I am just getting started working with small electronics. I have soldered together a few kits and recently converted a car stereo into a home stereo. With all of this soldering, I found this schematic for a soldering fan/lamp that is made out of a computer fan and two LEDs. enter image description here

from Instructables

In the description that goes along with the instructions, they explain that the zener diode allows for the LEDs to receive a fixed voltage. I looked up zener diode and saw that it was a diode that allowed for current to flow in the opposite direction if the voltage was high enough. I am having a hard time understanding how being able to switch the direction of the current fixes the voltage?

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    \$\begingroup\$ Does "Instructables" tell you how not to build something ?Sure the zener is round the wrong way but why not place the LEDs inseries so they will have equal brightness and you will save some current. \$\endgroup\$
    – Autistic
    Jan 31, 2016 at 2:14

1 Answer 1


The description in your link is totally bogus.

Assuming the switch is in the positive lead, the LEDs are shown backwards - the cathodes should be towards the negative supply lead, and there should be one resistor for each LED. (Sharing one resistor between two paralleled LEDs is a Bad Idea, particularly if the LEDs are different colours.) There will be 4.7 volts dropped across the Zener diode, but that does nothing to stabilize the voltage across the LEDs, or otherwise compensate for varying supply voltages - it just makes the voltage across the LEDs and their resistors about 4.7 volts less than the supply voltage.

  • \$\begingroup\$ ok wow, thank you that makes a lot more sense to me now! \$\endgroup\$
    – celeriko
    Jan 22, 2016 at 4:17

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