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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

JYelton
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Dmitry Sergeev
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2 Answers2

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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?

winny
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  • So this one flyback diode connected to both relays to protect them? Can it be that if this diode is broken, than both relay switched off imidietly? – Dmitry Sergeev Jun 29 '23 at 21:27
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    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. – winny Jun 29 '23 at 21:55
  • it's 0 across every pins combination. I tried another diode with same "W6"(btw what does it mean?) and got 150 between some legs. And what really confuse me on another diode with "W6" mark, two legs on same side are connected – Dmitry Sergeev Jun 29 '23 at 22:15
  • and unfortunately there is no schematic in internet for my 2002 sanyo air conditioner. Just basic service manual with connection diagram. – Dmitry Sergeev Jun 29 '23 at 22:31
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    Strange. Try unsolder it first, then measure. – winny Jun 30 '23 at 06:56
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    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](https://www.mouser.com/datasheet/2/308/EN1895C-9753.pdf)). – fgrieu Jun 30 '23 at 10:33
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    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. – Dmitry Sergeev Jun 30 '23 at 12:05
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    For direction, please trace out a schematic around the coil. What is connected to Vcc/+12 V? What goes elsewhere? – winny Jun 30 '23 at 12:47
  • Yes, i tried to find polarity on relay, but there are no marks, even in datasheet. https://www.fcl.fujitsu.com/downloads/MICRO/fcai/relays/ftr-f3-5a.pdf As you said, I will try to draw schematic and than i will think what to do, but w/o schematic it will be long process. – Dmitry Sergeev Jun 30 '23 at 12:55
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    Good find, but you need to trace it out from the PCB. – winny Jun 30 '23 at 12:58
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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.

user263983
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