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I'm working on a project that uses this board I made and the Arduino Mega 2560, where the board will need to be powered by the Mega's +5V pin at certain times. A connector will be used to bridge the bus on the Mega (containing the +5V pin) with the board.

When the board is being powered by a DC supply (+12V regulated to +5V), and not the Arduino, I want to make sure that the Arduino +5V pin is protected from reverse voltage (seeing as the +5V pin is not an input.)

Questions:

  1. Would a Schottky diode be the most ideal solution to make this work?
  2. I do not want to draw more than 500 mA of current, what part of the diode spec sheet would this apply to?
  3. +12V will be the largest voltage input for the project, so what range should I consider for the reverse voltage breakdown?
  4. Is it considered good practice to add a capacitor to ground on the cathode side of the diode? Or is that a specific design consideration (e.g. AC to DC rectified situation.)

Thanks.

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  • \$\begingroup\$ What is the minimum allowable supply voltage for your "custom board"? What exactly does "ideal" mean to you...do you care about things like size and cost? \$\endgroup\$ – Elliot Alderson Feb 6 at 15:21
  • \$\begingroup\$ Minimum allowable voltage will be 5V for the "board" (edited). "Ideal" as in types of diode options (e.g. schottky, zener, rectifier). If diodes aren't the way to go, then other suggestions are welcomed. In terms of size, I've been given a max of 6x6 inches for the board (I will not be using all that space.) Cost - as cheap as it can get for the job required. \$\endgroup\$ – Anthony the Kid Feb 6 at 15:33
  • \$\begingroup\$ If the custom board requires 5V and the Arduino supplies 5V then you can not use any real-world diode for this purpose. You really need to think more carefully about the minimum voltage that the Arduino may supply and the minimum voltage your custom board requires. \$\endgroup\$ – Elliot Alderson Feb 6 at 15:35
  • \$\begingroup\$ My apologies here, let me clarify. Whatever voltage is coming from the Arduino 5V pin (it could be 4.875V as I have measured in the past), that will be regulated to 3.3V. Technically, there is no minimum allowable supply voltage - if it's 1V, the thing won't work properly - if it's 4.875V, so be it. The 3.3V regulator I'm using is an AP130-33Y, which has a minimum input voltage of 2.7V...so if anything, that would be the minimum. \$\endgroup\$ – Anthony the Kid Feb 6 at 15:42
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Would a Schottky diode be the most ideal solution to make this work?

That depends, if you want the cheapest solution then I think a Schotty diode might be the best choice. The disadvantage of the single diode solution is that some voltage (about 0.5 V) will be dropped across the diode. So the 5 V will become about 4.5 V. That might or might not be an issue for you.

If the 0.5 V voltage drop is an issue then you have other options like using a MOSFET based solution, see this blogspot.

I do not want to draw more than 500 mA of current, what part of the diode spec sheet would this apply to?

It depends on the diode's datasheet what this figure is called. For the SS12 (that's a very commonly used Schottky diode) it is called:

Maximum Average Forward Rectified Current I(av)

For the SS12 this current is 1 A so more than enough for your needs.

+12V will be the largest voltage input for the project, so what range should I consider for the reverse voltage breakdown?

Anything above 12 V but add some margin so at least 15 V. You will be hard pressed to find a diode that cannot withstand more than 15 V. The SS12 can handle 20 V so also more than enough.

Is it considered good practice to add a capacitor to ground on the cathode side of the diode? Or is that a specific design consideration (e.g. AC to DC rectified situation.)

You mean a bypass capacitor after the diode. Yes, that is good practice. You can almost never have too many bypass capacitors. As the diode does introduce some voltage drop depending on the current through it, the voltage after the diode will have a little more ripple on it. A bypass capacitor will help to eliminate part of that ripple.

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  • \$\begingroup\$ Very thorough explanation, thank you very much for your help. I'll look into a MOSFET based solution just to see how that compares. And I will add a bypass capacitor \$\endgroup\$ – Anthony the Kid Feb 6 at 15:52
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Here are several schottkey diodes rated for 220mV drop at 1A, rated to 15V, that would work well.

https://www.digikey.com/products/en/discrete-semiconductor-products/diodes-rectifiers-single/280?k=schottkey&k=&pkeyword=schottkey&sv=0&pv1989=0&pv96=24&pv2261=u220mV+%40+1A&sf=1&FV=ffe00118&quantity=&ColumnSort=0&page=1&stock=1&pageSize=25

These are less expensive if you can tolerate 300mV drop https://www.digikey.com/products/en/discrete-semiconductor-products/diodes-rectifiers-single/280?k=schottkey&k=&pkeyword=schottkey&sv=0&pv1989=0&pv96=24&pv2261=u300mV+%40+1A&sf=1&FV=ffe00118&quantity=&ColumnSort=0&page=1&stock=1&pageSize=25

There are thru-hole options as well.

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  • \$\begingroup\$ CrossRoads, thank you for the links to these diodes, the 300mV is absolutely something I can stomach given the range of voltage inputs for the rest of my components. \$\endgroup\$ – Anthony the Kid Feb 6 at 15:54

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