Re: Simple PSU stuff

Assuming each motor may operate simultaneously, the transformer for your power supply is not large enough to handle 3 - 2 amp bipolar motors. On each full step 2 amps will flow through each of the motors coils making a total of 4 amps per motor or a total of 12 amps for 3 motors. This does not allow for fan current.

If you parallel connect the transformer coils you will get 12.6 amps at 12 volt ac. After rectification you will have about 17 volts dc.
Depending on type of drivers this may be quite ok as the supply voltage is over 5 times nominal motor voltage. Though higher voltage would probably improve the performance it may not be necessary as various factors including your machine design all inter-relate.

The minimum capactor size should be 2000uf per amp or 24000uf, so using a 47000uf capacitor could help reduce ripples a bit more. If you were only using say half the rated current of a transformer its quite possible the larger capacitor could deliver a higher output voltage, perhaps up to say 19 volts dc, as it would be working higher up the rectified sine curve and resistive losses etc in the transformer would be lower due to a lower current.

If it was me I would try it out and see what happens, it may work very well or you may have to get a bigger transformer to get absolute peak performance.

Regards
Bob Thomas


The conventional size of a fuse in a normal electrical circuit is twice the size of the current, so a 2 amp system should have a 4 amp fuse.

Regards
Bob Thomas

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Jon Elson <elson@p...> wrote:
>
>
> 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