Re: Zap a volt regulator w/ no cap??

pbdelete@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx wrote:
I converted this part of the machine's control to be read from a PLC
analog input module, so I had to come up with my own 12 V source. What
I did was add an LM2940CT-12 voltage regulator (TO-220 package) to the
circuit right before the analog sensor. This provided my 12V and
worked great for about a day and a half, then my analog sensor fried.
When I measured the voltage from the regulator I found that it was now
the full 24V input...so apparently the regulator failed first and then
cooked the sensor with 24 V.

When I took a look at the spec. *** on the LM2940, I see that they
recommend putting capacitors from the input and the output to
ground...I didn't do this. Other than this mistake, I can't see
anything else wrong with the circuit. Could the absence of these caps
have caused the regulator to fry? If not, what else could have caused
it? I'd like to know exactly what my problem is before I toast another
sensor (they're $175 a pop).

Low drop out regulators (of which the LM2940 is one) are
only stable with certain values of capacitance (and
sometimes only with certain values of series resistance in
that capacitance).
The graph on page 11 shows how tricky this is.
http://info.hobbyengineering.com/specs/NATSEMI-LM2940.pdf

Guess it's the 00882206 diagram "Output Capacitor ESR".

Makes me wonder if these LDO really are good. Seems more like liability :)

They are essential when you actually need real LDO operation! e.g.
dropping say 5V to 4.5V with low noise.

Dave :)

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