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Using Arduino => General Electronics => Topic started by: sterretje on Jun 23, 2018, 08:26 am

Title: Preventing parasitic power; how?
Post by: sterretje on Jun 23, 2018, 08:26 am
Hi all,

this is a theoretical question that was triggered by another thread (https://forum.arduino.cc/index.php?topic=554552).

If the power to a LCD is removed, it might very well still be powered via a microcontroller's output pins.

I could think of two solutions to prevent the parasitic powering, but I'm not sure if they are correct.
1)
First thought was to use a bus driver like a 74HC244 and tristate the outputs before powering off the LCD
2)
Thinking about (1), thought came up to set all data and control pins on the microcontroller to input (high impedance).

I think that both will work but like to know if I missed something or if there are implications using those approaches.
Title: Re: Preventing parasitic power; how?
Post by: ChrisTenone on Jun 23, 2018, 08:46 am
The rule on IC components such as op amps, is to not power any input high if the chip itself is not powered. Same goes for a board level component, I'd say. So yout (2) answer seems to satisfy this.
Title: Re: Preventing parasitic power; how?
Post by: Grumpy_Mike on Jun 23, 2018, 11:00 am
An other soloution is to use opto isolators on persistent inputs.
Title: Re: Preventing parasitic power; how?
Post by: sterretje on Jun 23, 2018, 11:10 am
Thanks all for the replies.

An other soloution is to use opto isolators on persistent inputs.
How many optocouplers in a package? I'm only aware (if not mistaken) of dual ones but haven't looked at those for ages.
Title: Re: Preventing parasitic power; how?
Post by: septillion on Jun 23, 2018, 11:39 am
Or by adding resistors in a way the current is limited enough :) Couple of 100k's will probably do it.
Title: Re: Preventing parasitic power; how?
Post by: wvmarle on Jun 23, 2018, 05:35 pm
How many optocouplers in a package? I'm only aware (if not mistaken) of dual ones but haven't looked at those for ages.
Commonly available in packages of 1, 2, 3 and 4 (e.g. the PC817, PC827, PC837 and PC847).