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Context:

I got this fancy freezer from Liebherr a while ago and it stopped working. The compressor would fail to run continuosly. I have narrowed down the issue to the control board (behind the front panel). The micro-controller would constantly attempt to turn the triac ON, but fail to maintain the ON state. The triac is responsible for switching on the compressor.

Suspect:

The following power supply circuit (might be wrong though... multilayer PCB):

schematic

simulate this circuit – Schematic created using CircuitLab

The C1 is X2 MPF capacitor. R1 is a 0.5W resistor. C2 is an electrolytic (already replaced). The two diodes next to the ceramic capacitor are two schottky diodes.

If I attach a regulated 3V power supply across the capacitor (the polarized one), the micro controller is happily chooching. The current draw from the external power supply does not seem to be high. but as soon as I switch off the external feed, the leds on the display dim and the micro-controller fails to keep the triac in the on state.

So the question is: how do I troubleshoot this further?

Picture:

enter image description here

Another Detail:

The X2 Metalised polyester foil capacitor is rated 680nF but my multimeter reads 104nF. Could that be the issue?

Update

Failed Component: C1. Thanks to Simon B

Denis
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2 Answers2

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It looks like a relatively standard capacitive dropper circuit. C1 provides the current limiting. R1 prevents a surge on power-on. The two diodes, plus the two pairs of diodes (why pairs?) form a bridge rectifier, with C2 providing the smoothing.

With the correct load applied, there should be an appropriate voltage on the + and - terminals. Without the load, the voltage could rise to the point where C2 goes bang.

You should be able to test each of the components in turn with a multimeter. If C1 is failing, it may not be letting through enough current.

Beware, there would normally be a high value resistor across C1 in similar circuits. Without that resistor, if the circuit has been plugged in recently, there could be a substantial voltage across C1.

Simon B
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  • The X2 capacitors are designed to fail open (that is what i read on the net). Could the C1 capacitance deviation be causing this? – Denis Dec 23 '16 at 23:18
  • Thanks for explaining the circuit by the way. I kind of assumed the C1 is just for filtering purposes only. – Denis Dec 23 '16 at 23:20
  • Yes. If C1 is going open circuit, then the current through it will be dropping below what was designed. It's being used as a simple constant current source, based on its capacitance, the mains voltage and the mains frequency. – Simon B Dec 23 '16 at 23:21
  • Damn brain! I speak the words, but do not understand them. I say Open and test for short, don't see the short and assume everything is fine and dandy. Let me solder some things and come back to you in a sec. – Denis Dec 23 '16 at 23:27
  • Thanks for kicking my brain, Simon. The Freezer is running again! – Denis Dec 23 '16 at 23:30
  • Found some more info on the circuit in question [link](http://ww1.microchip.com/downloads/en/AppNotes/00954A.pdf) – Denis Dec 26 '16 at 07:55
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C1 may be the reason yes. The pairs of smd diodes indicate that there should be a "great" current flow (compared to a micro controller only PCB) as it is driving LEDs and triac, there may be two main reasons:

  • C1 and R1 are in a HighPass filter configuration, therefore a C1 deterioration that lower the capacity and therefore higher the cutoff frequency that may introduce noise to the power line. Dimming LEDs and "ciclicaly" reset the uController.

  • If you measure a 6x lower capacity with a multimeter (that induces a very low current flow while measuring) there may be major material deterioration inside the capacitor, that under stress will behave unpredictably.

If diodes are not burnt, changing the two main capacitors will solve it.

Newbie
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