Electronics > Beginners

Mains earth and MCUs ?!

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Hi guys

I've designed and build an adjustable dummy load, kind of what Dave showed in one of the blogs..
I use some other components, and included a PIC16F877a and a 16x2 lcd, for measurement.

I found that when the circuit is powered form a non-earthed power supply, the digital part doesn't work, and sometimes erases the microcontroller (all the voltages looks fine?!). But when powered from a 9v battery, or an earthed supply, it works fine.
I have no idea why!

I've tried both a standard 9v adapter, and my adjustable power supply, neither works.
We don't have earthed wall outlets! But I tried burying a pipe in the garden, with a wire going to the negative terminal on my power supply, and it worked fine :o .. ( yea, i actually did that, was desperate  ;) )

I'm not sure how the DC relates to the mains earth, when it's been through a transformer, and been rectified ? 

Can anyone explain why this is happening ? And how to avoid the problem, so I CAN use a non-earthed power source ? :)
I'm sure it's just a basic thing I don't know about :P I'm still a beginner in electronics :)
I did add caps at the Vdd/Vss of the micro, at the power input, and after the 5v regulator.

I get the same problem with almost any MCUprojects, I tried breadboarding a few afterwards.

Do anybody have the same problem ?
More info available, if needed ;)


C7 is too small

Hmm, C7 ?
Sorry ;)

Based on the enormous, detailed information your provided I would say your power supply is junk.

I think people are trying to say: POST A SCHEMATIC !!!


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