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Is there any reason not to test these rectifiers in circuit?


Chris Wilson

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I have a device which died the other day, it's been dry stored for years, powered up OK, then died on soak test after three hours with part of the front panel LED's and other stuff not working. I am now getting 5.9 volts on the 7 V + line with an accurate DMM. I tested the diodes circled in green in circuit and some showed bad (conductive both ways the top two), and one on another rail seemed bad, but forget that for now. Am I right in thinking this circuit should allow on board testing? I intend to replace the IN4001 's with similar or higher rated, and change all the electrolytics, but was wondering if I am right in thinking this circuit would allow accurate on board testing of the rectifiers circled? Cheers.

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Without running some calcs, I'd not recommend testing the rectifiers in circuit due to the size of the electrolytics. Even though capacitors should appear as open circuits when taking DC measurements (e.g. diode testing), they do pass a current as they are charging. With the small currents used in testing diodes, it could take a long time to charge the capacitors so they could be affecting your measurements.

 

I'd recommend testing with your oscilloscope. Connect your ground lead to "CON 2 2" or any other ground point and start probing. You should see sine waves at the two inputs to the bridge rectifier. If it's working correctly, you should see a waveform similar to this (the black line) across the capacitor. Make sure you have a load connected when taking these measurements otherwise you won't be able to see failed diodes.

 

If you have any findings, post up some captures and I can suggest further measurements to pinpoint failed components.

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Thanks for another great reply Andy. I suspect an electrolytic may have killed these. I will have a play with my scope for the sake of experience and see what i see. then I will pull them out and test them isolated from the circuit. I will probably just change them to 3 amp ones and cahnge all the PS electrolytics, for what they coat, but as you know, i am trying to brush up my electronics skills, so eager to learn new testing methods and their pros and cons. I am also restoring an old tektronix 475 scope, with faulty triggering, and have bought a Philips / Fluke Combi Scope (digital and analogue combined). Doing something very different from my norm is quite refreshing, even if i am struggling to take it all in ;) Thanks again, how's the Adaptronic ecu doing?

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