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I have a short ( i think) and don't know where to start
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metrologist:
You said when you put your bench supply to U1 - U2, it went to CC mode. You need to identify what caused that. In that state, if it is not drawing too much current, because you set the current limit to a decent value, you might be able to detect something getting hot. A thermal camera is great for that, or maybe a finger...
Hextejas:

--- Quote from: metrologist on December 19, 2018, 07:38:24 pm ---You said when you put your bench supply to U1 - U2, it went to CC mode. You need to identify what caused that. In that state, if it is not drawing too much current, because you set the current limit to a decent value, you might be able to detect something getting hot. A thermal camera is great for that, or maybe a finger...

--- End quote ---
Excellent advice and i will give it a try.
nick_d:
Yes you need both resistors (for the test at any rate, I am not sure I would recommend the resistors for production). Now let us think about this. A fully discharged capacitor charging from 35V through 2 ohms will take 17.5A initially, this dropping off as the capacitor charges. According to http://www.mouser.com/ds/2/149/RURP860-244175.pdf the peak non-repetitive surge current of MUR860 is 100A. So I believe you have proved that inrush current isn't the issue. You may want to check RC time constant to be sure that this counts as a "surge" but realistically I think that it would.

I think more likely you have an intermittent short or a PCB design rule issue or a reject PCB or all three. Can you make up another PCB the same and test it? And in case the halves are shorted together, maybe try running with the top half (+ve supply) from the first PCB and the bottom half (-ve supply) from the second PCB.

Otherwise, the original advice applies. Validate everything with the 9V battery and possibly small signal diodes instead of the MUR860s (but keep the MUR860s if they are already soldered in and pass multimeter tests on diode setting).

Then, if that's okay give it low voltage AC with much current limiting. What I do sometimes for this is grab a 12V AC plugpack and then use the 12VAC as if it was 240VAC. So basically you have an extra step down and then your normal transformer, result about 2V. And put some resistors in too. Then see if you actually have a reliable 2V supply over time.

cheers, Nick
ebastler:

--- Quote from: Hextejas on December 19, 2018, 05:30:52 pm ---Picture attached.

--- End quote ---

But you did not actually install the resistor as shown in your schematic (in parallel with the wire), right? It needs to be in series, i.e. you must disconnect the original wire or trace. Apologies if this was already clear.
nick_d:
Another thought: check your sums carefully. If you increased transformer voltage increase capacitor voltage in proportion. That's because 35VAC means 35V RMS so the peak is 1.414 times 35 in VDC, I often see 400V capacitors used in 240VAC supplies, 450V or 500V are also common. If you use say 35V rated capacitor with 35VAC input, the capacitor charges normally until the AC gets near the peak and then the capacitor begins to short as the AC peak exceeds 35VDC. It will short as long as more than the rated voltage is across it, which tends to limit the voltage stored in the capacitor to approximately its rated voltage. There may be some margin of safety but typically the margin is tiny.

Another possibility is that your capacitors could be damaged from previous experiments causing them to either short (easy to detect) or short above some voltage that is less than the rated voltage (much harder to detect). Although it would be a major pain a good test would be to try running the circuit with all capacitors removed. The low voltage test won't pick up this issue. The low current test MIGHT but the problem is fairly benign at low currents so may be missed.

cheers, Nick
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