Clamping 9-volt to 8-volt?


#1

I realize this is not a question about KiCad itself. It’s more of an electrical design question. I have asked this on Stack Exchange / Arduino sub group, but the response I got didn’t seem favorable. The question:

Have a DFRobot Arduino Pro Mini.

Seems odd to me, but according to their specs the voltage regulator on board cannot handle more than 8-volt input. I can find no documentation, or have they answered me concerning what specific regulator they use on the board.

I would like to be able to run this board from a 9-volt battery. What is the best / most eloquent way to do this, keeping the smallest component footprint for a home etched board ? An additional regulator ?? Could a diode clamp the voltage down ?

Many thanks in advance.

Screenshot of the response:

My base question wasn’t answered.
Apparently, the guy doesn’t like those regulators.

Any help ??
Greg


#2

Have you considered removing the regulator on the board and replacing it with a more capable regular in the same package? You might need to replace the caps also.


#3

Thank you for you reply, kiki… But this board is small. Here it is on one of the smallest Radio Shack boards, hand-soldered. Works… Programs successfully… Just can’t run on 9-volt:


Replacing regulator and bypass caps would be a delicate task.


#4

The goal is… Because the thing was so tedious to hand-solder, is to design a working, stackable board for it with KiCad:

But… The best to clamp 9-volt to 8-volt ??


#5

Not quite the terminology that I would use.
You can simply add 2 silicon diodes in series between the battery and the device.


#6

Thank you Sprig !!
Terminology may be off… Still learning.
Will try that on a breadboard.
Any suggestions on which type diodes ??


#7

Silicon rectifier diodes with a current rating higher then your design draws.


#8

Then that would be max current rating for the Pro Mini itself. Correct ??


#9

I did not see your second photo till now.
If you are gong to design a pcb to hold the module, then I’d recommend using an adjustable voltage regulator on the pcb. I thought you were wanting a simple method on your breadboard.

Also, the datasheet for the regulator on the board, is good for 10V, not the 8V as stated by the DFRobot web site.

Datasheet


#10

Thank you, Sprig.
Then there may be something wrong with the regulator on this board.
Hit it with 9-volt and get nothing… 5-volt does fine.

Thank you for the data sheet link !!!


#11

By the way, Sprig…
The board dimensions on “my hopes” is the same as the the Radio Shack board…
It’s small.


#12

Yes, but most fab houses don’t charge much, if any, more for a 2 sided board. You can put the extra parts on the bottom copper layer of the board.


#13

If your goal is to run a 5-volt device off a 9-volt battery, another option you could consider is SnapVCC. It’s a switching regulator rather than a linear regulator, so it’s more efficient and the battery will last longer. (That does make it pricier, though.) I have one and find it handy.


#14

Thank you, ppelleti.
I will look into that.
:wink:


#15

Why the 9V battery requirement?
Unless one has long cable runs it is desirable to supply with a voltage close to what one needs. Regulators of any kind cause losses, which aren’t desirable for battery driven applications.

Not to mention that energy density of 9V blocks is rather crappy.
If you take 4x AAA alkaline cells (gives you 6V) even with conservative assumptions you wind up with more energy for the AAA version.
9V blocks are a niche cell for applications that need ‘high’ voltage, but not much energy in one package.


#16

I guess, Joan… it’s not a requirement. However, it would be nice to be able to function on that without being tethered to a wall wart, or something of that sort.


#17

Think @Joan_Sparky may be subtly asking why not use 4AA cells, or maybe even just 2 LiPo cells?


#18

I nailed it! LOL.

20 characters.


#19

Yeah, SE also suggested to use Li-Ion with a boost converter.
But that’s only desirable if the application can be recharged every now and then.
So I (hope) the application @grpace has in mind is one of those fire&forget deals, that require alkaline or even lithium primary cells.


#20

Does your version have a 3.3V supply rail, or 5.0V? (DFRobots says they produce both variants.) This value affects the range of viable design options available to you.

This strikes me as the most straightforward solution. OK, I understand you’re not comfortable working with SMT devices at that scale. I’m not either. But I can be patient long enough - with the help of a toolmaker’s microscope, or at least a very good desk magnifier - to get the job done on one or two boards.

Doesn’t the official Arduino Pro Mini design incorporate a jumper (solder bridge) that completely disconnects the on-board regulator? Does your board have that feature? With the on-board regulator out of the circuit, you would be free to drive the Arduino Pro Mini from a regulator circuit using a device of your choosing. The venerable 78L05, or LP2950, are probably adequate for your application.

Or simply buy a version of the Arduino Pro Mini that can handle power input from a 9V battery. I believe there around half a dozen companies building the thing, and I know they don’t all use that sorry excuse for a 3-terminal regulator IC.

Even prehistoric relics like the 1N400x series would do the job. But, the two junction drops would ALWAYS be in series - compromising your ability to extract the full electrochemical charge from the battery.

10V is the ABSOLUTE MAXIMUM rating - the “smoke point”; the voltage at which permanent damage is likely to occur. 8V is the guaranteed operating voltage - the maximum voltage for the device to meet its published performance specifications. What happens between 8V and 10V? We aren’t told. Perhaps the device will drop out of regulation; perhaps totally disabled, or perhaps passing the full input voltage to the output; or maybe the device breaks into oscillation; or maybe it fails to meet some obscure specification that doesn’t matter to 99% of applications. We don’t know what the behavior will be, but as Clint Eastwood once asked, “Do ya feel lucky, punk?”

And keep in mind that some factory-fresh 9V batteries will measure 10V, or a bit more, across their terminals.

Dale