Figure 5-15 shows a zener regulator. When the circuit is working properly, the voltage between node A and ground is +18 V, the voltage between A node B and ground is + l0 V, and the voltage between node C and ground is + 10 V.
Now, let's discuss what can go wrong with the circuit. When a circuit is not working as it should, a troubleshooter usually starts by measuring node voltages. These voltage measurements give clues that help isolate the trouble. For instance, suppose he or she measures these node voltages
VA = + 18 V VB = + 10 V VC = 0
When you are trying to figure out what causes incorrect voltages, trial and error is useful. That is, you play the what-if game. Here is what may go through a troubleshooter's mind after measuring the foregoing node voltages.
Figure 5–15 Zener Regulator
What if the load resistor were open? No, the load voltage would still be + 10 V. What if the load resistor were shorted? No, that would pull nodes B and C down to ground, producing 0 V. All right, what if the connecting wire between nodes B and C were open? Yes, that would do it. That's got to be it.
This trouble produces unique symptoms. The only way you can get this set of voltages is with an open connection between nodes B and C.
Not all troubles produce unique symptoms. Sometimes, two or more troubles produce the same set of voltages. Here is an example. Suppose the troubleshooter measures these node voltages:
VA = +18V VB = 0 VC = 0
What do you think the trouble is? Think about this for a few minutes. When you have an answer, read what follows.
Here is a way that a troubleshooter might find the trouble. The thinking goes like this:
I've got voltage at A, but not at B and C. What if the series resistor were open? Then no voltage could reach node B or node C, but I would still measure + 18 V between node A and ground. Yes, the series resistor is probably open.
At this point, the troubleshooter would disconnect the series resistor and measure its resistance with an ohmmeter. Chances are
that it would be open. But suppose it measures okay. Then the troubleshooter's thinking continues like this:
That's strange. Well, is there any other way I can get +18 V at node A and 0 V at nodes B and C? What if the zener diode were shorted? What if the load resistor were shorted? What if a solder splash were between node B or node C and ground. Any of these will produce the symptoms I'm getting.
Now, the troubleshooter has more possible troubles to check out. Eventually, she or he will find the trouble.
When components burn out, they usually become open, but not always. Some semiconductor devices can develop internal shorts, in which case, they are like zero resistances. Other ways to get shorts include a solder splash between traces on a printed-circuit board, a solder ball touching two traces, etc. Because of this, you must include what-if questions in terms of open components, as well as open components.
Example 5–8
Assume an ideal zener diode and work out the node voltages for all possible shorts and opens in Fig. 5-15.
Solution
In working out the voltages, remember this. A shorted compon-ent is equivalent to a resistance of zero, while an open compon-ent is equivalent to a resistance of infinity. If you have trouble calculating with 0 and ∞ then use 0.001 and 1000 M . In other words, use a very small resistance for a short and a very large resistance for an open.
To begin, the series resistor RS may be shorted or open. Let us
designate these RSS and RSO, respectively. Similarly, the zener diode
may be shorted or open, symbolized by D1S and D10. Also, the load
resistor may be shorted or open, RLSand RLO. Finally, the
connecting wire between B and C may be open, designated BCO.
If the series resistor were shorted, + 18 V would appear at nodes B and C. This would destroy the zener diode and possibly the load resistor, but the voltage would remain at + 18 V. Then a trouble- shooter would measure VA = +18 V, VB = + 18V, and VC = +18V.
If the series resistor were open, then the voltage could not reach node B. In this case, nodes B and C would have zero voltage. Continuing like this, we can get the remaining entries shown in Table 5-1.
In Table 5-1, the comments indicate troubles that might occur as a direct result of the original short circuits. For instance, a shorted RS
will destroy the zener diode and may also burn out the load resistor. It depends on the power rating of the load resistor. A shorted RS means there's 18 V across k . This produces a power of
0.324 W. If the load resistor is rated at only 0.25 W, it will burn out. Study the table. You can learn a lot from it. Also, use the T-shooter at the end of this chapter to practice troubleshooting a zener regulator.
Table 5–1