Physics · Electronics

Interpreting diode behaviour

You are told the diode only conducts one way, but the circuit diagram still confuses you.

A semiconductor diode lets current pass in one direction only. The symbol has an arrow pointing the way conventional current can flow, and a bar that blocks the other way.

This lesson is part of SPM Physics electronics. It leads into explaining rectification.

How do I decide forward or reverse bias?

Trace the conventional current from the battery’s positive terminal. Check where it meets the diode.

Where the current enters	Bias	Result
The arrow side (anode)	forward	current flows
The bar side (cathode)	reverse	almost no current

Worked example: a lamp and a diode in series

A 6.0 V battery, a diode and a 100 Ω lamp are in series. Treat the diode as ideal when forward biased.

Forward bias. The positive terminal connects to the anode. The current is I = V ÷ R = 6.0 ÷ 100 = 0.060 A, and the lamp lights.

Reverse bias. Swap the battery connections. The positive terminal now meets the bar side, the diode blocks, and the current is close to zero. The lamp stays off.

Swapping the battery connections switches the lamp off, which shows that the diode, not the lamp, sets the direction.

The mistake that loses marks

A student reads the arrow as the direction of electron flow and decides the diode blocks. The arrow shows conventional current, which goes from positive to negative.

Always decide from the positive terminal of the battery. If the arrow points the same way as the current travels from that terminal, the diode conducts.

Check yourself

A diode is connected with its bar side toward the positive terminal of a cell, in series with a lamp. Does the lamp light, and why?

Answer

The lamp does not light. The current from the positive terminal meets the bar side of the diode, so the diode is reverse biased.

A reverse-biased diode blocks current, so the lamp gets almost none.

What to study next

Put diodes to use in explaining rectification. You can also review the circuit rules in electricity.

Test yourself with the electronics practice set. If you want a teacher to give you new diode circuits, see online one-to-one Physics tuition.

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Common questions

What does the arrow in the diode symbol show?

It points in the direction of conventional current, from the p side (anode) to the n side (cathode). Current flows easily the way the arrow points, and is blocked the other way. It does not point along electron flow.

What is forward bias?

The diode is forward biased when the anode is connected to the more positive side of the supply. Current then flows and the diode acts almost like a closed switch. In most school problems, treat the voltage drop across it as negligible unless told otherwise.

What is reverse bias?

The diode is reverse biased when the anode is on the more negative side. Almost no current flows, so the diode acts like an open switch, and a lamp in series stays off.

How do I decide the bias quickly?

Follow the conventional current path from the positive terminal of the battery. If it enters at the bar end of the symbol, the diode blocks. If it enters at the flat back of the arrow, the diode conducts.

Keep going

A teacher working one-to-one can give you a new diode circuit each time and listen to how you decide the bias, which shows whether the rule or only one diagram has been learned.

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