Re: Question on Power Supply - regulated notes

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

> m0nkey0ne wrote:
>
> >This is an interesting and informative thread. Please don't stop!
> >It may have been repeated, or way beyond the technical knowhow of
> >some of us, or so utterly simple it should have already been known,
> >
> >
> The only thing I want to point out is the use of the zener diode, as
> I've done in my supply.
>
> Here is a description of the power supply setup I'm using.
>
> My transformer is a 750VA 440-120 stepdown transformer that I found on
> ebay for $10. With USPS flat-rate shipping it cost me $17. I run the
> primary at 120VAC to get a secondary voltage of about 30VAC. After the
> rectifier and the filter cap its a bit over 40VDC. To this I add a 39V
> zener (the wattage of zener is chosen based on how much current it will
> be handling when the supply is unloaded). This clamps my final voltage
> to no more than 39VDC. Any overvoltage at the primary resulting in
> greater voltage on the secondary is dumped through the zener, providing
> some protection to the controller board.
>

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.

Zeners are typically used in low current circuits to protect sensitve
devices and/or provide a constant reference voltage for control
circuits. As a power shunt they have to be of the stud mount varity
with a series resistor. More common is to use a Zener to trigger a
power device (Transistor or MOSFET).