Isolating a multiplexing circuit from AC voltage

Thread Starter

diogoribeiro_

Joined Feb 7, 2020
6
Hey guys,
I'm currently designing a multiplexing circuit based on the MCP23017-E/SP for digital I/Os and the MC14051BCP or CD4051BE for the analog I/Os. My concern is that I want to connect a current sensor (ACS712) connected to the analog multiplexers. In case they fail and somehow the 220V get to the pins of the multiplexers everything will go south really quick. What would be the best approach to keep everything safe?

Thanks in advance!
 

Thread Starter

diogoribeiro_

Joined Feb 7, 2020
6
Hi!
1581103885427.png

So, the inputs/outputs of the MUX are connected to a header. This header will only receive the OUTPUT signal of the ACS712. The other side of this ACS712 has 2 screw terminals with the live wire. My concern is if something goes wrong and somehow the 220V shorts from the screw terminals to the OUT pin. This is the current sensor ill be using :
 

crutschow

Joined Mar 14, 2008
25,248
You could use a 1kΩ resistor in series with a 0.1A fast-blow fuse.
Also connect two small Schottky diodes (e.g. BAT54) after the resistor-fuse, one going anode to ground, and the other going cathode to the V+ mux supply.

Note that the 1kΩ resistor will appear in series with the mux output load, so will reduce the signal voltage proportional to their ratio.
Using a non-inverting buffer opamp at the mux output will eliminate that. and also any signal loss from the mux on-resistance.
 

Thread Starter

diogoribeiro_

Joined Feb 7, 2020
6
Thanks for the valuable inputs! Is this what you mean ( A1_X7 is the header that connects to the sensor) ?
1581154453639.png

My main concern with this is that this board will have 40 signals in the same situation. This will make me use 40 fuses but if it works im ok with it.
 

Thread Starter

diogoribeiro_

Joined Feb 7, 2020
6
Additional input: About the opamp is this what you meant?
1581155882549.png

Correct me if im wrong, but this only works for a signal comming from the mux to the sensor. My goal is to be able to read from the sensor and write to it too.
 

crutschow

Joined Mar 14, 2008
25,248
Is this what you mean ( A1_X7 is the header that connects to the sensor) ?
No.
The series order is sensor-fuse-resistor-diodes-mux input.
About the opamp is this what you meant?
No, the opamp would be added to the analog mux output with the sensor going to the single mux input.
But that won't work if you need to write to the sensor also.
(Why would you want to write to an analog sensor?)
 

Thread Starter

diogoribeiro_

Joined Feb 7, 2020
6
Thank you very much for the help.

No.
The series order is sensor-fuse-resistor-diodes-mux input.
1581182661350.png


I think the order is now correct.

No, the opamp would be added to the analog mux output with the sensor going to the single mux input.
But that won't work if you need to write to the sensor also.
(Why would you want to write to an analog sensor?)
I'm so sorry but I couldn't understand what you meant by connecting it to the single mux input. With the circuit that I presented where would that be?

Out of the 40 analog IO, only 30 are going to be used with that type of sensor. I was thinking of letting 10 other pins with the ability to write to them just in case I need it in the future.
 

crutschow

Joined Mar 14, 2008
25,248
I couldn't understand what you meant by connecting it to the single mux input. With the circuit that I presented where would that be?
I meant single mux output for the opamp.
The sensor is connected, as you show, to the mux input, and the opamp would go to the A4 mux output.
(Why do you have the opamp with a gain of +1.4?)
I was thinking of letting 10 other pins with the ability to write to them just in case I need it in the future.
Then you can't add the opamp to the mux output.
 

Thread Starter

diogoribeiro_

Joined Feb 7, 2020
6
So, i changed the design and i came up with this:
1581193340645.png

Is this what you mean ? I dont even know why i had a 400 ohm resistor, i mean to have 1k to have a 1 gain. If this is the correct result how would i connect the remaning mux outputs to the opamp ? Every single "OUT" would connect to the A4?
 
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