Need Help!- Building a power supply

[boxwood]

Hi building an unregulated linear power supply. My question is I bought a Triad 24volt 10amp transformer but when I apply 120v volts to it the readings I am getting are 26.6 volts output. This is a cheap digital meter I don't access to another meter is it possible the transformer is putting out more than its rated voltage. this is gonna put me over on my DC volts after rectifying and smoothing capacitor. any input would be welcomed

[Al_The_Man]

Most transformers put out slightly higher off load voltage, if it is a concern, look into taking about 5 or 6 turns off.
Al.

[boxwood]

ok I unwound about 5 turns of wire from the transformer out put voltage from the caps going to the xylotex board is 31 volts DC but nothing happens when I turn everything on. The motors dont lock the transformer gets slightly warm the terminals at the board input show 31 volts. No smoke or fire was a good thing but something is obviously wrong does any one have any ideas what to check next?

Ray

[blackbeard52]

lets start with the simplest question..Have you had the motors operating before... i.e. is it the transformer or the drive setup? generally if you have the proper voltage coming out then it is assumed everything is proper.

[CarveOne]

Have you set the Vref potentiometers according to the instructions yet?

There is a Vref adjustment for each axis driver circuit. They come from the factory set for 0v. They must be set for 3.35vdc (3 axis board) or 2.0vdc (newer 4 axis board) with no steppers attached to the driver board and with the specified less than 30vdc maximum input voltage. If the Vref pots are not set the steppers won't run when you connect them. If the Vref pots are set too high you can blow the driver chips.


CarveOne

[Rance]

Originally Posted by boxwood ...any input would be welcomed

I thought you said you had too much input already. Oh, that's right, you said you had too much OUTput. I'm sorry, I just couldn't help myself.

[odubob]

Hi,
First of all, most transformers I buy, say a 24vac 10 amp, it would be great - if (1) I know what the input line voltage is, it can easily vary 5%. That gets you a volt or so of unknown. (2) The transformer v spec usually indicates a voltage at full load =10 amps. If it would put out 10 amps at 24vac and your open circuit voltage is 26.6vac then the internal series resistance is 2.6/10 = 0.26 ohms. Of course the spec doesn't mention internal resistance and this is most important loss of power.

When you go to DC, say a full wave rectifier at 10amps, 24v results in about 34v which vary with load. Subtracting say a volt for drop on the bridge, then depending on the size of the capacitors, one could have quite a wide range of ripple and average voltage.

Without going into more about power supplies and boring you, let me suggest an old way that works. Buy a (24v/120v)x10A = 2amp 'variac' type 110 v panel controller which you just set to vary the input voltage to what dc you need to work.

best regards,
odubob

[Al_The_Man]

One thing to watch for in using Variac's, is for the most part they are not isolated devices, they are auto-transformers, at least if using one, this should be confirmed.
Al.

[TSJ]

I had planned on asking a question about using "Variac" transformers but you beat me too it. What do you mean by "auto-transformers" and is there anything else to be considered with the Variac type of transformer? I realize these transformers are more expensive, but for an amateur you can just turn the dial until the voltage needed is set.

[Al_The_Man]

If using a variac directly off a supply that has one side referenced to ground i.e. a 120vac outlet N.A. or a 240v outlet UK. if care is not taken to ensure that the 'common' conductor is connected to the neutral conductor, then both 'secondary' conductors are liable to be 120 or 240 above ground potential.
IOW if you had for instance the variac set for 24vac output, and the neutral happened to be connected to the wrong input connection, then both 24vac conductors will be at mains potential above ground.
This is eliminated when using a normal isolated type transformer with a 24vac secondary. there will be no potential to ground, and you could if you wish, ground one conductor.
Al.

[boxwood]

Thanks for all your help guys. I am still at a loss though. Just a little history to clarify somethings this was a functioning system I wanted to max out the volts for more performance. I disconnected a 20 volt Power One supply and purchased the following items from ebay plus a transformer from allied electric
I have instructions on how to wire it I have the proper volts at the input to board. even the vrefs show 3.0 vdc but know power to motors at all. One thing that seems rather odd to me I probed the inputs at the board with my meter set to AC and I get 67 volts notice I said ac maybe the rectifier is bad. maybe I BROKE something when I unwound the transformer I dont wanna give up I have about 60 dollars in this now and about a bazillion burned brain cells thinking about. thanks again

Ray


http://cgi.ebay.com/MAKE-YOUR-OWN-CN...QQcmdZViewItem

http://cgi.ebay.com/Bridge-Rectifier...713.m153.l1262

[martinw]

Dear boxwood,

There are far better men than me to give you advice about the behaviour of transformers.

Try these people about load regulation.
http://www.atc-frost.com/products/design/
Best wishes,

Martin