4. Diode: reverse-polarity protection
Step 4 of the automatic light: D1 in the + line right after the battery. The LED gets 0.7 V less (a little dimmer); connect B1 backwards and the circuit simply stays dark, no damage. Component 4 of 14: the diode. A diode conducts one way only (anode → cathode) and drops about 0.7 V while doing it. In reverse it blocks, up to its peak reverse voltage. The cheapest protection there is: if the battery goes in backwards, nothing flows.
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Parts list
Test procedure
Each test has an expected value and a pass range, like a factory test sheet:
- Normal: LED lit (0.7 V less)
- Battery connected backwards
How it works
The diode. A diode conducts one way only (anode → cathode) and drops about 0.7 V while doing it. In reverse it blocks, up to its peak reverse voltage. The cheapest protection there is: if the battery goes in backwards, nothing flows.
Reading it. The band marks the cathode. 1N4007: 1 A, 1000 V. 1N4148: small signal, 0.2 A, 100 V. Schottky (1N5819): only ~0.3 V drop, but less reverse voltage.
Testing it. Meter on the diode range: 0.5–0.7 V with red on the anode, OL the other way. OL both ways = open; 0 both ways = shorted.
In the build. D1 in the + line right after the battery. The LED gets 0.7 V less (a little dimmer); connect B1 backwards and the circuit simply stays dark, no damage.
Builds on lesson 3: everything from before is still there.
Next: lesson 5, capacitor.
Things to try
- Measure the voltage across D1 while the LED is lit: about 0.7 V.
- Swap D1 round (it then blocks the right way): the LED stays dark. That is what it does for a reversed battery.
Background
One circuit, built up one part at a time: by lesson 14 it is a solar-powered, dusk-sensing, motion-triggered timed lamp with an alarm.