Author Topic: Powering one section of a circuit when another comes up  (Read 724 times)

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Offline fabiodlTopic starter

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Powering one section of a circuit when another comes up
« on: August 12, 2022, 07:29:57 am »
I have a board (A) which exposes a parallel bus. I want to make a second board (B) which connects to (A) but does not draw too much current from it. I want (B) to come on only when (A) is powered, to avoid having logic gates on (B) that are powered to feed current to (A) through the parallel bus lines. (B) maximum current draw is around 500 mA, so a small mosfet should be ok to power it.
I came up with the following schematic. 5V_pwr is where (B)  draws current from, while 5V_sense is from (A). Do you see any problems / possible improvements?
« Last Edit: August 12, 2022, 07:31:56 am by fabiodl »
 

Offline jmelson

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Re: Powering one section of a circuit when another comes up
« Reply #1 on: August 12, 2022, 03:05:52 pm »
You need a resistor on source to gate of Q3, so that it will turn off when the 5V sense goes off.
Jon
 
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Offline jmelson

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Re: Powering one section of a circuit when another comes up
« Reply #2 on: August 12, 2022, 03:07:01 pm »
You need a resistor on source to gate of Q3, so that it will turn off when the 5V sense goes off.
Jon
 

Online JustMeHere

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Re: Powering one section of a circuit when another comes up
« Reply #3 on: August 13, 2022, 01:56:00 pm »
You need to have a high value pull down on both of your gates.  10k or 20k should do.  These will turn off your mosfets when the power is gone. 

This may do the job for you, but I don't know if there's a power off delay (or if that's a problem in your case.)

https://www.teaching.eng.ed.ac.uk/open-educational-resources/rc-delay-use-timer
 
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Offline TC

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Re: Powering one section of a circuit when another comes up
« Reply #4 on: August 13, 2022, 04:51:19 pm »
Look up bus isolator ICs and learn about their purpose/function. Also learn about power sequencing methods and latchup as a phenomenon.

The ESD diodes of the unpowered devices can become forward biased and try to source power from the signal pins for the unpowered IC. Not good.
 

Offline fabiodlTopic starter

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Re: Powering one section of a circuit when another comes up
« Reply #5 on: August 14, 2022, 08:53:02 am »
Thank you everybody. I modified it to the attached schematic
 


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