Feet of clay

A customer emailed about an LM2936 regulator that worked with a 9V battery but got hot with a 13.8V supply, yikes !  As mentioned in previous blogs, the LM2936 is a fave of mine, it is low power and hard to kill.  I've never had a report of failure until now.

https://wb9kzy.blogspot.com/2022/02/ldo-regulator-ic-chips.html

I emailed the customer back, requesting more info.  

Then I tried seeing if I could duplicate the problem.  First I tried reversing the LM2936, swapping the output for the input.  It used about 18 uA and no output voltage.  

I put the LM2936 back the correct way but then reversed polarity from the supply.  There was a brief blip of current (maybe from the input cap ?) and then the meter just read 0.

Finally I tried inserting the LM2936 as if it were a Seiko regulator (remembering that the reverse situation had caused a problem previously). 

https://wb9kzy.blogspot.com/2022/03/a-kit-weakness-revealed.html  

Bingo !  The HP supply output wilted to 4.5 volts and the LM2936 started using 35 mA (I had set the HP 6281a supply to full current limiting).  After putting the LM2936 back correctly it functioned fine.

But my customer reported that he had inserted the LM2936 correctly, so possibly I've sent him a bad part or ??  He confirmed that the LM2936 was bad above 9V.  He replaced the LM2936 with the pin compatible 78L05.  Since the keyer would be installed within a rig and operated from 13.8V and not with a battery the higher standby current of the 78L05 didn't matter.

Well there is a first time for everything and the LM2936 apparently can have feet of clay like any other device.  Or possibly a counterfeit part got into the batch ?  But that seems unlikely.  The Iddq of the bad LM2936 was a little high at 10 uA (versus 7 uA), possibly due to some damage ?  More likely the 10 uA reading was due to the precision of the meter being 10 uA.

In any case I have more work to do on my documentation and also adding a plastic label on the circuit board to cover the extra hole for the Seiko regulator when the LM2936 is used.

Best Regards,
Chuck, WB9KZY
http://wb9kzy.com/ham.htm