Smoke test - intriguing outcome.

Thread Starter

atferrari

Joined Jan 6, 2004
3,642
Around six months ago I managed to burn my middle finger big time, by touching a TO220 transistor in a constant current source out of control. Yesterday I repeated the burning experience and dare to say the effect was even worst when testing the circuit here below.

Smoke test.png

Both channels were outputting a continuous negative voltage of around -3,76V. The test lasted maybe two or three minutes until I checked how warm both opamps were. Imagine cursing!

After finding out the reason I was resigned to replace both opamps TL071 and TL074 so when they had cooled down completely I connected them properly to see how much they were damaged. Long story short, both channels are giving the expected 5 V (+ 3 to 4 mv) for an input of +3,7V right now on my bench.

How comes...?

SANY9377.JPG
 

Lestraveled

Joined May 19, 2014
1,946
By now I am sure the fingerprint of my right index finger looks like a mirrored transistor part number. As far as what was going on with your circuit, don't have a clue. But....In your schematic there are no TLO71 op-amps.
 

Thread Starter

atferrari

Joined Jan 6, 2004
3,642
="paulktreg, post: 964001, member: 23647"]Because you connected them correctly? ;-)
Yes, as I said, I did it later.

The schematic shows the power supplies reversed. Sorry, I should have said that explicitly. Minimum 2 minutes!

="Lestraveled, post: 964004, member: 239699"]By now I am sure the fingerprint of my right index finger looks like a mirrored transistor part number. As far as what was going on with your circuit, don't have a clue. But....In your schematic there are no TLO71 op-amps.
Yes, there is one TL074 and one TL071.
 

SLK001

Joined Nov 29, 2011
1,543
I imagine that the op amps have some form of reverse polarity protection diode that was getting hot. Your schematic shows the powers are connected in reverse. Is that the way you connected on your protoboard?
 

crutschow

Joined Mar 14, 2008
24,115
I imagine that the op amps have some form of reverse polarity protection diode that was getting hot. Your schematic shows the powers are connected in reverse. Is that the way you connected on your protoboard?
The "reverse protection" diode is simply the normal parasitic substrate diode that most IC's have, which conducts when the power is reversed.
 
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