I found the following circuit next to a switching power supply in an electrophysiology amplifier (AXOPATCH 200A, an instrument for a technique called "patch clamp").
References to power outlet:
- L: line
- N: neutral
- G: ground
SEAL is then connected to an AND gate input (pin 9 in a 74HC08), the other input (pin 10) of the gate comes from a selector in the front panel: "SEAL TEST (5 mV line frequency)".
I think the purpose of this circuit is to generate a square wave with line frequency (50 or 60 Hz), 5V and 50% duty cycle, but I'm not completely sure. Can any of you analyze the way this circuit works? What should be the ratings on the zener diodes?
Edit: I desoldered D1 and D2 to see the markings and found that it reads 1N914 which is an older version of the general purpose diode 1N4148. I mistakenly thought it was a zener for its transparent appearance. Resistors are just one (not two or three in series), they are 1% precision, 1/4 W as you can see on the following picture (with one of the desoldered diode).