I am using LM2902 as a unity gain voltage follower to isolate measurements of a sensors voltage. pin out ( datasheet: http://www.ti.com/lit/ds/symlink/lm2902.pdf )

the sensors i wan to measure operate at a voltage of 0-5v , for one of them, this voltage is inputted into pin 3 of the op-amp, pin 1 and 2 are connected together and also to the analog pin of a ATmega328p to measure it (or so i thought).

i messed up and wired pin 1 and 2 together and also to the 5v pin of the Atmega 328p instead of A7, pin 3 was connected to the sensor. this op-amp is used on 3 sensors, and they all gave junk information when this happened.

i am a mechanical engineer, and this sort of situation is out of my league ATM. can someone explain to what ended up happening when i miss-wired the op-amp to the Atmega? can i wire it properly and run it, or is the op-amp fried?


According the datasheet:

Absolute Maximum Ratings

(4) Short circuits from outputs to VCC can cause excessive heating and eventual destruction.

Since the IC is quite cheap, I'd recommend to replace it and double-check the wiring before powering the circuit.

  • \$\begingroup\$ The short circuit was between pin 1 and 2 and the 5v pin of the Atmega328p, it was not a short between VCC and output. The VCC has it own supply for the op-amp, but the same ground as the Atmega. its time consuming to de-solder/solder the op-amp on this prototype, id rather not do it unless there is a good chance the op-amp is fried. \$\endgroup\$ – haxxor Jun 30 at 18:38
  • \$\begingroup\$ Pin 1 of the LM2902 is output. If the 5V pin of the Atmega is from a power supply, my answer still holds. The output pin 1 will unlimitly sink from the 5V supply and gets heated up and maybe fried. \$\endgroup\$ – Huisman Jun 30 at 19:01
  • \$\begingroup\$ the 5V pin of the Atmega is not from a power supply. i am using the Arduino nano with 9V applied at Vin. \$\endgroup\$ – haxxor Jun 30 at 19:11
  • \$\begingroup\$ @haxxor I read "I messed up and wired pin 1 and 2 together and also to the 5v pin of the Atmega 328p " as: pin1@LM2902 = pin2@LM2902 = 5Vpin@atmega. If i understood you correctly, the comment above holds. \$\endgroup\$ – Huisman Jun 30 at 19:12
  • \$\begingroup\$ @haxxor If things are easily to rewire, you could try to isolate and test pins 1&2&3 of LM2902 as opamp follower. \$\endgroup\$ – Huisman Jun 30 at 19:18

IC is current limited but there may be issues if reverse power voltages were applied , but I don’t see that from your explanation.

These can sense down to 0V but only up to V+-2V .

for Linear output. Both input to Vin- and sensor ought to be Biased to get linear output at V+/2 then eliminate offset in software or use rail to rail type Op Amps to get 0 to Vref out for ADC. But if output range doesn’t go to 0.0V or say 5V then these work fine. show exact circuit with links to all parts for a better answer.

So you can attempt rework with hot air gun and thermometer or as you require to wire correctly.

  • \$\begingroup\$ The sourcing current is limitted, the sinking current not. OP is sinking current. \$\endgroup\$ – Huisman Jun 30 at 19:07
  • \$\begingroup\$ nope, power voltages were applied correctly. can you explain what you mean by "biasing to get a liner output at V+/2". these sensors are usually operating at 1V - 4.5V, the limits of the sensors are avoided. my Vcc to the op-amp is 8v, data sheet said it needs +2V above what needs to be outputted, 8v seems more than enough and there is no heat issue. \$\endgroup\$ – haxxor Jun 30 at 19:08
  • \$\begingroup\$ Ok . Bias is to determine Output offset from input Vref - Vsignal with gain \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jun 30 at 19:11
  • \$\begingroup\$ @Huisman True there is no active current sense limiting but 6uA bias current goes to 3 stages, one saturated to limit output current plus Rs of PNP output to about < 10 Ohms so it will heat up with To output and if it exceeded 260 ‘C momentarily which is applied in solder profiles then It’s ok based on you quick response to seeing it if it got hot. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jun 30 at 19:20

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