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I've made this circuit to detect de zero cross but in out pin (pin 7) the voltage are about 3,7V fix. I've already measured pins 5 and 6 and the delta voltage ( vin+ and vin-) are inverting. I had inverted pins 5 and 6 and in this case pin7 is fixed in 0V.

Circuit:

Oscilloscope measurement problem:

--First Situation

Probe1 connected only: enter image description here

Scope img: enter image description here

--Second

Probe1 and 2 connected: enter image description here

Scope showing perfectly the signals: enter image description here

I've already tried to measure with other Scope (TDS 2002B) add caps on various places in this circuit. I will try to put this signal on uC INT pin to see if is a osc problem or a circuit one.

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    \$\begingroup\$ what is "> BRIDGE" connected to? \$\endgroup\$
    – Curd
    Commented Jan 13, 2017 at 14:01
  • \$\begingroup\$ Also: you should take care that inputs of OpAmp are whithin max. specified voltage range. Vcc is 5V but it seems that input at (+) gets much higher (>12V, w/o current limiting) maybe destroying the IC. \$\endgroup\$
    – Curd
    Commented Jan 13, 2017 at 14:04
  • \$\begingroup\$ I saw that the max diff voltage are about 36V, but put a divider in the circuit and now is working. Thank you! \$\endgroup\$
    – Henrique
    Commented Jan 13, 2017 at 17:45

2 Answers 2

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The LM358 is powered from 5 volts and you are forcing an input signal from the bridge that is rectifying a 12 V AC voltage. This usually means the LM358 will never work again.

The operational input range for the LM358 powered from 5 V is 0V to 3.5 volts. Try adding a potential divider to keep the voltage range between 0V and 3 volts then it should work as a comparator (Vbias2 should be set accordingly also within this range).

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  • \$\begingroup\$ Hi Andy! In fact i think i've discovered the problem. My oscilloscope =[. I saw that the maximum diff voltage are about 32V and for that i've connected directly on the bridge. But i will preprare the circuit for a res. divider. For de output signal, i did that because i'm working with a 3v3 uC. What i'm not undertanding is why i olny can see the in channel1 the circuit working when channel2 is in the bridge signal. I've already change all configurations in trigger menu and restored all scope for default config as well. I will post the photos... \$\endgroup\$
    – Henrique
    Commented Jan 13, 2017 at 16:05
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Solutioned Circuit:

Added the resistor divider.

enter image description here

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