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Author Topic: Powering 3 servos & what's everyone using on their workbench?  (Read 2022 times)
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London, UK
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Hi everyone,
I'm creating a very basic robotic arm (3 futaba S3001 servos, 2 DOF+1 claw), and I think the arduino board may not be able to power all 3 servos under load. I'm looking into power supplies, and am thinking of using either SLA batteries or 7.2V hobby batteries stepped down to 6V to power the servos. The projects will not leave my workbench, so I don't need mobility. Would this be a good idea? I'd like to hear alternative power sources!

Also, what is everyone  using to power their projects? Would be great to learn more about the kind of projects and the power sources you used, tethered or not smiley

Thanks!
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If it's tethered, and non-mobile, then go for a fixed power supply; a PC power supply would be the easiest and cheapest to get working, although you would be limited in your voltage selection (use a 7806, or a similar 6 volt regulator on the 12V rail). A good rule of thumb for standard servos is 1 amp current capability per servo (current requirements go up as you get into the larger servos). Keep this in mind as you plan your regulator layout (you may need one regulator per servo, which isn't that big of a deal - you might also run the servos off of the 5V rail of the supply, if you can work with the reduced torque of the servos at this lower voltage).

I have in my shop a couple of small adjustable power supplies that I bought used from a local electronics junkyard; they can output over 20 VDC @ a little over 1A, they also have a few fixed taps. The nice thing about such supplies is that you can set a current limit on them, and if the load goes past that for any reason, the supply switches the output "off" and blinks a light to let you know you've exceeded the setting. However, for my small supplies, its limits me sometimes in testing.

For other testing, I have a 2 A, 12 VDC "brick", connected to a box that has some DPDT switches on it, which I can use for a lot of purposes; however, it isn't current limited, and not adjustable in any way. But for testing 12 VDC loads, it works really well. I picked up the whole lot at Goodwill for a few dollars.

Lastly, though I haven't played with it yet, I have a 50 A, 13.8 VDC battery charger/starter - it's designed to allow you to crank-start your car as well as charge the battery. I got it "on sale" at Harbor Freight (for $30.00), and intend to use it to test/run some motors for an on-going project; the motors are large 540 size brushed DC motors in a PowerWheels Hummer H2 ride-on toy, and I didn't have anything that could supply that kind of amperage, short of an SLA or a car battery (the SLA that came with the toy is dead and needs to be recycled, and I don't fancy lugging a car battery into my shop).
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I did as Cr0sh suggests and use an old computer power supply that I rewired. The instructions to do that on the Internet work well. As Cr0sh says, you only get 12V, 5V and 3.3V. Not 6V. But you could use a regulator to go down from 12V or up from 5V, or run the servos at 5V.
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I also use a couple of old PC power supplies. They can put out a lot of current and have very steady voltage. I also have a collection of old wall warts in various voltages from old electronics like cell phones, routers, laptops, etc. Sometimes you can find one exactly the voltage you need, but they usually only put out from 200ma up to maybe 1.5a.
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cr0sh, Daanii, Brakk,
thanks for the advice! To be honest I am a bit afraid of having a psu on my bench smiley However, I think price/performance it may be the best. Off to look into one, cheers!
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I found this desktop power supply: http://www.powersuppliesonline.co.uk/product/30w-5v-4a-desktop-adapter/bpa-301-5/default.htm
they also have a 5V, 8A model. Would this be suitable as well?
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Lab power supply "instek PS-3030", 30V, 3A with high/low range and coarse and fine adjustment on both voltage and current limit.  Very reliable.

For portable low power I tend to use 4 NiMH cells which gives 5.3V or so and drive Vcc directly.

For higher power 12V or 24V I have some deep-cycle batteries connected to a solar panel.
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cr0sh, Daanii, Brakk,
thanks for the advice! To be honest I am a bit afraid of having a psu on my bench smiley However, I think price/performance it may be the best. Off to look into one, cheers!

Why? Are you afraid of being shocked? Maybe you need to experience it...

When I was a kid, I got shocked more than a couple of times - only twice with a level that could have been fatal. On one of those occasions, I was reaching for the cutoff grounding switch on the top of a lawnmower engine on my gokart, and wasn't looking carefully (in my defence, I had just crashed my kart and was shutting down the engine), and grabbed the bare sparkplug connector (no rubber insulation) by mistake. The other time, I was messing with a cooler pump (110 VAC) to pump out some water from a kiddie pool my parents used to keep our dogs cool on our back porch in the summer (it was my job to clean the pool every weekend, and pumping the water out was waaaay quicker than bailing) - and the motor tipped over but kept running. Unthinking, I reached into the water to tip it back upright...

Also as a kid, I used to make "shocking" devices, by inverting various salvaged transformers to a step-up configuration. Adding a relay buzzer supplied the AC input current/signal needed (I don't recall if I ever added a capacitor across the input - probably not). Gather a bunch of kids (or a single kid), tell them to hold on, then "zot!". Very few could hold on for long. Got shocked several times by those kinds of things (I had a few geek friends that did such things as well, so we were all playing with this stuff).

As an adult, I'm more cautious around electricity - though there was one time I got shocked by an ignition coil on a fur-covered car at Burning Man...

What all this means is simply to be cautious, respect what you are doing, and if you are really concerned, keep one hand in your pocket, and probe/work with the other (and honestly, you should always try to avoid random probing on a live circuit, if at all possible - and if you must, probe with a single probe -only-). At the same time, you gotta live a little too, even if it means potentially getting shocked (and perhaps hard). Its a hazard, certainly - but being fearful does you no good, either.
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One last plug for the rewired computer power supply. You can get a lot more current out of it than you can from a typical wall-wart power supply.

The one I have is small. Still, it puts out up to 15 Amps of 12 Volt power, and at the same time up to 35 Amps of 5 Volt power. The 3.3 Volts power shares a current source with the 5 Volts power, so you can get up to 28 Amps with it, but the total available current will be divided up between the two.

That's a lot of current. With a little effort, you could run your Arduino off the 12 Volts, and run your servos off the 5 Volts.

Still, for your application, maybe a couple of wall-warts will do you fine.
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I'll just throw out that thrift stores are nice to look for bargain wall warts, but even better are lap top power modules. These are usually regulated and anywere from 9-15vdc and a couple of amps at least. I've seen them for two bucks at times.

One thing that really helped me with DC power for experimenting around on my desk. I bought a switching DC to DC step down voltage regulator module, about 2" x 1" x1" on E-bay for  $10 ( http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=220648404910&ssPageName=STRK:MEWNX:IT )

If you wire up 35vdc to it's input, there is a ten turn pot that lets you set the output voltage anywhere from 1.25 to 30vdc at 2 amps (3 amps if one adds a heatsink). This thing has been really cool to have around for quick check out of circuits, calibrating stuff, etc.

Lefty


« Last Edit: March 02, 2011, 08:15:10 pm by retrolefty » Logged

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Daanni, Retrolefty, thanks! @Retrolefty: That dc-dc step down seems interesting, I may go that route. I've seen some laptop power supplies too, I'm just not wanting to mess too much with the power side of stuff smiley

@cr0sh: Yes, I am actually very afraid of being shocked. About 12 years ago, I accidently touched a live switch's wire of a regular psu for a pc I was fixing. Apart from being stunned for a fair few minutes, I had a small piece of skin several mm deep come off. Trust me, it is not fun to see and feel a part of one's finger missing.... thankfully over the years it has healed.

Regards.
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Quote
When I was a kid, I got shocked more than a couple of times
This explains a lot!

 smiley-yell smiley-razz smiley-yell
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I vote for PC power supplies.
You can do a lot with them.
Heres mine as is stands now.
http://scratchpad.thisandthose.org/scratchpad/article.php?story=20110224160501815

Gordon
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Nice PSU there.

I notice you mention ink-jet-printable sticky-backed plastic.  Where do you get this wonderful stuff?  Does it smear if it gets wet though?
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@cr0sh: Yes, I am actually very afraid of being shocked. About 12 years ago, I accidently touched a live switch's wire of a regular psu for a pc I was fixing. Apart from being stunned for a fair few minutes, I had a small piece of skin several mm deep come off. Trust me, it is not fun to see and feel a part of one's finger missing.... thankfully over the years it has healed.

Only fear it if it manages to go across your heart or brain; other than that, it's just pain. Paying attention to the "one hand" rule will keep it from crossing your heart; not much can be done about the brain part, except "don't do that" - LOL.

Regarding one's fingers, I have three cresent-shaped scars on the first three fingers of my right hand that were caused during a fall from the bumper of one of my trucks - I had reached out to grab something as I fell backwards, and managed to grab a sharp steel support. Lots of blood, and several stiches, but no tendon damage...then there was the one time my right-angle grinder "bit me" through my gloves.

Rather than fear it, learn from the experience of what you did wrong, and try not to make the same mistake twice - prepare for it the second time. For instance, in your case it would mean not fiddling around with the switch while the power supply was plugged in. In my case, I should've worn gloves and used a proper platform to work on the engine - or in the case of the grinder, used thicker/better gloves for more protection (or something else). Needless to say, I keep these incidents in mind for the future, but it won't stop me from working on my truck or grinding some metal.

Good luck with your project. smiley

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