Breadboard Power Supply 5V/3.3V (DFR0140)

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“Breadboard Power Supply 5V/3.3V”

This is a very simple board that takes a 6-12V input voltage and outputs a selectable 5V or 3.3V regulated voltage. All headers are 0.1" pitch for simple insertion into a breadboard.

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I just recieved this DFRobot breadboard power supply. Before connecting it to anything, I checked the supply voltage with a multi-meter. With 12V in the barrel plug, I measured 3.3V on one side, and 5V on teh other, as well as correct voltagles on the pins on top.

However, after attempting to power a ESP32 devboard, I was suddenly measuring 12V on teh 5V rail (still 3.3V on that rail). After some time, the board is not being powered at all via the barrel connector, only via USB 5V in.

Is this board meant to ouput 12V on the 5V rail at any time, because mine certainly was.


Hi Ryan.
The board is very simple. A 5V regulator for the 5V and a 3.3V regulator hanging off that for the 3.3V.
The schematic is available here

It looks like the 5V regulator has failed short circuit which would still provide 3.3V via that regulator but 12V on the 5V line. Then the 5V regulator has gone open circuit which will remove the 12V from the 5V line but you would still be able to power the system with USB as that 5V connects directly.

Having said all that I think the whole thing is pretty wrong (or to use another term, garbage). If that circuit is correct you can’t use a XX117 like that. If you connected a 117 like that all you would get out of the end would be 1.2V which is the chip reference. Check the XX117 data sheet. You will find it is an adjustable regulator (and thus very versatile) and needs a voltage between “adj” connection and ground, usually obtained with a resistor arranged to use some of the output current. Refer sample in data sheet.
Cheers Bob


The LM1117 is available in several fixed-voltage versions which are used exactly as detailed in that schematic, ie Adj tied to ground. It is only the adjustable versions that rely on the Adj resistor to set the output. voltage. See, for instance:
LM1117 - 800 mA Low-Dropout Linear Regulator (
at page 1.


Hi Jeff.
True the 3.3V regulator on the schematic is quoted as LM1117. I was not aware of the difference until now, thanks for that. Lost touch over the years. Was not even aware of the existence of a LM1117. Always have just used a 117.

BUT the 12V regulator is marked as LM117 which is NOT a fixed regulator. This could be a typo but if not and it is indeed a LM117 fitted then this would explain Ryan’s problem.
Cheers Bob


It’s actually marked as LM117-5V, but there is no such thing. All four examples I have here use LM1117 for both regulators. OP has indicated that it worked at 5V before testing with the ESP, so a shorted or open regulator is the more likely explanation. However it is also possible that the measuring is being done with insufficient load on the regulator to make it regulate correctly. Some devices have undefined behaviour for a load of less than about 10mA. In this case the 3.3V regulator should ensure this minimum load exists, but perhaps not.


Hi Jeff

I don’t have that luxury so I have to rely on published information. If it is incorrect on the published circuit it should be fixed.

True, I couldn’t find it, but I also did not get pointed to a 1117 in my search.

A LM117 normally would have a minimum load of 10mA which is provided by a 120Ω resistor from OP to ADJ then another resistor is used to obtain the required ADJ voltage to Gnd passing this 10mA as you are probably aware. The LM1117 is probably a LM117 with these resistors fitted internally to provide a fixed voltage regulator.

My lack of awareness is probably due to my habit of just grabbing a 78XX or 79XX device when I need such a thing or I usually have 1 or 2 LM317 devices around in case I can’t find a 78XX unit or I need something a bit different.

Just had a look at the data sheet you linked. There is another interesting application where you can change the output voltage in the upward direction bu inserting an appropriate resistor from ADJ to ground. In other words it can still be used as an adjustable regulator with the minimum voltage as designated in the type number.

Anyway thanks for setting me right and for that info. Try to learn something new every day, that’s today taken care of.

The published circuit is still incorrect though.
Cheers Bob


Thanks all for the replies. I’m not that experienced with electronics, just having fun programming some ESP32 modules.

Not sure what happened, I guess something shorted, unfortunately it seems to have damaged my ESP32 board, as I can’t communicate with it when connecting the USB. Oh well.

Thanks for the help anyway folks.

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Hi Ryan,

Welcome to the forum!!

Sorry to hear that the breadboard power supply hasnt lived up to its specs, feel free to reply to your order confirmation email and we’ll help you get your projects back on track!


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Total noob here.
I am not sure why but using the same source (I have tried 2), when I plug it into the 2.1mm I measure about 20% less voltage than when plugged into the USB socket. I assume this is not by design ?

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You can’t use the same source for either input. The 2.1mm round socket requires a minimum of 6V while the USB socket has a maximum of 5V (as for any USB).

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Hi Mark
What are you plugging in and what are you measuring and where.
If plugging 5V into the 2.1 socket this is not enough. For the 5V regulator to work this has to be something greater than 6.2V (I think). If you plug 5V into the USB socket the 5V regulator is by passed and not used. hence you will measure the full 5V.

to prevent guesswork (which the above is) please provide a bit more information and be as exact as you can. Makes diagnosing a lot easier.
Cheers Bob

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Hi Mark,
Welcome to the forum!
To add onto Bob’s questions here. Photos work wonders when troubleshooting. A picture says a thousand words here.
We’re happy to help you troubleshoot we just need a bit more to get started.

Thank you chaps, that’s the information I was after.
Intuitively I had assumed that 5V on either would produce the same result out, since the difference in the variables was a socket.
As a noob I would be grateful if anyone explain why this occurs. Is there different plumbing for each socket ? Oh wait… I also assumed that only one socket at a time would be used but that’s not the use case is it ? Although that doesn’t make any more sense, I am obviously missing some shred of understanding here.

  • Mark

The 2.1mm round socket can handle a voltage of between 6V and 9V, because the supply is routed through a regulator to provide the 5V output. A regulator will always introduce a small voltage drop, so that input cannot work properly at less than 6V - if it is less then the output will be the input voltage less 1V or thereabouts. The USB input is routed directly to the output, so it simply passes through the 5V from the USB source.

Ahh… that makes sense now and thank you.
As a side question, it occurred to me that you could conceivably plug two inputs in at the same time, I’m guessing this would not be wise ?

Hi Mark

You guess correctly. A good practise for everything unless you know what you are doing and do it correctly.
Cheers Bob
Some devices have the ability to connect 2 supplies at the same time but one has priority and is controlled by electronic switch circuitry. This device is not one of these.

The regulator monitors the output voltage to control its internal regulation. If the output voltage was being maintained from some other source then the regulator would not see the expected relationship between what it was trying to do as a regulator and what it was seeing as the output voltage. Whatever it did in response to this unexpected situation would likely not be good. The same situation applies with respect to whatever is regulating the 5V USB supply.