Audio Engineering

Components 127

Make sure the current meter’s positive terminal goes to the battery’s positive terminal.
The current will be just under a tenth of a milliamp. The actual current value doesn’t
matter. If the meter’s needle kicks against the end stop, reverse the meter polarity
(assuming you’ve got an analog multimeter); a digital multimeter will automatically
compensate whatever polarity is present. When using a digital multimeter to measure DC
voltage, there is no need to worry if you get the test leads reversed. The multimeter will
still show the correct voltage; there’s just a negative sign in front of the numeral. That
tells you that the multimeter red test lead, for example, has been connected to the negative
voltage potential. There is no damage done to the digital multimeter. If you now take
the feed of the positive battery terminal via a diode (it doesn’t matter at this stage which
way round it goes), one of two things will happen. Either the current will be the same as
before or the current will be zero. Whatever it is, take note of it. Then reverse the diode
polarity; just reverse the diode’s connection in the circuit. An effect opposite to the one
you fi rst observed will now take place. You’re seeing the rectifying action of the diode.

One really useful function for the diode is as a protective device. Electronic circuits are
invariably powered with the positive voltage supplying the power rail. If the voltage is
inadvertently reversed, there is a high probability that the components will suffer some
damage. Placing a diode (this would be a power type called a rectifi er) in series with the
positive supply voltage would do the trick. When the polarity is correct, insert the diode
in such a way that current starts to fl ow (trial and error is the quickest way to learn which
way to attach the diode if you’re not sure about the markings). Now if the voltage polarity
should be reversed, no current will fl ow, thus providing the protection. Try it and see.

Symbol

Cathode Anode

Component









Forward bias

High current flow

Reverse bias

No current flow

Figure 4.6 : Diode symbol and bias conditions.