16 years ago
Here we go. Consider a resistor and a zener connected as a shunt regulator
between +ve and -ve supply rail. The resistor is sized so that a safe
no-load current of, say, 25mA passes through zener. Naturally, we measure
the nominal zener voltage or thereabouts at the point where resistor and
zener connect. If we apply a load at this point, how much current can the
load draw before we lose regulation? Surely not the full 25mA? There must
be a minimum current we must leave for the zener so that it continues to
operate as a zener.
If this vital information can be gleaned from the zener data-sheet, what's
the term I should be looking for?
I did look up my old ARRL manual and it says that the feed resistor should
be calculated by SupplyV minus ZenerV divided by 1.1 Load current. That
sounds like a rule of thumb of "add another 10% for the zener".
Does that sound like a wise rule to follow? I can think of situations where
the load current varies widely and there's a fine line between losing
regulation at high loads and burning the zener up at low loads. In such a
case, the amount of current I must leave for the zener will affect the
maximum load current, given a zener of a particular wattage. On the other
hand, it's dumb to design a power-gobbling zener current of 80mA when the
load will never exceed 10mA.
Apart from that, I've looked at zener data sheets and Googled around the
electronics education websites and am no wiser.
Maybe I'm just tired and dense after a hard day so I'm asking you
knowledgeable gentlemen to think for me again.