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Topic: Preventing parasitic power; how? (Read 245 times) previous topic - next topic

sterretje

Hi all,

this is a theoretical question that was triggered by another thread.

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.
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Electronics engineer by trade, software engineer by profession. Trying to get back into electronics after 15 years absence.

ChrisTenone

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.
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Grumpy_Mike

An other soloution is to use opto isolators on persistent inputs.

sterretje

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.
If you understand an example, use it.
If you don't understand an example, don't use it.

Electronics engineer by trade, software engineer by profession. Trying to get back into electronics after 15 years absence.

septillion

Or by adding resistors in a way the current is limited enough :) Couple of 100k's will probably do it.
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wvmarle

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).
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