I looked at DALI application node and stumbled upon this circuit. Everything but the Zener diode D4 makes sense. What is the function of D4?
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4The opto is activated/switched at higher voltage level than without diode. You can do it by increasing R6 but you will loose the nice vertical edge of output signal. – Nov 10 '21 at 14:07
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And how does the output of the opto-coupler ever get high when the supply voltage is 5 V? – Elec1 Nov 11 '21 at 15:31
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A DALI input must understand voltages up to 6.5V as OFF reliably, and voltages above 9.5V as ON reliably.
If there is no Zener diode, the optocoupler input would turn on at much lower voltage.
The Zener diode simply allows the LED to turn on between 6.5V and 9.5V.
Justme
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How does the diode do so? I have read a lot about Zener diodes, but haven't found anything on such a use case – jonas Nov 11 '21 at 18:27
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That's a Zener diode with 5.1V Zener voltage. The simplified explanation is that if there is less than 5.1V over the diode, it won't conduct current and LED is off, and if there is 5.1V over the diode, it conducts current and LED is on. – Justme Nov 11 '21 at 19:21
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Voltages lower than 6.5 V on the DALI bus are defined as logical low and should not cause any reaction in the optocoupler. The avalanche diode stops any current flowing reaction for those voltages (the bridge rectifier already causes a drop of 1.2 V, so with the diode drop of the LED itself, the start of light will be safely in the "undefined" voltage range between low and high).
Null
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A silicon diode has a forward voltage drop of about 0.6 to 0.7 V. A bridge like this one will always have two (2) diodes in series with the load, so 1.2 to 1.4V drop across the diodes. – John R. Strohm Nov 11 '21 at 19:22
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Ahh okay, I just assumed it was an ideal diode and the voltage drop was 0.7, so I was confused as 2*0.7 != 1.2 haha, but thanks – jonas Nov 12 '21 at 09:42