What it shows
A diode lets current through in one direction only. Four diodes arranged as a bridge always pass the current the same way through the LED, so the lamp lights even if the battery is connected backwards.
The science
In a bridge, whichever input terminal is positive, two diodes conduct and two block. The current always leaves the bridge at the same output terminal, so the LED sees a fixed polarity. Each conducting diode drops about 0.7 V, so two drop 1.4 V; the LED takes about 2 V, and the remaining 5.6 V is across the 680 Ω resistor, giving about 8 mA.
Materials
Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Four 1N4007 diodes (or 1N4148)
- One LED and a 680 Ω resistor
- A 9 V battery with snap connector
- A breadboard and jumper wires
- A marker to note the diode bands
- A multimeter if available
Safety
For this project: Battery power only. Check the LED direction before snapping on the battery.
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
8 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.
Look at the diodes: the band marks the cathode (the side current leaves).
Place two diodes in a row with their bands facing the same way and two in the second row also facing the same way, so the four make a diamond.
Join the cathodes of the first pair together: this is the positive output of the bridge.
Join the anodes of the second pair together: this is the negative output.
Connect the AC-style inputs: the junction of one first-row diode and one second-row diode is input 1, the other junction is input 2.
Connect the positive output through the 680 Ω resistor to the LED long leg, and the LED short leg to the negative output.
Connect the battery to input 1 and input 2, then swap the two leads and see that the LED still lights.
Measure the voltage across the LED and across the bridge if you have a meter and compare with 1.4 V of diode drop.
What you should see
The LED lights with the battery connected either way round, showing a bridge makes polarity not matter.
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 a diode?
A component that lets current flow in one direction only.
2Why do two diodes conduct in each direction?
The bridge always forms a path through one diode on the high side and one on the low side.
3How much voltage is lost in the bridge?
About 1.4 V, 0.7 V in each of the two conducting diodes.
4Where is a bridge used in real devices?
In mobile chargers and power adapters, to change AC into DC.
Take it further
Add a 470 µF capacitor across the LED branch to see the effect of smoothing, then connect a hand-cranked toy motor as the source.
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
Robotics
Electronic Components Handbook
Resistors, capacitors, LEDs, diodes, transistors and more, with how to use each