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Electronics => Electronics => Topic started by: Half Shell on September 21, 2009, 10:06:46 AM

Title: Mystery voltages appearing in my op amp
Post by: Half Shell on September 21, 2009, 10:06:46 AM

I am currently working on the following op amp:
(http://dl.getdropbox.com/u/533803/tmp_schem_1.png)
Notes about this: all grounds are tied together. The object plugged into the + sign on the op-amp is a piezoelectric tab, which generates +/- 70 Volts (up to, but rarely that much) upon being shook. I am trying to make a simple vibration sensor.

The chip is the 8 dip LM741CNFS, from Fairchild Semiconductor.

The resistor values are 220 Ohm both, though I've tried it with a number of additional resistors to see if the resistor values were causing trouble.

When powered, my op amp has a constant 2 volt surplus, as measured by my oscilloscope.

One might say it is the fact that the vibration sensors can generate such wild swings of +/- 70 Volts. I also run this circuit with a zener diode in parallel to the sensor, making it so it can only generate 0-5V. I have tested this myself and it does work.

What is going wrong? What am I missing?

Thank you for your time.

Title: Re: Mystery voltages appearing in my op amp
Post by: billhowl on September 21, 2009, 10:38:47 AM

LM741 minimum supply voltage is ±4.5v and It's not a single-supply op amp.Your circuit requires a separate negative supply (a minimum of 4.5V) on pin 4.

Title: Re: Mystery voltages appearing in my op amp
Post by: Half Shell on September 21, 2009, 10:42:19 AM

Quote from: billhowl on September 21, 2009, 10:38:47 AM

LM741 minimum supply voltage is ±4.5v and It's not a single-supply op amp.Your circuit requires a separate negative supply (a minimum of 4.5V) on pin 4.

Thanks! I see my mistake now.