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power switching with mosfet and comparator

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rompelstilchen

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Power switching with mosfet and comparator

Hello,

the arduino I use does not deal with switching between the USB power and the battery connected to VIN pin

I made this design using an LM3393 comparator and an IRF7319...since I am not used to do this, I wanted to know if the schematic made sens, maybe I missed soething

Untitled.png

the 10k res is a pulldown resistor, not sure if this is needed

thanks

http://skory.gylcomp.hu/alkatresz/irf7319.pdf
http://www.onsemi.com/pub/Collateral/LM393-D.PDF
 

Doesn't work for several reasons, at least these:
- LM393 output is open collector an can't source voltage
- comparator threshold of 0V makes no sense

LM393 power supply isn't clear, I also fear that comparator and MOSFET substrate diodes may thwart the intended operation.
 

Hi,

Additionally to FvM:
If you want the N-Ch Mosfet low impedan e to get 5V at the source, then you need at least 5V_gate to source, which means 10V at gate referenced to GND.

My recommendation:
This is no new idea to switch between different power supplies, thus there are a lot if proven circuits around. No need to reinvent the wheel.
To make it simple with low dropout voltage: look for "ideal diode" circuits.

Klaus
 

sheers, I'll try to find a ready made circuit

- - - Updated - - -

I did no find any circuit to order online but I found explanations on how to make one

https://arduino.stackexchange.com/q...-with-both-the-usb-and-external-power-voltage

I also found an alternative schematic that says the same

**broken link removed**

NCP1117 has low drop out as you mentioned too

I still dont understand why would people prefer the battery power supply over the usb, I would wire it the other way around, like an UPS system
also the lp2985 seems overkill to me
 
Last edited:

Hi,

If that U5A is a comparator, as stated in post #2 it needs a pull-up resistor to operate (and the PMOS won't turn off without the pull-up either); if it's an op amp, they don't make good comparators. Minor criticism is the "mirrored" and therefore confusing layout of the schematic - i.e. USBVCC is "out"?
 

pfffff....I guess, I'll use agood ole f@#king mecanic switch...

dont have much choice here, the only breakout board I found for purchase involve usb/li-ion battery charging...which I dont use
 

Hi,

Give some specifications:
We know USB voltage should be 4.5V ... 5.5V.
* what is your PWRIN voltage range?
* what is your output (load) voltage range?
* what is the load current range?
* what is the switch function? You want to enable PWRIN as long as it is > 5V? (Independent of USB voltage?)

Klaus
 

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