Page 85 - Troubleshooting Analog Circuits

72 6. Understanding Diodes and Their Problems

most 10- or 50-W zeners. If you need a really high-current zener, a power transistor
can help out (Figure 6.4). As mentioned earlier, a diode tends to fail by becoming a
short circuit when overpowered, and zeners cannot absorb as much power as you
would expect from short pulses. How dreadful; but, can IC designers serendipitously
take advantage of this situation? Yes!
The Vos of an op amp usually depends on the ratio of its first-stage load resistors.
IC designers can connect several zeners across various small fractions of the load
resistor. When they measure the Vos, they can decide which zener to short out-or
zq-with a 5-ms, 0.3- to 1.8-A pulse. The zener quickly turns into a low-impedance
(= 1 short), so that part of the resistive network shorts out, and the Vos is improved.
In its LM108, National Semiconductor first used zener zapping, although Precision
Monolithics (Santa Clara, CA) wrote about zener zapping first and used it extensively
later on. Although zener zapping is a useful technique, you have to be sure that no-
body discharges a large electrostatic charge into any of the pins that are connected to
the zener zaps. If you like to zap zeners for fun and profit, you probably know that
they really do make a cute lightning flash in the dark when you zap them. Otherwise,
be careful not to hit zeners hard, if you don’t want them to zap and short out.
These zener zaps are also becoming popular in digital ICs under the name of “ver-
tical fuses” or, more correctly, “anti-fuses.” If an IC designer uses platinum silicide
instead of aluminum metallization for internal connections, the diode resists zapping.

Diodes That Glow in the Dark, Efficiently
Once I needed 100 LEDS, so I bought 200 LEDs from the cheapest supplier. I hoped
to find some good ones and maybe just a few units that were weak or performed
poorly, which I could use for worst-case testing. I lost out; every one of the 200 was
of uniformly good intensity. In a variation on Murphy’s Law, worst-case parts will
typically appear only when you are depending on having uniform ones.
,
So long as you don’t fry LEDs with your soldering iron or grossly excessive mil-

o HIGHTNT RECTIFI;

9v 2N3055
COMPOUND
ZENER EQUIVALENT

HIOHCURRENT RECTIFIERS OR BRIDGE

W
COMPOUND 2N3055
SYMMETRICAL OR
ZENER EQUIVALENT

Figure 6.4. The power rating of this compound zener (a) is that of the power transistor. The second
compound zener, (b). is almost the same as (a) but acts as a symmetrical, matched, double-
ended compound zener.