Variable reluctance Sensor

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I found this variable reluctance sensor circuit during a search, and it's not obvious (to me) why the zener diode has been included.

During the positive portion of the input waveform, the diode within the opto limits the voltage (at the opto input) to about +1V
During the negative portion of the input waveform the 1n4003 diode conducts and limits the voltage (at the opto input) to about -0.7V or so

What is the Zener doing? It seems like the other two diodes will limit the input voltage to about +/-1V

 

hi,
The Zener will conduct, forward at approx 1.2V so it will shunt any excessive current and so protect the photo diode in the 4N35 from being over driven.
It will also clamp the input voltage/current level to the internal photo diode, giving a more consistent current drive
E

EDIT:
I should add, the above were the designers intentions, but sadly the circuit will not work as expected
 
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Whatever the motivation for a zener diode in this circuit may be, the schematic is just erroneous. With this zener diode connection, the opto coupler LED will be never conducting forward current.
 

FvM said:
Whatever the motivation for a zener diode in this circuit may be, the schematic is just erroneous. With this zener diode connection, the opto coupler LED will be never conducting forward current.
Click to expand...

Agreed.

The input waveform is typically about 6-20Vp-p, I'm going to:
- delete the Zener
- replace the 1n4003 with a 1N4148W-TP
- perhaps find a smaller version of the opto
- test it
 

If you worry about possible opto overload with large input voltages, you'll use a stagerred protection: R - ZD - R - OC
 

chemelec said:
The Zener is rated at 5.1 Volts, So the circuit should work.
Click to expand...
No, the Zener is backwards. Remember that a Zener diode conducts in the forward direction just like a regular diode, so it is shorting out the LED, which is why the LED will never light up. And in the reverse direction the Zener will never conduct because it is shorted out by the 1N4003.

If the Zener diode were reversed, these problems would go away, but it still would not work as intended because if the forward voltage on the LED ever reached 5.1 v, the LED would burn out first.
 

Not Sure Why the Zener or the Diode was put in there.
Other than possibly to Help Square the wave.

Just Remove Both of these diodes.
And the Resistor should limit the current enough to the LED to protect it from damage.
 

The opto is an IR diode operating near 1.2V and only rated at -5 to -6V ( no matter how short the duration). So for protection from long wire inductance and high side driving a negative spike from turn off will occur. Thus only the reverse diode is suggested and the zener should be omitted.
 


I agree, the VR sender will actually drive the input signal negative possibly more than the -5 to 6V so definitely need the reverse bias diode to protect the opto.

Who ever added the Zener did not think it through.

Thanks for your comments.
 
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