Diode Configuration in Protection Circuit: Nose-to-Nose or Tail-to-Tail?

[nbo10]

Hi All,
I'm replacing a switch in a piece of equipment. I'm looking at the circuit diagram and the actual circuit, and I notice that the diodes in the circuit aren't wired as they are in the diagram. The HI signal is dc from 0-1 Volt and LO is ground. Is there going to be any significant difference between the two? I think they do the same thing, but in the real circuit one of the diodes is going to be conducting. While in the diagram neither of the diode are going to be conducting.

 

[berkeman]

I don't understand the drawings. What does the midpoint of the diode connection go to? Usually diode clamps to the rails have a diode up from GND to the circuit to be protected (anode side to GND), and a diode up from the circuit to the + supply (with the cathode connected to the + suupply). That way, positive transients on the circuit will be clamped at the + supply plus a diode drop, and negative transients will ce clamped at GND minus a diode drop.

 

[nbo10]

The midpoint of the diodes goes nowhere. the diodes are zener diodes. If I understand correctly both circuits limit the differenece between HI and LO to +/- 12.5 volts. Is that correct?

 

[berkeman]

If they're about 12V zeners, then yes, the configuration with nose-to-nose or tail-to-tail zeners will clamp at about 12.6V at room temperature.