Choosing a Power Supply

[sparky961]

I have a long term project on the go that involves servo drivers, and as I get farther along on the project and add more devices that need power I'm thinking I'll need to get a pretty big power supply at some point soon.  Can anyone offer advice on what to get (and what not to)?

The drivers can accept unregulated 20 to 60 VDC.  48 VDC seems to be a common voltage and I think that would work well.  The four drivers are rated for 12A peak, 6A continuous... so if I'm somewhere just above 24A I should never have a problem with current requirements under normal conditions.  The problem I see is that things get expensive with those sorts of ratings.

I've been leaning toward something like this (Ebay link): 350W 48V DC 7.3A

... but I'd need to put 3 of them in parallel if I were to meet the calculated demand.... which seems possible, but doesn't seem to be the best solution.

Am I thinking about this the wrong way?  Do I really need to have that much current available to the system?

[awemawson]

Buy a suitable transformer  bridge rectifier and capacitor. Far the cheapest way to go at that power and unregulated. Output voltage will be 1.414 times transformer voltage. So take the voltage you want, divide by 1.414 and that's the transformer secondary voltage you need. Then just choose one who's power rating is right

[sparky961]

Buy a suitable transformer  bridge rectifier and capacitor. Far the cheapest way to go at that power and unregulated. Output voltage will be 1.414 times transformer voltage. So take the voltage you want, divide by 1.414 and that's the transformer secondary voltage you need. Then just choose one who's power rating is right

You've confirmed what I figured may be the case.   I already have a suitable bridge rectifier and caps, but I was having a harder time finding a big enough transformer than I was a pre-fab switching power supply.  Freight will be a killer on the size of transformer I'd need.  Alas, I'll poke a bit deeper here and there.  Maybe I can find one at a local surplus outlet, though most places I used to frequent have turned into Chinese crap resellers.

[John Rudd]

sparky,

Cant you get hold of a defunct microwave oven and rewind the transformer?

Might be a bit onerous but its for nothing but the cost of the copper wire.........

[kwackers]

Have you measured the current pulled at 48v?

As an example if you have a 2.5A stepper motor running at 48v the average current is a lot less due to the driver 'chopping' the supply. As the voltage drops the 'on' ratio increases and the average current goes up.

You'll probably find something similar with your servo drivers. Just make sure you've got enough capacitance to supply the current for the initial start!

You can also get 15A 48v supplies from fleabay.

[sparky961]

sparky,

Cant you get hold of a defunct microwave oven and rewind the transformer?

Might be a bit onerous but its for nothing but the cost of the copper wire.........

The idea certainly crossed my mind.  I got rid of a pile of them a few years during a cleaning spree because I hadn't used them in years.  This solution is on my short list.

[sparky961]

Have you measured the current pulled at 48v?

As an example if you have a 2.5A stepper motor running at 48v the average current is a lot less due to the driver 'chopping' the supply. As the voltage drops the 'on' ratio increases and the average current goes up.

You'll probably find something similar with your servo drivers. Just make sure you've got enough capacitance to supply the current for the initial start!

I've got some pretty scary huge caps here.  That shouldn't be a problem. ;)

I wasn't sure of the exact relation between current and voltage, but what you describe is logical.  I'm running two drivers and servos right now from 2 computer power supplies in series (for 24V).  I think the ones I'm using are good for about 5A.  So far I'm testing with very little load, but haven't seen any issues yet.  I'll try to get some current measurements and go from there.

[Bluechip]

OK, another wild diversion ....
 
Suppose you need 48VDC at some 15A or so nominal, but it's a bit peaky ......
 
Four 12V car batteries ???
 
Yes, I do know what they cost, but you don't need new ones. I had one a few years ago which would crank a 2l diesel when the engine was warm but refused at 5AM with the temp. around -5C. It was replaced as you might expect. ( There are plenty of them here, I just have to ask one of my motor trade mates  ). 
 
I still have that battery to play around with. It still quite happily chucks out 15A even if the 400-600A or whatever is utterly beyond it.
 
You could lash up a 56V 6-8A ish current limited charger with a L200 regulator, this would bring your transformer into the 300-500VA range which are expensive but not lunatic prices. If you fiddle around a bit with the charge rate, most of the time the batteries will be doing all the work, charger just topping up when the load is low enough.

Upside is you need no wild looking low ESR electrolytics.
 
Downside is you won't have precisely 48V and you will have four wretched batteries to fall over.
 
Dave BC
 
 

[sparky961]

Batteries, yes... but maybe at the bottom of my list for now.

I had a bit of a "DUH!" moment while thinking about the various transformer suggestions here.  These servos and drivers I'm working with came from an industrial system that I acquired, and I actually _have_ a 1kVA transformer already that came with it.  I had overlooked the possibility of using it, or at least considered it a last resort, because the input voltages don't match what's available in my house.  Just to remind you folks overseas, that's 120VAC and 240VAC.  The former is much easier to work with since our "standard" power outlets are 120VAC.

The transformer is wired for two input voltages, 208V and 460V.  When I input 123.1VAC to the 208V terminals, I got 19.5VAC out.  Close, but close enough?  I was thinking I might be able to get away with 240VAC (15% higher than the spec sticker), and it should yield 38VAC.  I'm not one to disregard recommended ratings, but I don't think I'd be pushing it too far on this one.  Anyone think I'd be foolish to try?  Anything specific to watch out for?

Also, I looked at maybe rewinding or partially rewinding this transformer.  It looks possible, but if I can use it without changes that would be my preference.

Pictures attached show the spec sticker, as well as the size of this beast.

[sparky961]

If the above setup is "safe", would my calculation be correct that the max safe current on the output would then be ~26A @ 38VAC?

I've found some references to interchanging 208V and 240V in some equipment, but I only have one of these transformers and I'd prefer not to have to purchase another because I let the smoke out...