RE: [CAD_CAM_EDM_DRO] Re: Question on Power Supply

This thread on power supplies is excellent. Thanks to everyone. Does anyone
have a source for the capacitors - I've been able to find bridge rectifiers
and transformers, but no caps so far.

Thanks

Dave Davies


-----Original Message-----
From: CAD_CAM_EDM_DRO@yahoogroups.com
[mailto:CAD_CAM_EDM_DRO@yahoogroups.com] On Behalf Of jymmm
Sent: Wednesday, June 08, 2005 12:02 PM
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: [CAD_CAM_EDM_DRO] Re: Question on Power Supply

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Luke1027 <luke1027@p...> wrote:
> When you say "Just check for the maximum that you can use."
> You are talking about the max. voltage that the Driver can handle
> ... right?
> Which in my case will be 30 Volts. So I would need a transformer
> that will put out 30V. I would like to build my own, just for the
> sake of learning.



Hi Luke,

Since you said you want to learn, I thought I would toss in my two
cents. Take it for what it's worth =)


A plain old transformer ONLY does AC voltages, NEVER EVER DC voltages.


Below are the unregulated voltages. Just using a transformer, bridge
rectifer (or four diodes) and capacitors.

If you need (for example) 24VDC you can simple divide the desired DC
voltage by 1.4 to obtain the AC voltage you would need from a transformer:

24VDC / 1.4 = 17.14 VAC transformer

The same goes if you have a AC transformer already and want to know
what DC voltage you'll get out of it:

12VAC * 1.4 = 16.8 VDC


You can read up a bit here:
http://www.sciencelobby.com/general-theory/rms-and-peak-to-peak.html

The difference from 1.414 and 1.400 is the 0.07 (fwd) voltage drop
across the diodes in the bridge rectifier: 1.414 - (0.07 * 2) = 1.400

Some folks say to use unregulated power supplies mostly to deal with
back voltages (BEMF) from steppers.

This is the MAIN reason you don't drive steppers to the max potential.
They HOPE that not all 3 axis are deaccelerating all at the same time.

If you want to try something kinda neat...

Grab a stepper motor (any kind will do, not connected to anything -
just bare wires and nothing connected to the shafts) and turn the
shaft with your hand. Then, short out any two wires and try to spin
the shaft again.

If you feel resistance, then you've shorted two wires from the same
coil inside the motor. (This is a simple way to find out the wiring
scheme when no diagram is available).

The 'resistance' you feel is the BEMF that would be sent from the
stepper motor to the power supply. If the power supply is regulated,
it'll try to fight this as well (so to speak).

I hope this helps a little in understanding the basic concepts.

Jim...




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