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Part of a PCB I designed looks like this:

Phototransistor

Simple phototransistor with a resistor to measure the voltage over. More light => more voltage over the resistor.

However, the circuit behind the output doesn't like it when the impedance of it's input is too high. Lacking a better idea, I decided to try what effect an opamp as isolation buffer would have:

Amplified

The opamp (LM358N) got hot and didn't perform as expected.

VDD is 3.3V, not even close to the maximum Vin the LM358 is rated for.

Did I cut a corner I shouldn't have cut or did I make a seriously stupid mistake here?

To avoid any confusion: the circuit without opamp works exactly as intended. Just not in combination with the next stage of the design.

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  • \$\begingroup\$ How did you connect the other op-amp in the LM358 package? If you left it's inputs unconnected, it's possible that it's oscillating and that's what's heating up. \$\endgroup\$
    – The Photon
    Mar 26, 2016 at 21:43
  • \$\begingroup\$ @ThePhoton All unused inputs are directly connected to GND. \$\endgroup\$
    – Mast
    Mar 26, 2016 at 21:44
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    \$\begingroup\$ That's still a potential problem. Configure the other op amp as a follower with the input grounded. \$\endgroup\$
    – WhatRoughBeast
    Mar 26, 2016 at 22:02
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    \$\begingroup\$ When T6 conducts it will pull the +ve input to the op-amp up towards Vdd. But an LM358 is not a rail-rail op-amp, and it doesn't work well when the +ve input gets within 1.5V of Vdd. You need to use a rail-rail op-amp. \$\endgroup\$
    – Steve G
    Mar 26, 2016 at 22:24
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    \$\begingroup\$ The 358 is a clever design which is often extremely useful in single-supply circuits, since it will operate with inputs at the negative rail (ground). But that's not actually rail-to-rail. \$\endgroup\$
    – WhatRoughBeast
    Mar 26, 2016 at 23:53

1 Answer 1

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When T6 conducts it will pull the +ve input to the op-amp up towards Vdd. But an LM358 is not a rail-rail op-amp, and it doesn't work well when the +ve input gets within 1.5V of Vdd. You need to use a rail-rail op-amp. – Steve G Mar 26, 2016 at 22:24

Replacing the op-amp with a proper rail-to-rail capable one did indeed fix the problem. I had picked the wrong type of op-amp.

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