Electronics > Repair
Giant MLCC cap in CCFL HV supply, what could be its capacitance?
capslock:
I tried to kill the high pitch noise eminating from Bosch/Blaupunkt Nav Radios used in 2010-2014 Ford S-Max, Galaxy and Mondeo. My prime suspect was an MLCC cap that seems to connect two transistor collectors (they are marked 435ZDG) near the transformer. Unfortunately, I didn't measure the cap in circuit, and in desoldering, one of the contacts started sliding off.
It's a raw MLCC stack, so no molded case. It is 7.0 mm long, 6.38 mm wide and 4.8 mm high. This does not seem to match any known SMD case. Any idea what the casee might be called and who made those?
For context, here's a thread in a car forum and a picture of the PCB in question that sits on the back of the LCD touch screen.
https://www.motor-talk.de/forum/navi-nerviges-fiepen-t4268623.html
https://www.motor-talk.de/bilder/navi-nerviges-fiepen-g77078414/004-i208918361.html
inse:
It seems to be the resonant capacitor in the HV inverter for the backlight.
Unfortunately you screwed it, the inverter will probably not run stable without the right replacement.
Ceramic capacitors do have a piezo electric effect which creates audio noise when excited electrically.
Even with the original component you will not get rid of the noise.
You would have to shift the operating frequency into a non audible range.
Without schematic there is little chance to get the inverter back running.
You could try hacking in a separate backlight inverter search term ccfl inverter those are available with various specs - good luck!
capslock:
Interesting, probably something like this:
https://www.google.com/url?sa=i&url=https%3A%2F%2Fdanyk.cz%2Fccfl_en.html&psig=AOvVaw0hYbeKMLc9HeLMEtglxoz9&ust=1735848170088000&source=images&cd=vfe&opi=89978449&ved=0CBEQjRxqFwoTCNjEifaK1YoDFQAAAAAdAAAAABAE
Why do they use a 160 V rated cap in this example when the operating voltage is 12 V? Resonance of the LC circuit or even back-induction on switching transients?
It also means I was on the wrong track looking for a high capacitance, low voltage cap. It also begs the question why Bosch did not use a proper MKT cap that would have been acoustically quieter.
I guess I'll just try some film capacitor of similar size. If I change the resonant frequency, that would actually be welcome.
inse:
Size is probably not the proper selection criterium..
Are you from Germany?
You can have this one, for 7 inch TFT, automotive
capslock:
Yes, I am, but unfortunately, that is not the board. In your pic, those also seem to be MLCC caps, but they are greenish-gray rather than light brown.
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