Re: Question on Power Supply - regulated notes

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Codesuidae <codesuidae@c...>
wrote:

> caudlet wrote:
>
> >--- In CAD_CAM_EDM_DRO@yahoogroups.com, Codesuidae <codesuidae@c...>
> >wrote:
> >
> >
> >>The only thing I want to point out is the use of the zener diode, as
> >>I've done in my supply.
> >>
> >>
> >A zener is an interesting device. I conducts virtually no current
> >until it reaches it's designed voltage than rapidly (over less than a
> >few millivolts goes into full conduction (infinite current). A Zener
> >placed across a DC level will absorb ALL of the current the source can
> >supply as long as the voltage remains above the turn-off voltage of
> >the device. The bad part of this is most zeners (you can afford) are
> >rated at either 1W or 5W. They could easily see 10 times that amount
> >of dissipation in a short period of time. In the case of a line surge
> >or back EMF the zener will last in the microsecond time frame.
> >
> I did neglect to mention that I have a low value resistor in series

with

> the DC supply before the zener to limit the current the zener must pass
> when the transformer is unloaded. This page describes and explains the
> configuration i'm using:
>
> http://tinyurl.com/8kvds
>
> Dave

Things can get out of hand in a hurry where the load is several amps.
The resistor will drop (and dissipate as heat) the current flowing to
you motors (and limit the current in the process). To keep
dissipation low you need a low value of resistor. For example if you
are drawing 3A and the value of your resistor is 3 ohms it will
dissipate 27 Watts and drop a 39 Volt head voltage down to 30VDC. If
the load is removed and the head voltage were to rise from 39 to just
42VDC the zener would drop the 3 volts difference using the same
resistor to limit current. 3 volts though 3 ohms limits the current
to 1A through the diode. 3 watts of dissipation. The numbers get
worse the lower the resistor value.

The series resistor in the DC path to the motors is not a good idea if
you want max performance unless the value is very low (< 1 ohm for a
3 or 4 AMP supply) and the same 3 volt surge as before then places
over 9 watts of dissipation on the zener. Also back EMF from the
motors greater than the zener breakover will be clamped at a current
level only limited by the motor impendence.

I know this is a pretty small supply but used as a shunt regulator
(across the outputs) zeners are not a good choice for any supply that
can source over a few hundred milliamps.

In a power supply for motors that draw their currents in a non-linear
fashion you want a power supply with a "low impedence" meaning "stiff"
voltage source. A series resistor raises the impedence and defeats
part of the purpose of the power supply.

A better way would be to replace the series resistor with a much lower
value (say .1 ohm) and to replace the Zener with an NPN darlington
power transistor. Put the zener in series with a 1K resitor and tie
the cathode to the transformer side of .1 ohm resistor and and the
anode through the 1K to the base of the NPN. This in essence is an
"amplified" Zener that will dissipate 30 to 50 Watts without burning up.

I'm not trying to throw rocks at your design but merely point out the
"physics" involved.