When +12VDC….Isn’t or Why to avoid cheap wall warts

First excuse the err…crude “Screen Shots” – I don’t have my ‘scope setup yet for proper screen captures so I just took pictures.

For a while now I’ve had a +12VDC wall wart that I use occasionally.  It always caused weird heating though in regulators.  I was never sure why, the multi-meter read it as having a fairly stable +12VDC.  I finally hooked it up to an oscilloscope today…well now atleast  I know why…

It’s supposed to be producing +12VDC, center positive.  What you’re looking at is the center pin (tip) on the yellow (CH1) channel, and the ring  (or barrel) on CH2.  The trigger is on CH1 @ the rising edge of the wave at 20V.  As you can see it’s getting voltages as high as +65V and as low as -110V.  It follows the AC line perfectly, the cycle time measured by the scope almost exactly matches that of the AC line.  In fact if I trigger on the AC line (this scope can trigger from it’s AC line input) the waveform stays completely steady.

The second picture shows in red what a multi-meter reads.  Which the scope reproduces by differencing CH1-CH2.  The ripple is a bit better when it has a load but it’s still producing these ridiculously high voltages.

So, it’s producing +12V…sorta.  Only as a function of the difference between it’s own +/- reference.  Even a good multimeter can be fooled by a bad signal.  Normally you’d probably never even notice this because you’d hook your multimeter up to your projects GND which is usually the – on the power supply.  If you’d tied this wall warts – to GND, you’d either cook it, or hit it’s current limiter, or both.  This little sucker is probably going to be dismantled and tossed.








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