Isolated zero crossing detection with logic level output

1 year ago 923
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Description

Wide voltage range 85-265V 50/60Hz Low current consumption (~0.15mA @ 120V) Pulse width 50-110us Opto isolators tested 4N25, PC817, ACPL-217 (and H11L1 with modifications) 3.3V or 5V output

Documents

Zero crossing detector

BOM

ID Name Designator Footprint Quantity
1 MMBT2222 Q3,Q2 SOT-23(SOT-23-3) 2
2 MB6S D1 MBS-4.9X4.0 1
3 TLP291 U2 SOP-4_P1.27 1
4 4.7nF C6 0805 1
5 470nF C7 0805 1
6 1N5711WS-7-F D10 SOD-323_L1.8-W1.3-LS2.5-RD 1
7 220K R9,R8,R11,R10 0805 4
8 75K R12 0805 1
9 10K R18,R16 0805 2

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Comments (2)

hypersonik Reply

Sasha, could you please explain how come does Q2 enters in conduction?
BTW, nice design, simple and efficient.

sascha23095123423 Reply

hi, i have not looked at this for a long time but as soon as the zero crossing is almost reached and the polarity changes (dc voltage at the diode increases above 0.7V and at the base of Q2), the capacitor dumps all its charge into the opto isolator firing a short pulse. the diode D10 keeps the transistor from conducting after the polarity change until the next zero crossing has been reached while the capacitor is being charged. depending on the type of diode you get different results over a wide voltage range. increasing the capacity of C7 is probably a good idea, maybe 1µF/50V and checking that the pulse does not exceed the max. current of the opto isolator... ~50mA. 2-3mA are sufficient and will extend the life of the opto isolator significantly. 0.3-0.5mA are usually the edge case when you can see a stable TTL signal on the other side. to reduce the peek current, a resistor can be added between C7 and U2. that increases the pulse width as well.
a bigger capacitor might need a few cycles to charge up before it can trigger the opto isolator, depending on the discharge resistor R12.


unfortunately it is a bit susceptible to noise and filtering the digital output signal is recommended. you can find some improved version in my trailing edge dimmer circuits, which do not have any misfires with software filtering

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