Do I need diodes for my circuit? If yes, please suggest! (tolerating different power sources)

Hi everyone!

I'm building a circuit that will allow the user to select a power source (5V regulated or 7-18V unregulated).
Please take a look at the attached image!

The questions are simple:

  1. Do I need to use the diodes (marked green) in my circuit?
  2. If yes, can you suggest any diodes (1x dual or 2x single) [need to handle 2A, and has to be as tiny as possible]

+1) If I will use diodes, would the voltage drop affect the fuctioning of "Module"? (I mean if a [standard silicone] diode has 0.7V voltage drop, "Module" will only get 4.3V instead of 5V, and this doesn't sound good)

Example diodes I've found:
1x MBRD650CTG or
2x SK24A or
2x BYW96

No if that is your actual circuit and you are truly physically switching between the two power sources you shouldn't need diodes. Diodes are typically only used like that when both power sources are kept on. For instance if you have a device plugged into mains and also have a battery backup.

So isn't it a problem that without the diodes

  • if 5V is selected, the external voltage regulator gets 5V on Vout?
  • if 7-18V is selected, Arduino gets unregulated power on VIN and 5V on its 5V pin from the external v.regulators Vout?

You schematic labeling is condusing. You have a 5V 2A module but the box labeled "Power Source/5V or 7V " what is that ?
is it 5v or 7v and if it is 7v , why do have it labeled 5v ?
What is a "5V 2A module ? is that a 5v wal wart ? If so your diodes are backwards and you can't use diodes there anyway.

Sorry if it wasn't clear enough.

So once again: The plan is to make the whole circuit work with
a) 5V power (like a wall wart, or a 5V battery pack, etc), OR
b) 7-18V (this also can be a wall wart, or a battery pack, and anything that can supply 7-18V)
That is why I have a toggle switch as a voltage selector.
If 5V is selected, "Module" is powered directly from power source, just like Arduino (through 5V pin).
If 7-18V is selected, "Module" gets power from the voltage regulator, Arduino gets 7-18V (through VIN).

I hope you get the idea now : )

With the design you have shown, if you power it up with the switch in the wrong position you will blow the Arduino up. This seems like a bad idea since it would be extremely easy to do. There are several ways you could reduce the risk:

Provide separate outputs so that the 5V output can never be connected to more than 5V and vice versa. Depending where your power comes from, that might not be possible.

Provide a voltage sensing switch. This would be feasible, although it seems like overkill for such a simple function.

Incorporate a boost/buck converter in your power supply so that it always generates a smoothed regulated 5V output regardless of the input. This seems by far the best approach if you want this device to be tolerant of different power sources.

Thanks for the advice!

Unfortunately providing separate outputs is not possible here.
Voltage sensing switch would be also a good idea, but yes, it would be an overkill in my case.

About the buck-boost converter:
I've never worked with or built buck/boost converters, but as I know it would replace the voltage regulator(s), right?
Do you think I can find (or do you know any) buck-boost converter that outputs 5V @ 2A, from 5V-18V (or at least 12V)?
As I wrote, size really matters in this project, as I am very limited by the size of the PCB so I think I will need an IC for this.

Boost/buck converters tend to be compact. Since this approach would remove the need for almost all of the rest of your hardware, it seems pretty likely that this will give you the smallest solution, as well as the simplest and safest.

frzsombor:
Do you think I can find (or do you know any) buck-boost converter that outputs 5V @ 2A, from 5V-18V (or at least 12V)?

The converter needs to provide slightly more than 2A since you need 2A for your 'module' plus the current demand for the Arduino.

wanderson:
The converter needs to provide slightly more than 2A since you need 2A for your 'module' plus the current demand for the Arduino.

Nice, thanks for pointing it out!

So the only thing I need is a (small, IC sized) converter with

  • 3A
  • 5V output
  • 5V-18V input

Finally can you guys help me find anything like this?
I've tried searching on octopart.com, but nothing.
All the converters I've found are too big, or their inputs are not in my range.

Thanks in advance!

Well, after long hours of (re)search, I think I will not be able to use dc-dc converters, because:

  • I couldn't find any suitable converter with the mentioned requirements
  • I found that I would also need a few other parts (diodes, capacitors, even resistors in some cases) for these converters which is more than I can find free space for, on the pcb.

So guys - sadly - I think we are back to the original question.
Of course if anyone knows a single converter IC that can do all the work, I would be glat to hear about it!

I don't really understand what your problem is. Everything you have said you want to do is possible but not the way you are proposing it .
The 7V battery can be switched to the barreljack.

http://www.ebay.com/itm/Waterproof-DC-DC-Converter-12V-Step-Down-to-5V-Power-Supply-Module-3A-15W-BLACK-/321405617382?pt=LH_DefaultDomain_0&hash=item4ad54484e6

Use two of these in parallel
http://www.ebay.com/itm/2pcs-DC-DC-3A-Buck-Converter-Adjustable-Step-Down-Power-Supply-Module-LM2596S-/200964337125?pt=LH_DefaultDomain_0&hash=item2eca686de5

What exactly is the problem you are talking about ?

I couldn't find any suitable converter with the mentioned requirements

  • I found that I would also need a few other parts (diodes, capacitors, even resistors in some cases) for these converters which is more than I can find free space for, on the pcb.

I'm building a circuit that will allow the user to select a power source (5V regulated or 7-18V unregulated).
Please take a look at the attached image!
The questions are simple:

  1. Do I need to use the diodes (marked green) in my circuit?
  2. If yes, can you suggest any diodes (1x dual or 2x single) [need to handle 2A, and has to be as tiny as possible]
    +1) If I will use diodes, would the voltage drop affect the fuctioning of "Module"? (I mean if a [standard silicone] diode has 0.7V voltage drop, "Module" will only get 4.3V instead of 5V, and this doesn't sound good)

The whole diode thing is a moot point.
I don't know what you mean by other components.
I don't see anything in your post that says you are laying out a pcb and are looking for a single chip dc to dc converter.
If you said that and I missed it could you show me where you said that ?

So once again: The plan is to make the whole circuit work with
a) 5V power (like a wall wart, or a 5V battery pack, etc), OR
b) 7-18V (this also can be a wall wart, or a battery pack, and anything that can supply 7-18V)
That is why I have a toggle switch as a voltage selector.
If 5V is selected, "Module" is powered directly from power source, just like Arduino (through 5V pin).
If 7-18V is selected, "Module" gets power from the voltage regulator, Arduino gets 7-18V (through VIN).

You should just use a similar circuit as used on the UNO so switch ON or OFF a mosfet switch that allows the USB 5V to pass to the arduino Vcc IF AND ONLY IF there is NO VOLTAGE on the DC BARREL JACK EXT PWR INPUT. (a comparator is used for this)

Hi raschemmel!
See my answers in bold.

raschemmel:
I don't really understand what your problem is. Everything you have said you want to do is possible but not the way you are proposing it .[...]
These converters you've linked are good, they do what I want, and in most cases they would solve the problem, but in my case the first one is almost bigger than my whole PCB design.

What exactly is the problem you are talking about ?
[...]
The whole diode thing is a moot point.
I don't know what you mean by other components.
Again, my problem with the converters is their SIZE. By "other parts" I meant that all the small converter ICs I've found, required some other components too. I've checked their PDF documentation, and found a "Typical Application Circuit" part, where you can see a circuit built from the IC and diodes, capacitors, etc. So they were not meant to be used alone, and again I don't have free space on my PCB for more than 2-3 components. (example)

I don't see anything in your post that says you are laying out a pcb and are looking for a single chip dc to dc converter.
If you said that and I missed it could you show me where you said that ?
As I wrote in my original post, I even need the diodes to be "as tiny as possible". Then in my reply to PeterH I do wrote "size really matters in this project, as I am very limited by the size of the PCB so I think I will need an IC for this".

Actually, to end the confusions, all the free space left on my pcb (if I remove the voltage regulator) is the size of a to-220 horizontal footprint, and a few diodes (that I'm looking for)

[...]

You should just use a similar circuit as used on the UNO so switch ON or OFF a mosfet switch that allows the USB 5V to pass to the arduino Vcc IF AND ONLY IF there is NO VOLTAGE on the DC BARREL JACK EXT PWR INPUT. (a comparator is used for this)
I will take a look into this, but I'm not sure I will be able to figure it out myself, and maybe size will be a problem again.

U5A (small , but not tiny( switches off T1 (FND340P; SuperSOT pkage)
SEE ATTACHED

FDN340P.pdf (207 KB)