Need to power 7.4V servo with wall wart through breadboard

Using Arduino General Electronics
#1

I am trying to run a 1400mA 7.4V servo from a wall wart. I have read through some posts saying that I will need to provide ~2 more volts than I need to power the servo(not sure why?). Can a breadboard(like this one: http://farm9.staticflickr.com/8196/8109455106_e7da664576_c.jpg) handle the current/voltage(say 12v)?

What I would like to know is how I can get the power from a 12V wall wart into a couple of 7.4V servos? I have seen things that attach to the breadboard that will take a DC/AC jack. Do I need a voltage regulator and capacitors?

Anyone have some circuit drawings for something like this or links to products that might be of use to me?

Cheers, Jason

#2

I believe the idea of ~2V more is actually referring to Vin applied to a common voltage regulator like a 7805. For servos, you just apply the voltage straight-on.

Also, when talking about 'unregulated' wallwarts, the voltage out varies significantly with the load current, and typically by 4..6V from 0 to full load. It's not a good idea to use unregulated wallwarts to power servos.

For 2 1400 mA servos, that's 2.8A, so you might look for a wallwart with 'regulated' output voltage, and find one as near to 7.4V as possible. Preferably, it should be rated at somewhat more than 2.8A, so 4-5A to give some leeway.

The other thing is NiMH AA rechargeable cells make a good power source for servos, and can be had with 2500 mAhr ratings, which will run your servos for a while. You can also look into Lithium batteries.

Running off of 12V, you might use a voltage regulator to drop to 7.4V, but linear v.regs are not very efficient, and will get hot. The other thing to check is an adjustable dc-dc converter, maybe your best bet here.

#3

First off, thank you very much for your response! I learned a lot by reading through your post and researching a bit.

If you don't mind I have a couple more questions...

  • Is there a general rule of thumb for how much leeway to give allot for the current? Is there a risk of supplying too much current?

  • I'm thinking of using some of these NiMH batteries. I will need one per servo correct? http://www.servocity.com/html/7_2v__2500mah_nimh_battery.html

  • A general question, how can I hook up these power sources to my breadboard? Can I just go in and snip off the ends of a regulated wallwart or battery pack and hook it up directly to the breadboard? Is there a max voltage or amperage that I can supply to a breadboard? I've seen people use diodes or capacitors when using this stuff on the breadboard, I'm guessing to prevent any surges or shorts.

Thanks again!

#4

Is there a general rule of thumb for how much leeway to give allot for the current?

In this case, the 1400mA stall current is the "worst case". Assuming you are not putting a heavy load on the servo, or running it a maximum speed all the time, etc., your typical/average current draw will be quite a bit less. You probably don't need any more than 2800mA for both servos, plus whatever your Arduino and other stuff (laser?) is using.

Is there a risk of supplying too much current?

No. The current rating on a power supply is it's maximum capability. The current flow depends on the supplied voltage and the resistance/impedance (i.e. resistance to current flow) of the load. The specs for the servo won't show you the resistance (and it changes with the mechanacal load), but they will give you the maximum current the servo will "ask for".

Voltage, current and resistance are related by [u]Ohm's Law[/u] (1 Volt across 1 Ohm results in 1 Amp of current flow).

With a power supply, the voltage is constant (or almost constant) and the current flow depends on the load. If we try to "pull" too much current from the power supply (too low of a load resistance), Ohm's Law is always true, so the voltage will drop (and the power supply might burn up).

A general question, how can I hook up these power sources to my breadboard?..

...Is there a max voltage or amperage that I can supply to a breadboard?

I'm not sure what the ratings are, but I'll guess. :D I wouldn't run 3 amps through one of those plug-in breadboards. I suggest wiring the servo's power supply & ground without going through the breadboard. (The signal line can come from the breadboard.) The voltage shouldn't be a problem. I'd just avoid running 120VAC or 240VAC on a breadboard...

Can I just go in and snip off the ends of a regulated wallwart or battery pack and hook it up directly to the breadboard?

Sure! Of course, you'll void your warranty, but that's normal... I just made an "unauthorized" modification to a magnifying lamp this morning! And I've got plans for modifying something else I'm working on. :D Or, you can use a matching connector.

I've seen people use diodes or capacitors when using this stuff on the breadboard, I'm guessing to prevent any surges or shorts.

It's hard to say what the capacitors/diodes are for.. They won't really provide that kind of protection, but they can be used to somewhat isolate the electronics from motor noise on the power supply line.

#5

Besides what DVD said,

First off, 2500mah means milliamp-hours. This is the amount of "energy" in the battery, and NOT the amount of current [amps] you can draw from the battery. That might be as high as 4-6 Amps max.

With a 2500 mah battery, you can draw around 250 mA for 2500/250 = 10 hr, or 500 mA for roughly 2500/500 = 5 hr. For larger currents, like 2.5A, the time will decrease disproportionately because there will be greater loss due to internal battery resistances, so you'll get somewhat less than 2500/2500 = 1 hr, maybe only 1/2 or 2/3 that time. So, all in all, the 2500mah battery will power both servos servos ok, but only for 1/2 or less time than for 1 servo.

Secondly, as far as wallwarts are concerned, those ratings are in Amp and not mah [milliamp-hours], and I try to use something that has some margin, eg 2X the max current you might expect.

Thirdly, those whiteboards are probably not very reliable with larger currents, ie over a few-100 mA. You need nice tight screw terminals for currents 1-2 Amp or greater.

#6

Thank you guys once again for the valuable information.

After reading what you've written as well as consulting some other sources, I think that the best option for me is using a voltage regulator, heat sink, and a 9V wallwart.

A couple more questions if I may:

Since I will be using a voltage regulator and capacitors for a circuit that I would normally build out on a breadboard but can't since I should't run 3 Amps through one, how can I build out this circuit? I.E. Do I need to solder everything together on some kind of conductive material?

How are screw terminals used?

When you mention connecting the servo's ground and power directly to the power source, how would that work with multiple servos? What if I am using a voltage regulator? Also, shouldn't I have the ground from the Arduino, my breadboard circuit, and the ground from the servos connected?

Thank you for your help!

#7

Ok… So I am actually deciding to go with analog servos now because I’ve realized that the current draw is more manageable for my current application. Also they are much cheaper. :slight_smile:

The servos will probably be drawing 300mA max.

The regulated power supply I’m looking at is a 6V 3.5A. If I understand correctly, the 3.5A is the max current draw. So if I have two servos drawing 600mA total at any point, I should be ok connecting these to a breadboard since I will probably never draw more than that. Correct? Any risk of current spikes greater than 1A melting my poor breadboard?

Or is it good practice to still hook these up directly to my power supply?

Thanks again! Almost got it all figured out!

#8

Ok… So I am actually deciding to go with analog servos now because I’ve realized that the current draw is more manageable for my current application. Also they are much cheaper.

The servos will probably be drawing 300mA max.

They will also have less torque/mehcanical power. I hope that’s OK… Sometimes it’s hard to calculate and you don’t know 'till you try. I have a project in mind (that I won’t start for awhile) and the load is going to be balanced, so there is almost no constant-force required by the servo. But, it’s going to require torque to start/stop the thing. I could probably dig out my physics/dynamics books, re-learn some stuff, and make some calculations based on the mass & acceleration, but I’m just going to buy a small motor, cross my fingers, and see what happens! :smiley:

The regulated power supply I’m looking at is a 6V 3.5A. If I understand correctly, the 3.5A is the max current draw.

Right… That’s the maximum you should draw from the power supply. You won’t be stressing it at 600mA.

So if I have two servos drawing 600mA total at any point, I should be ok connecting these to a breadboard since I will probably never draw more than that. Correct? Any risk of current spikes greater than 1A melting my poor breadboard?

Like I said… I don’t know what the ratings are. You’ll probably bo OK. What’s the worst that can happen? :smiley: A “spike” doesn’t generate much heat, since it takes a few seconds for heat to build up.

Or is it good practice to still hook these up directly to my power supply?

Yeah… I’d say it’s good prectice, but if it’s more convenient to run the power through the breadboard, I go-ahead and give it a try!

#9

OK great! Thanks so much for your help. I'll post when I finish the project in a couple weeks :)

Good luck with yours! Jason

#10