EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: fabiodl on August 12, 2022, 07:29:57 am
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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?
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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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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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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 (https://www.teaching.eng.ed.ac.uk/open-educational-resources/rc-delay-use-timer)
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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.
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Thank you everybody. I modified it to the attached schematic