Re: newbe needs help?

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Barrie Hopewell" <bazo@b...>
wrote:

> hi all
> been reading most of the groups emails for about year now and have

steadily been byeing the bits to build my cnc m/c but have hit a
problem with the power surrply which is I don't know what parts to
bye i.e. caps and rectifiers . the transformer I intend to bye is a
toroidal type 1000va 11.25A 48V with the secondary winding at 500va .
so my questions are what does VA stand for, what sort of rectifiers
do need and what size and sort of capacitors do I need ,for any uk
people out there I am hoping to bye these from RS so if you know any
stock numbers would be most appreciated for all the boys in the
USA , I don't mind byeing from there if you have any ideas all your
help will be vary helpful

>
> many thanks baz
>
>

First some basics: VA is Volt-Amps. For a load with a power factor
of 1 (pure resistive) its the same as Watts (volts * amps). From
what I have seen in a lot of the posts there is a tendency to
oversize the power supply on most of these designs (here comes the
rocks!). Having more current available than you need is like having
250 liters of petrol in a can in the back of your lorry for a trip of
20 kilometers. It will make you feel better but the motors will not
run faster or better. The thing to do is determine the voltage you
will need based on the motors you are going to use (this is where the
Gecko white paper will help). The rule of thumb is 10 to 20 times
the name plate voltage. The current is the rated current of all of
the motors less the duty cycle. Once again 60% is very
conservative. In most hobby conditions seldom are the motors
running at full speed, full load and all at the same time. Which
brings me back to my first point. IF all of your motors add up to
12Amps and you want to run them at 48v AC (about 67 Volts DC at the
filter cap) then you can probably live with a 48V 6A power supply.

There is a formula in the white paper for the size of the cap but we
(me and some other crusty power design engineers) always used 2000
mfd per amp of peak max load. Totally unscientific but we designed
and built hundreds of different linear and switching power supplies
that had a low failure rate.

Transformers and big filter caps are very forgiving of overloads.
Big inductive loads like motors are tolerant of more ripple than
something like an audio circuit.

There are a lot of your fellow countrymen that have taken the CNC
plunge and they may be able to guide you on sources. You might want
to checkout a US source of surplus powersupplies at www.MPJA.com.
There is nothing wrong with buying a regulated power supply cheap and
either tapping the raw voltage off the filter cap or using the
regulated output (even better if it has current limiting). A large
Diode in series with the positive will prevent any back EMF from
taking out the series regulator if you elect to use the regulated
output.

Hope this helps some and the white paper sheds more information.