Tzarls
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Help with this circuit

Tue Jul 02, 2019 6:53 am

Hi guys. I´m looking for help analyzing this circuit: (simple drawing)

https://www.dropbox.com/s/rijqpf96i4fxh ... t.png?dl=0

- Batt is putting out 5V (actually it's less than that but just say 5V to make thing simpler for now)
- Grounds are tied (but I gues it would still work if they weren´t)
- There is a resistor between GPIO2 and Opto1. The resistor is of the correct value.
- GPIO2 is just a reference, not the actual GPIO I'd be using. It's just a GPIO used to drive Opto1.

SW1 is in "OFF" state, not pushed. So:

- When GPIO2 is OFF (0V) then En is pulled to ground thru the 10k and 100k resistors.
- When GPIO2 is ON (3.3V) then En is pulled to Batt (5v), thru the 10k resistor and Opto1, right?

Now, what would happens if I push SW1?
- Would En still be pulled to Batt? (directly this time?)
- What would happen to Opto1´s emitter? It will suddenly jump to 4.5V - will it destroy Opto1, the device to the right, or both? Or nothing will happen?

In case you are wondering, I'm messing around with this project: https://github.com/NeonHorizon/lipopi/b ... er_down.md and I was trying to find out how to decouple the RPi from the Powerboost pins.

Thanks for your help.

drgeoff
Posts: 9759
Joined: Wed Jan 25, 2012 6:39 pm

Re: Help with this circuit

Tue Jul 02, 2019 7:09 am

When SW1 is closed En is at battery voltage. Opto's emitter is at a bit less than battery voltage. No damage to the opto, irrespective of GPIO state.

Tzarls
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Re: Help with this circuit

Tue Jul 02, 2019 7:16 am

Great, thanks for your answer. And what would happen if Opto's emitter was at battery voltage? In other words, what would happen if I removed the 10k resistor? Would it still work?

drgeoff
Posts: 9759
Joined: Wed Jan 25, 2012 6:39 pm

Re: Help with this circuit

Tue Jul 02, 2019 8:19 am

If you remove the 10k resistor then the GPIO cannot control the state of En. If you replace the 10k resistor with a zero resistance link, the circuit will still work as intended and no damage to the opto.

Tzarls
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Joined: Tue Feb 26, 2013 6:59 am

Re: Help with this circuit

Tue Jul 02, 2019 6:53 pm

Replace the 10k with zero resistance link, that's what I meant with "remove".

Thank you, I can go on with my project now. :)

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