "30 amp supply" is capable of supplying up to 30 amps at some voltage. It won't make the circuit draw 30 amps.
I get paid to play with robots - can't beat that with a stick.

http://www.ltu.edu/engineering/mechanical/bachelor-science-robotics-engineering.asp

#### jwatte

• Supreme Robot
• Posts: 1,343
##### Re: current supply for servos
« Reply #5 on: January 29, 2013, 07:09:19 PM »
I agree with jkerns. Amps are never "pushed" into a circuit, they are only "pulled" by the circuit. If the circuit tries to pull more amps than are available in the voltage source, the voltage will typically sag (drop) and you'll get brown-outs from electronics, and weaker performance from motors. You'll also end up stressing your batteries a lot, leading to shorter lifetime.

You should measure one of your servos in idle, and under load. Then multiply that by 14, and you'll know how much the entire robot will draw under maximal load. It may be that the 2300 mAh battery is delivering a lot less before it runs out, because the servos draw more than it's capable of delivering. That would change the equation for how much current you need in total. Thus, get a digital multimeter, and measure one of the servos first.

Or, if you get a bench power supply, it typically has an amp meter built in. You can get a 18 volts, 5 amps, supply pretty cheaply (as well as bigger things, like 30 volts 10 amps.) This will let you measure the servo, and deliver some amount of power -- it may be that 10 amps is enough for "normal" load on your robot, and you don't need the more expensive 30 amp supply.

Also, bench supplies have adjustments for voltage, and current. Typically, they will attempt to reach the given voltage, but if more current than the set limit is reached, they will supply a lower voltage. You can typically short-circuit these supplies without damanging them; they'll just limit the voltage to something very low, to keep the current under the set limit. For any hobbyist serious about electronic circuits, the three must-have tools are:
- digital multimeter
- soldering iron
- bench power supply