Lessons · Engineering · A device that conducts one way
A component that conducts one way
A diode allows current in one direction and blocks it in the other, dropping a small roughly constant voltage when it conducts. That one-way behaviour is what turns alternating current into direct current and what protects circuits from reversed supplies.
Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.
In beta. This lesson was written for Hone and has not yet been checked by a licensed engineer. Practice material, not professional advice. What that means.
What it is for
It is the first component whose behaviour is not a straight line, and understanding it as a one-way valve with a small toll explains rectifiers, protection diodes and most power supplies without any mathematics.
How to think about it
Decide which way round the diode is and whether the circuit is trying to push current that way. If it is, subtract the forward drop and continue; if it is not, treat the branch as open.
Worked example
Forward: conducts, with a small voltage drop.Roughly constant while conducting.
Reverse: blocks.Treat the branch as open.
Four diodes make a bridge rectifier.Both halves of the wave become one direction.
Reversed supply protection is the same property.The diode simply does not conduct.
Your turn
A 5.0 V supply feeds a load through a forward diode dropping 0.7 V. Write the load voltage in volts.
5.0 - 0.7 = V
Solve one, graded on the server
The trap
Forgetting the forward drop in a low-voltage circuit. Seven tenths of a volt is nothing at 240 and is 14% of a 5 V supply.