Electronics > Beginners
I keep killing mosfets - what I do wrong?
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PMA:
I don't disagree, but could you explain or point to articles or other sources?
Yansi:
I think that a VI characteristic of that zener is telling enough.  The zener does not magically turn on exactly at the specified voltage. It begins to conduct well blow the rated voltage, skewing the measured voltage due to the voltage drop on the series protection resistor.

This phenomenon is especially notable with low voltage zeners.
Per Hansson:
Thanks for this useful thread and the links to further discussions!
My Weller WSD81 provides a 0-ohm connection to ground.
However my old and venerable Weller WECP-20 had been modified by someone with an IEC plug.
I thus assumed it was grounded but found out a few weeks ago it was not, no ground connected inside, yikes!
Originally it must have come with a two-lead prong, I attached the ground now to the internal screw for the banana ground jack.
Before there was ca 24VAC on the tip!
That soldering iron is interesting though, there is a jumper inside and if you cut it you get a R/C filter 120 KOhm/22nF for the ground.
I assume this might be beneficial if the device to be soldered actually does hold some charge above ground.
I left the jumper intact for now though, what are your thoughts on it?
http://remotesmart.wdfiles.com/local--files/weller-wecp-20/WECP_manual.pdf
PMA:
This has been taken into account (kind of).

I have assumed that 5V1 zener starts to conduct in about 4.5V on small currents. However, I have never measured it. Because of the scrutiny and the fact that I had test setup practically ready made, I decided to take some measurements.

There is four identical circuits side by side on the same board. To get different voltages to ADC circuit I used 2k4 potentimeter and 5V supplied by the board.

Circuit on PCB:


Measurements:


Max/min are taken from the channel that gave max/min reading to that specific voltage.

The data shows that 5V1 zener starts to effect measurements about one volt below nominal voltage, but this of course depends on many things and my sample size wasn't large enough to any definite conclusions.
Yansi:
Better than getting affected by a zener, is to use a Si diode clamp (BAV99, BAV199, double 4148, etc). These will work even for very high impedance measurement circuits. Leakage is very low, and when they are operated with a floating clamp voltage, the leakage is almost nonexistent.

But be aware that neither of these solutions is very rugged against ESD events, additional protection may be required.
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