Atmega328P provides logic signals to the AQH1213 solid-state relay to control AC load (contactor).

Task: Protect the Atmega328P in case the solid-state relay fails and supplies high voltage to the low voltage part. Although the technical data sheet shows insulation up to 5000 volts, I would like to provide additional protection measures.

What I use now: - TVS diode (600W) between the relay control line and ground. Theoretically, if suddenly 220 V falls on a low voltage circuit, the TVS diode turns off, the 220 V fuse blows, and the microcontroller does not receive a high voltage.

What I found: - Digital isolator. Example: TI ISO7220ADR

The datasheet says about using separate power supplies (VCC1 / GND1 and VCC2 / GND2). I have one line with a voltage of 5V on the board.

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Question: Will the digital isolator work if I use one power line on both sides?


Depends on your definition of "work"

If all are interested in is signal continuity then yes, a "digital isolator" would work. However it would no longer be isolating and acting more like a buffer. If all you are after is buffering then yes this will work but there are cheaper alternatives.

An isolated buffer is used when you need to break a ground loop or the reference points are different.

You are after mitigating a failcase where the relay breaks over. In this case powering both sides of the digital isolator from the same power source will not provide isolation.

Two options

  1. Provide a floating/isolated PSU for the secondary side (https://power.murata.com/mev1s2405sc.html is an example)

  2. Use a digital isolator with integrated power ( http://www.ti.com/product/ISOW7821 )

NOTE: Whatever approach you go for, ensure that the output drive current capability is good enough for the solid-state relay ( 25 mA ). If you look at the isolators listed they are only really viable for ~ 4-10mA output drive capability as they are meant for signal isolation.

Personally I would go for a SIL DC:DC and utilise an OPTOCOUPLER to provide the 2nd layer of isolation, if I was concerned about this failure mode


Task: provide additional Insulation voltage > 5kV of Optoisolator using a common 5V to a digital isolator.

Will the digital isolator work if I use one power line on both sides?

No. This will not increase your insulation protection if > 5kV transient is applied to the output.

The Filter attenuates say an 8kV impulse which is the breakdown arc of the power meter has electrodes that breakdown at 8kV. An LC filter can attenuate this to < 5kV which in turn is isolated from the DC opto driver side.

The line filter can then use a 600V MOV to suppress filtered transients.

The Digital isolator serves no purpose for protection from lightning transients with a common supply.


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