The Shop > Electronics & IC Programing

Choosing a Power Supply

<< < (2/2)

sparky961:

--- Quote from: John Rudd on August 30, 2013, 01:33:39 PM ---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.........

--- End quote ---

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:

--- Quote from: kwackers on August 30, 2013, 01:39:55 PM ---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!

--- End quote ---

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...

Navigation

[0] Message Index

[*] Previous page