Re: [CAD_CAM_EDM_DRO] Simple PSU stuff

abbylynx wrote:

>Hello again Folks,
>
>I have a nice heavy toroidal transformer here, rated 160VA, with two
>windings of 12V @ 6.3A each, a 20A full-wave bridge rectifier (100V
>or 200V I think), and a big 50V 47,000uF capacitor. I'm going to hook
>the windings in series, to get 24V. I want to build a power-supply,
>but am wondering if I need to use bleeders on this? I'll also be
>chucking a 24V fan onto the whole rig, so that should do the job of
>bleeders right?
>
>

The problem is you will get 33.9 V out, as the 12 V rating is almost
certainly RMS, the equivalent
heating power you would get from such a transformer into a resistive
load. But, the rectifier
charges the capacitors to the peak voltage, which is 1.414 x the RMS
equivalent.

That will likely be great for a stepper drive, but may smoke your DC
fan. You could put
14 1 Amp diodes in series with the fan to drop the voltage down to 24 V.

>As for the load, it'll be 3 bipolar drive 2A, 3.2V stepper motors,
>ala the type Sherline provide. Should be enough juice, no?
>
>The next question, what rating and type of fuse would be recommended?
>Should I put fuses on all the motors grounds, or would one single
>one be sufficient?
>
>

Certainly DON'T fuse the grounds. That could cause all sorts of
problems, depending on the
exact design of the stepper drivers. You want to fuse the (+) power
input to the stepper drives,
in most cases. Don't put fuses on the motor wires at all, this could
damage the stepper
drivers if the fuses ever pop. If the maker of the stepper driver gives
such info, that would
be the best authority. If not, there can be all sorts of ramifications.
For instance, with
Gecko drives, you should put a large, low-ESR electrolytic capacitor
directly on the
Gecko power terminals, and have the fuse between that and the power
supply. Fuses
present substantial resistance, and can cause problems with devices like
stepper drivers
that draw power in short current bursts.

As for size, probably a 3 A standard-blow fuse would be a good starting
point. It is
unlikely that the drive will ever draw more than 1.5 x the current in
one motor winding,
unless you run it at very high speeds. Anyway, the fuse will not save
the drives from
most breakdowns, it is there to stop current flow AFTER a transistor has
blown.
This will limit the fireworks substantially if a transistor in the
driver fails.

Jon