Software engineer here :) Never work with electronics before and now my air conditioner is not working and I decided to check it myself.

This is a PCB from old air conditioner. And when the power is on, the relay RY02 switched on for a split second and tuned off again. Should be ON all time so the fan will work.

Explanation of photo:

  1. Red line is a signal to turn on the fan.
  2. Orange line is a signal to turn ac compressor.
  3. Blue - probably 12 V load for relays RY02 and RY03.
  4. RY01 - is a main power relay with 220 V.
  5. Yellow lines are lines to the ac compressor and fan accordingly to the relays.
  6. RY02 - relay for FAN.
  7. RY03 - relay for AC Compressor.
  8. D02 - diode (W6)

I don't understand why there is a diode D02 in RY02 relay? What is its purpose? And why there is no diode on second relay RY03?

enter image description here


2 Answers 2


Most probably a flyback diode across the relay coil. You need to trace out a schematic to tell for sure.

If it is, it’s there to prevent inductive kickback from the coil which will store energy when energized and when you turn it off, you need to provide a suitable path for that current to commutate. A flyback diode is the easiest way to do that.

See here for more information: Use of Flyback Diodes

Is a flyback diode required for this relay?

  • 2
    \$\begingroup\$ It does not protect they relays, they don’t care. It’s to protect the electronics which drives them. Set your DMM to diode mode and measure every pin and lead combination and report your measured values. \$\endgroup\$
    – winny
    Jun 29, 2023 at 21:55
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    \$\begingroup\$ Strange. Try unsolder it first, then measure. \$\endgroup\$
    – winny
    Jun 30, 2023 at 6:56
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    \$\begingroup\$ Most probably this is a dual diode with common cathode connected to +12V and one side of each relay coil, and the anodes on the other side of each relay coil. It appear low impedance to a diode tester due to the relay coils. Perhaps this a DCB010 which is marked "W6" (see below "Electrical Characteristics" on the data sheet). \$\endgroup\$
    – fgrieu
    Jun 30, 2023 at 10:33
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    \$\begingroup\$ Well, I just burned it, was trying to test relay and probably missed polarity and diode is gone. And thanks to @fgrieu-onstrike i know the direction of this diode and will try to replace it with something similar or maybe build same schema outside of PCB. btw maybe someone has access to PCB for my air con too? It's 2002 Sanyo SAP-KR129EH. I have only service manuals w/o schematics. \$\endgroup\$ Jun 30, 2023 at 12:05
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    \$\begingroup\$ For direction, please trace out a schematic around the coil. What is connected to Vcc/+12 V? What goes elsewhere? \$\endgroup\$
    – winny
    Jun 30, 2023 at 12:47

Just addition to previous answer. There is 2 diodes in SOT3 enclosure with common anode or cathode. Check ratings for relays coils, if it is 12DC, most possible it is flyback diodes.

  • \$\begingroup\$ yes, it's 12V DC. \$\endgroup\$ Jun 29, 2023 at 21:20

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