What it shows
A silicon diode conducts almost nothing until the forward voltage reaches about 0.6 to 0.7 V, then the current rises steeply. In reverse it passes almost no current.
The science
A p-n junction diode has a potential barrier of about 0.7 V in silicon. Below this the forward current is tiny; above it the current grows roughly exponentially, so the graph bends upward sharply at the knee. In reverse bias the barrier grows taller, so only a very small leakage current flows, too small for a school meter to show. Because the diode is not ohmic, V / I is not constant.
Materials
Cost: under Rs 200. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A 1N4007 or 1N4148 silicon diode
- Three AA cells in a holder
- A 1 kohm preset or a 10 kohm pencil-lead slider as a potential divider
- A 100 ohm resistor to protect the diode
- Two multimeters (one for voltage, one for current in mA)
- A breadboard and wires
- Graph paper
Safety
For this project: An adult must check the circuit before connecting; never connect the diode directly across a cell without the 100 ohm resistor, as the current would be large.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
7 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Build the divider by connecting the cells across the preset’s two ends so the slider gives any voltage from 0 to 4.5 V.
Join the slider, the 100 ohm resistor, the diode (band end toward the cell’s negative side for forward bias) and the milliammeter in series; put the voltmeter directly across the diode.
Start with the slider at zero volts and raise it in small steps; at each step write the diode voltage and the current.
Take readings every 0.1 V from 0.3 V to 0.8 V where the current changes fastest, and stop before the current exceeds 30 mA.
Turn the diode round for reverse bias and take the same sort of readings from 0 to 4.5 V; note that the milliammeter hardly moves.
Plot the forward points with voltage on the x axis and current on the y axis, joining them smoothly, and put the reverse points to the left on the same sheet.
Mark the knee voltage where the curve leaves the axis and compute the dynamic resistance ΔV / ΔI on the steep part.
What you should see
The forward curve stays flat until about 0.6 V and then climbs steeply to a few tens of mA. In reverse the current stays near zero up to the highest voltage used.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What is the knee voltage?
The forward voltage at which the diode starts to conduct noticeably, about 0.7 V for silicon.
2Why is a series resistor used?
To limit the current so that the diode is not damaged.
3Is a diode an ohmic conductor?
No, its V-I graph is not a straight line, so its resistance changes with voltage.
4What is the use of a diode in a power supply?
It rectifies, letting current pass in one direction only, so AC becomes DC.
Take it further
Compare a red LED’s knee voltage, about 1.8 V, with the silicon diode on the same axes.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
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Electronic Components Handbook
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