Smoke test - intriguing outcome.

[atferrari]

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.

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...?

 

[kubeek]

Hmm, the opamps are to be running in steady state without an significant load yet you burned your finger, if I understand you correctly?
In your screenshot I don´t see any bypass capacitor, which can result into oscillation of the opamp and consequent excessive power consumption. TL07x can´t really be called fast, but don´t underestimate that in terms of power supply decoupling..

 

[atferrari]

Sorry kubeek, while it is clear in the schematic I should have said explicitly that the power supply connections were plainly reversed. Thus my intrigue.

The decoupling caps were there, even if not shown.

 

[dr pepper]

The op amps did well to survive at all.
The pass devices instead of being followers would have clamped the o/p of the op amps to 0.6v, not that it would make any diffo the o/p stages of the op amps having power hooked up backwards would have probably shorted the rails.
I maintain poly extruding plant, burns are a constant risk.

 

[schmitt trigger]

If you reversed the power supply connections, and the opamps overheated, your opamps will be severely degraded, even though they may appear to have survived.

Replace them.

 

[atferrari]

If you reversed the power supply connections, and the opamps overheated, your opamps will be severely degraded, even though they may appear to have survived.

That precisely made for "intriguing" in the title of this thread. They keep (or seem to keep) giving the expected output. In fact I expected them to be genuinely / completely destroyed.

Sure, I will replace them.

 

[Dr_Doggy]

licking your finger first , or getting it wet will create a temporary protective vapor barrier that will prevent burns upon QUICK & LIGHT brushes, full(pressure) contact or long exposure will still cause burns, by long exposure i mean anything over a few fractions of a second, by full pressure i mean anything more than a light brush,

this tactic was created in event of being in a fire , when you need to check a doorknob before grabbing it with full hand...

 

[atferrari]

licking your finger first , or getting it wet will create a temporary protective vapor barrier that will prevent burns upon QUICK & LIGHT brushes, full(pressure) contact or long exposure will still cause burns, by long exposure i mean anything over a few fractions of a second, by full pressure i mean anything more than a light brush,

this tactic was created in event of being in a fire , when you need to check a doorknob before grabbing it with full hand...

Trick I also used to extinguish candle's flame as a 6 years old altar boy (what you read).

Failed to remember it n the occasion.

 

[audioguru]

I burnt my thumb knuckle on my soldering iron a couple days ago. It made no smoke and no sound. Today it doesn't hurt anymore but it is still visible.