LM358: Overheating

Hello Marvin,

What about other side of 700 ohm resistor?

IN2+ is center tap of pot?

Is there any series resistance from output to FET? Is it FET gate?

Don't ground both inputs. A lesser mistake is to ground neither 

Does the top feel hot? Do this happen on every board?

Measure the voltage on every pin and report it back to me.

Hi Marvin,

a MOSFET can show a huge gate source capacitance. This would mean a high capacitive load at the output of OPAmp which can destabilize the OPAmp and make it oscillate. And wHen oscillating an OPAmp can dratamically heat up. One cure is to mount an isolation resistor in front of the MOSFET's gate. (Ron has already asked about the presence of such an isolation resistor in your circuit.)

From your description it's not quite clear how your circuit looks like. Can you show a schematic?

Kai

Hi Ron and Kai

Thanks for the response. Here is the response from the customer:

In1- / Capacitor 47 Mikrofarad (to ground) / Zener Diode 4,7V (to ground) / series resistance 700 Ohm / Signal 0 .... 19V
In2+ / pot is at a lower position. In2+ see 1,8V.
Out 1 / series resistor 1 kOhm/ FET gate

Pin Measurements:

Pin 1: 12V
Pin 2: 0...4,7V
Pin 3: 1,8V
Pin 4: 0V
Pin 5: 0V
Pin 6: 0V
Pin 7: 0V
Pin 8: 12V

I will provide the schematic once the customer sends a copy.

Regards,

Marvin

Marvin,

It looks like pin 1 is shorted to 12V for two reasons. With pin 2 changing, so should the output. 

LM358 output high should be close to 10.7V 

Are the voltages from a device that already became damaged?

Hi Ron,

Thank you for waiting.

Yes, the measurements were from the damaged device. Now the customer changed the layout and added 20k to all inputs. The + Pin was added by 440 Ohms. Now seems that the device is working properly and no abnormal heating can be detected.

Can you agree with this design? or can you suggest a better approach?

Regards,

Marvin

Hi Marvin,

a schematic would be helpful to answer your question.

Kai