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Input capacity of each bank

324 Views - Created 01/12/2019


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The spec sheet for the IO Pi Plus states that there is a maximum of 125ma for all i/o on each chip. If I understand it correctly, it implies that an input (applying 5v to a pin) will draw 25ma. Thus, there can be a maximum of 5 positive inputs, out of 16 potentials) at the same time.

It also seems that it is impractical to drive an optically-isolated 8-relay bank at 20ma per channel, if all relays could be active at once.

Is this correct, please, or have I misunderstood?


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Hello David

The 25mA current limit on each pin and 125mA limt total is for when the pins are set as outputs.  When using the pins as inputs and applying 5V to the pin it will draw less than 0.1mA so there will be no problem with using all 16 pins as inputs.

Using the IO Pi Plus to drive optically-isolated relays will exceed the maximum current if you try driving more than 6 relays.  To use the IO Pi Plus with a relay bank I would recommend using a buffer chip or transistor between the IO Pi output pin and the relay input to increase the current capacity.

With many opto-isolators they do not need their maximum current of 20mA to switch on so you may find that you can add a resistor in between the IO Pi output and the opto-isolator to limit the current.  A 800R resistor would limit the current to 6.25mA which may be enough to operate the opto-isolator while keeping the current low enough that you can use all 16 outputs on the IO Pi Plus.  If it does not work with a resistor then a buffer will be the best option.

AB Electronics UK Tech Support

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