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I am repairing an ultrasonic cleaner with a faulty circuit board. It has a blown out diode and capacitor that needs to be replaced. The board, however, has me curious and confused as to its function. I drew out the schemtic to the best of my abilities.

The large coil is a common mode choke according to its datasheet. It’s intended for filtering out high frequency noise, which is makes sense. There other boards drive the ultrasonic transducers at around 40 KHZ which may create interference.

schematic]([![enter image description here

The choke, diode and capacitor, however, appear to be stages in an AC to DC conversion. The choke is center tapped and rectified using only two diodes. This puzzles me since the choke’s datasheet only discusses its use for noise filtering.

Is the board both a DC filter and AC to DC converter? If so, shouldn’t the filtering be done after the AC to DC convention. Also, why would anyone want to use a choke instead of a transformer? I was under the impression that they were simple coils, not full blown transformers. Finally, not sure I drew it correctly, but what's the purpose of the two caps connected to ground?

EV35 EV35 traces

user148298
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    If it's a common mode choke it's not connected like that... DC measurements on the board should tell you how it's really connected. –  Jun 07 '20 at 20:35
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    Rotate the choke symbol through 90 degrees and it might be correct as a proper common mode choke. – Andy aka Jun 07 '20 at 20:35
  • too many schema errors. but cheap and dirty, unprotected from over-voltage or ground faults. – Tony Stewart EE75 Jun 07 '20 at 21:15
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    A transformer capable of handling the power involved would be 50x bigger. The common mode choke presents a high impedance to common mode noise and a low impedance to the normal mode power. You have drawn it rotated- it's basically one choke on each line but they share the same core so the current that goes through one choke and out the other subtracts from the magnetism and the inductance is very low, but if the voltage is on both lines the inductance (and therefore the impedance) high. – Spehro Pefhany Jun 08 '20 at 03:17
  • First, I couldn't find a component for the common mode choke. I could have used two coils, but I was uncertain as to how it was arranged and connected. Second, there is no longer a link for editing the diagram. CiruitLab has been going through some changes recently, it is now a pay to play site. – user148298 Jun 08 '20 at 20:48
  • @TonyStewartSunnyskyguyEE75. Are you saying the board itself is unprotected or are you saying my schema is unprotected? – user148298 Jun 08 '20 at 21:35
  • The schematic is wrong as a CM choke which burnt the board on 1 track only so it had to be a fault to ground rather than a short circuit differential loop . It has no over current protection like a PTC or a fuse. – Tony Stewart EE75 Jun 09 '20 at 01:57
  • I just used the transformer symbol http://i.stack.imgur.com/F43Gd.png – Tony Stewart EE75 Jun 09 '20 at 02:03
  • No wonder the company that made this device went belly up. They had a 1 star rating on google for bad customer service and for selling a highly defective product. I was wondering why there's a capacitor to ground and wether it would trip a GCFI or trip a breaker box. – user148298 Jun 09 '20 at 02:21
  • Weird serendipity! I was researching some more information on the board and discovered the following link in this site: https://electronics.stackexchange.com/questions/37514/replacement-transistor-for-ultrasound-cleaner The first answer from someone who claimed to be a former employee. – user148298 Jun 09 '20 at 02:24

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