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So I got this small circuit that plugs to the mains as power source. as for its DC converter, it's one of those transformer-less ones that drops the voltage with a resistors and filters before rectifying and smoothing it. the ones that can only deliver small currents.

well, after that, it uses two IN4737 zeners in series to create a reference of 15V. I needed to find how much current is being drawn between the 1) DC converter (as described above) and 2) the zeners and the rest of the main larger circuit. so I disconnected the positive wire between these two and put in series an ammeter.

the measurements were odd. using my multimeter, it's here as follows (as I recall it properly):

20A range - 0.06 A

200mA range - 0.006 mA

20mA range - 0.060 mA

2mA range - I can't remember

200uA range - I can't remember

It was a weird set of readings (and yes the number I gave above for each range are exactly as they showed up in my display, even the units). I restored the original connection and the whole thing won't work anymore. I then found out that the IN4737's were blown and shorts at a few milliohms in both direction each.

so what went wrong?? the only difference was the ammeter in series for the positive wire between the converter and main circuit. aren't ammeters specifically designed to be passive and attempt to be a perfect conductor in all cases??

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THE VOLTAGES INVOLVED IN THIS SORT OF SYSTEM CAN EASILY KILL YOU.

ALL PARTS OF THE SYSTEM INCLUDING THE LOW VOLTAGE OUTPUT MUST BE REGARDED AS ALWAYS BEING AT MAINS POTENTIAL.

A converter of this type MUST have a load at ALL times.
If you remove the load voltages can rise to rectified mains - about 150 Volts DC in a 110 VAC system and 300 VDC in a 230 VAC system. If you do not design the system to always clamp the output at a low voltage in both directions and if you do not ensure that current flow is low - voltage and/or current may be high. Usually for long enough to destroy components and/or to kill you.

If you do not know why it may have blown up then you clearly do not know enough to be using it safely. supplies like this can work well when designed properly with correctly rated parts. Many versions described on the internet are unsafe and liable to destroy themselves and kill you (or someone else).

You MUST post your circuit diagram - on a web sit elsewhere if needed - or email to me if you cannot post it. (email address on my profile page).

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  • \$\begingroup\$ thanks for the thorough answer. actually, blowing up the zeners, as mentioned above, was just one of the mistakes i've made (i had probably simply shorted the red wire with the black wire when i attached the multimeter, though i remembered being careful not to do so). i've also blown several capacitors used for smoothing in the converter just by plugging it in and not having any loads on it. even putting a 10K resistor across it didn't seem to help as much... or maybe that just allowed a very low current; didn't have much time to play around with it. \$\endgroup\$ – Dehbop Jan 11 '15 at 0:07

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