Re: Simple PSU stuff

The traditional 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