Is this solution correct for 23 servos ?

Hello guys,

I want to power 23 microservos Tower Pro SG90 (5 V, 1.2 A for a single servo), control them with Arduino Mega 2560 and using them all at the same time together, but I'm facing some problems:

  1. How much total current I needed?

I've searched in the forum and found some interesting topics, especially this one (at the 2nd page)
In order to have the total Ampere, have I to sum the single unit ampere? I would have 1.2 A*23=
27.6 A, so I need around 30 Ampere, is it right?
I've found also these two videos:
First (at 27th comment the author say that he needed 1 A to power 20 servo at the same time, how can it be possible?)
Second(in the 1st comment the author say he used 5 A to power 18 servo ?)

  1. What kind of power supply is correct for this use?
    I've found this model, what do you think about it?

  2. Is right to put the servos directly into the power supply with + and - ? Are the jumper cables safe to
    use with this high Ampere (nearly 30A)?

  3. Is safe to connect the Arduino Mega ground with the power supply - ?

  4. Am I missing something wrong and big due to my inexperience?

Thanks in advance for your kindness and wish you great times.

Albi

There are many different factors to consider, how many servos might be moving at the same time? How much load on each (% of full)? On startup, all servos will be attached at (nearly) the same time, so that’s 20 Amps, however that can be mitigated by putting a delay (about 50 - 75 mS) between attaches in setup(). You also have to think about managing that glob of wires. >:(

Is safe to connect the Arduino Mega ground with the power supply - ?

That’s a must, there has to be a signal return path from each servo to Arduino, that comes through the ground wire.

Just as a matter of interest, where did you get this number or did you measure it? Many if not most datasheets are silent on the current.

Albi_54:
Tower Pro SG90 (5 V, 1.2 A for a single servo),

Thanks edgemoron and kenwood120s for your time to answer:

how many servos might be moving at the same time?

Almost all of them, 20 on 23.

How much load on each (% of full)?

It's a kinetic sculpture, every servo is constantly lifting a plexiglass panel (80g each panel), is there a way I can know the % load?

putting a delay (about 50 - 75 mS) between attaches in setup()

Thanks so much for the advice :slight_smile:

On startup, all servos will be attached at (nearly) the same time, so that's 20 Amps

Why should not be nearly 27 Amps? 1.2 A*23 , isn't it correct? are the 7 amps less due to the 50 - 75 mS delay you advice me to use?

where did you get this number or did you measure it?

I've measured it with a multimeter, while one servo is moving with full load to be lifted, it's about 1.2 A

What do you think about the model I've chosen and the jumper cable use to connect power supply to servos?

Thanks for your kindness.
Albi

Albi_54:
It's a kinetic sculpture, every servo is constantly lifting a plexiglass panel (80g each panel), is there a way I can know the % load?

First you look at the servo datasheet again for the torque number, which is usually given as kg.cm (although many annoyingly call it kg/cm). I found this SG90 sheet for example which says 1.8kg/cm[sic].

Then you measure how far your 80g (0.08kg) load is from the axle of the servo and multiply the kg and cm together. (So it's 0.08kg.cm for 1cm away, 0.160 for 2, etc)

Lastly compare your number with 1.8 of the spec and see what leeway you have.

Conversely, take the torque the motor has (1.8kg.cm) and divide it by your load (0.08cmoops,kg) to see the longest arm you can get away with. In your case that's 1.8kg.cm/0.08kg so you can have a huuuuuuge arm of 22m(edit... crap, sorry being careless, cm not m) if I got the numbers right. (80g is bugger all.)

Your 1.2A seems very high for that tiny load. This datasheet says it stalls at 650mA +- 80, so 730mA max. But yours is nowhere near the maximum load, so should be closer to the no load current of 220+-50, so 270mA. Maybe 350-400mA with your small load.

I have a couple of those servos but alas have lent my decent meter out, and my cheaper one doesn't do current so I can't test it.

Ok, now it's everything much clearer, thank a lot for your time and dedication kenwood120s.

What kind of wire do you recommend to use for plugging + e - servo's directly to power supply? And is it safe ?

Thanks

Albi

Albi_54:
What kind of wire do you recommend to use for plugging + e - servo's directly to power supply? And is it safe ?

No idea! Maybe I'm naive but it would be nice to think that the wire already on the servo is ok, so go with something about the same.

Btw, when I say measure the distance from the load to the axle I mean horizontal distance. I'll do a quick pic and add in next post.

Pic to go with previous post

servo arm.GIF

servo arm.GIF

Thank you so much kenwood120s really too kind, yes, got it.
Would you recommend to use breadboard with that Amps or put directly servos into power supply?

Thanks for you time and effort.

Albi

I have no idea what current a breadboard can handle, but for your fairly complex wiring I think breadboard's too risky if only for the danger of a wire falling out and you not sure on the spur of the moment and in the heat of opening night of your exhibition with your audience in antici..... pation, where the damn thing goes. Some kind of terminals or terminal strip to join wires would be better I'm sure.

And yeah you'll need heavier wire to handle the current between where they all join and the supply. Someone else will hopefully tell you how to figure that out. Assuming it's the 30A you anticipate, something like the wire in your house, I'd say.

Really too gentle and kind kenwood120s, thanks a lot for your time and effort.
Wish you a great day :slight_smile:

Albi

Your suggested power supply can handle 60 A. It must have some heavy terminals. Use the thickest wire that can fit into the terminal and you're good.

Before investing in that power supply OP, I'd suggest measuring that current with another meter. At 1.2A it seems high for a micro servo, especially in light of the link I gave that it would stall with about 750mA.

I'd suggest measuring that current with another meter. At 1.2A it seems high for a micro servo

Yes, I've measured it again with another multimeter and it's around 800 mA with full load, thanks for the advice!

I'm connecting the power supply ground to the Arduino GND but: the power supply have "-" and "ground", which one should I choose to connect correctly the power supply to the Arduino GND?

Thanks

Albi

Albi_54:
I'm connecting the power supplies ground to the Arduino GND but: the power supply have "-" and "ground", which one should I choose to connect correctly the power supply to the Arduino GND?

I've made some tests and I have to connect to - not ground on power supplies.

I've bought two power supply, 5V and 12 A each one, my question now is: should I connect the two power supply - together ? And the Arduino GND to both power supplies -?

Albi

Never connect supplies directly in parallel, it won't work(*) and has the risk of oscillating to destruction(**)

(*) the one with the highest voltage will see the whole load.

(**) regulators are high gain amplifiers, LDO regulators are non-linear high gain amplifiers, I've seen
this happen - took out entire board of components...

Thanks MarkT for your reply, I would use around 11 servos for each power supplies so I can use them separately, so can I connect the first power supply - to the first Arduino Mega GND and the second power supply - to the second Arduino Mega GND without having signal problems?

Thanks a lot for your time.

Albi

All grounds commoned in a star-ground configuration would be the normal way to do things.

Thanks MarkT for your answer, on power supply I have ground and negative:

Do you mean to connect the grounds or the negative poles together?

In order to make one power supply function properly with Arduino, I have to connect Arduino GND to negative poles on the power supply, not the ground ::slight_smile:

Thanks for your time

Albi

Ground here means the negatives, not the ground as in the planet Earth, which is where the earth wire on your kettle goes.