Re: [CAD_CAM_EDM_DRO] Where to buy transformers?

ds_hutchins wrote:

> I'm thinking of building my own power supply, and I need to find a
> transformer for a reasonable price. While there's been a lot of posts
> on the list that discuss transformer types and/or sizes, I couldn't
> find any links to places that actually -sell- them at reasonable
> prices. The standard online electronics houses (e.g. mouser &
> digi-key) don't seem to carry large power transformers. I'm looking
> for something in the 500VA - 1000VA range. Toroidal would be nice, but
> I may go with the "normal" kind for price reasons.

Surplus Center, Brigar, Mendelson, C&H, Marlin P Jones are the
outfits that come to mind for power transformers.

>
> As to size... I just purchased 3 DC motors that run at 40V nominal,
> 60V max, with a peak current draw of 14A/each. The guy I bought them
> from recommended that I run them at 48V for CNC use. I know that I
> won't ever run all three at peak current, but would a 10A (500VA)
> transformer actually be sufficient, or should I shell out the $$$ for
> a 20A unit?

It depends on the size of the machine. 14 A at 60 V is 840 W, or about
one HP output from the motor. That is a lot of power, probably good for
a Series II Bridgeport machine to get up close to the same performance
as the EZ-Trak control. On a much lighter machine, you would likely
break something or get hurt by pushing the wrong button. If you DO have
a Series II, I would go for the 20 A transformer. If a smaller machine,
I think 10 A will probably be sufficient. The servo drive limits the voltage
to the motor, so a higher voltage DC supply will not harm the motors.

> Also, I've seen several posts that say I should multiply the AC
> voltage by 1.41 to get the DC rectified voltage. In my electronics
> books, however, it says that most AC voltages (such as the 110V
> household voltage) are already specified in rms form. When I'm buying
> a transformer, do I get a 48V secondary, or a 34V secondary?

Transformers are just about always rated at RMS, as you say. But, the
RECTIFIER + capacitor undoes that, and converts RMS from the transformer
to near the peak value. So, as others have said, you will get roughly 1.41 X
the RMS voltage from the secondary at the capacitor.

Many larger power supplies use a center-tapped winding, so that only
2 recifier diodes are needed. In that case, you will get 1/2 of the voltage
that would be obtained if you used the full winding into a bridge rectifier.

Jon