Diodes II
Electronics guide > Diodes II

In the last chapter, we took a detailed look at diodes and their characteristic curves. This chapter were going to take this one stage further and consider how we use the characteristic curve to define how the diode will operate in any particular circuit. Finally, well look at a number of circuits which show some of the many uses of ordinary diodes.

The components youll need for the circuits this chapter are:

  • 1 x 10 k resistor
  • 2 x 15 k resistors
  • 2 x 100 k resistors
  • 1 x 10 μF 10 V electrolytic capacitor
  • 1 x 220 μF 10 V electrolytic capacitor
  • 1 x 1N4001 diode
  • 1 x LED
  • 1 x 555 integrated circuit

Figure 7.1 shows the forward biased section of a typical diode characteristic curve. It has a transition voltage of about 0V7, so you should know that its the characteristic curve of a silicon diode.

The forward biased section of the characteristic curve of a silicon diode

Figure 7.1 The forward biased section of the characteristic curve of a silicon diode

Diodes arent the only electronic components for which characteristic curves may be drawn most components can be studied in this way. After all, the curve is merely a graph of the voltage across the component compared to the current through it. So, its equally possible that we draw a characteristic curve of, say, a resistor. To do this we could perform the same experiment we did last chapter with the diodes: measuring the voltage and current at a number of points, then sketching the curve as being the line which connects the points marked on the graph.

But theres no need to do this in the case of a resistor, because we know that resistors follow Ohms law. We know that:

where R is the resistance, V is the voltage across the resistor, and I is the current through it. So, for any value of resistor, we can choose a value for, say, the voltage across it, and hence calculate the current through it. Figure 7.2 is a blank graph. Calculate and then draw on the graph characteristic curves for two resistors: of values 100 Ω and 200 Ω. The procedure is simple: just calculate the current at each voltage point for each resistor.

A blank graph for you to fill in  see the text above for instructions

Figure 7.2 A blank graph for you to fill in see the text above for instructions

Your resultant characteristic curves should look like those in Figure 7.3 two straight lines.

Your efforts with Figure 7.2 should produce a graph something like this

Figure 7.3 Your efforts with Figure 7.2 should produce a graph something like this

Take note

Resistor characteristic curves are straight lines because resistors follow Ohms law (we say they are ohmic) and Ohms law is a linear r e l a t i o n s h i p . S o r e s i s t o r c h a r a c t e r i s - tic curves are linear, too. And because they are linear theres really no point in drawing them, and youll never see them in this form anywhere else we drew them simply to emphasise the principle.

We need to draw diode characteristic curves, on the other hand, because theyre non-ohmic and hence, non-linear. So to see what current passes through the device with any particular voltage across it, its useful to see its characteristic curve. This is generally true of any semiconductor device, as well see in later chapters.

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