ID-12 to Arduino Uno

Im trying to power my Sparkfun RFID-USB Reader board (it has an RFID ID-12 reader) using my Arduino Uno. I know the ID-12 only requires 5v. After some simple measuring of the current during normal operation and a read, I found that the Arduino Uno should be able to handle the current, but Im stumped as to why it wont (its 19.7mA Low, and 44mA High). The Regulator on the Uno can do up to 1A, and Im wiring straight to the 5V pin, so there shouldnt be any issues. The only thing that Im thinking is that because the 5v pin is hooked into the microcontroller, its limiting the current down to the 40mA per pin as the tech specs say per I/O pin. Is this so, or has anyone managed to get that board working on an Uno? Do you think running the ID-12 on its own might bring that current draw down?

UNTEngineer:
The Regulator on the Uno can do up to 1A,

Well Yes, the spec for the regulator says 1A. However, it depends on the input voltage. The regulator is limited because it is not well heat sinked. The higher the input voltage, the lower the output current.

UNTEngineer:
The Regulator on the Uno can do up to 1A, and Im wiring straight to the 5V pin, so there shouldnt be any issues.

What do you mean "handle"? What is happening?

UNTEngineer:
The only thing that Im thinking is that because the 5v pin is hooked into the microcontroller, its limiting the current down to the 40mA per pin as the tech specs say per I/O pin.

This doesn't make sense. The I/O pins are current limited because of the design of the ATmega328 chip. It has nothing to do with the 5V rail.

UNTEngineer:
... has anyone managed to get that board working on an Uno?

Yes: Gammon Forum : Electronics : Microprocessors : Simple RFID security system

Do you think running the ID-12 on its own might bring that current draw down?

I'm confused about what the problem is, here.

[quote author=James C4S link=topic=122054.msg918163#msg918163 date=1347144680]
Well Yes, the spec for the regulator says 1A. However, it depends on the input voltage. The regulator is limited because it is not well heat sinked. The higher the input voltage, the lower the output current. [/quote]

That would make sense if I was powering with external power. I am however powering it through USB, thus I am getting 5V directly.

[quote author=James C4S]
What do you mean "handle"? What is happening?[/quote]

When I wire the ID-12 board up to the 5V pin (and ground) nothing happens. It just doesnt work. When I wire it to an external 5V, it powers up just fine.

[quote author=James C4S]
This doesn't make sense. The I/O pins are current limited because of the design of the ATmega328 chip. It has nothing to do with the 5V rail. [/quote]

Your theory makes sense, but in practice thats not what I see is happening. Something is limiting the current, which in turn wont let the ID-12 power up.

Whoah, thanks a lot. This is actually exactly the application I was looking to use it for. :slight_smile:

Do you think running the ID-12 on its own might bring that current draw down?

I'm confused about what the problem is, here.

Im using the RFID-USB board from Sparkfun. The ID-12 plugs into it. Im curious to know if the board uses up more current than the ID-12 or would the current it consumes be miniscule?

UNTEngineer:
That would make sense if I was powering with external power. I am however powering it through USB, thus I am getting 5V directly.

In that case, the on board regulator has nothing to do with this. The 5V pin is connected to VUSB through a 500mA poly fuse. So you are limited to 500mA by both the USB spec and the on board fuse.

UNTEngineer:
When I wire the ID-12 board up to the 5V pin (and ground) nothing happens. It just doesnt work. When I wire it to an external 5V, it powers up just fine.

What is the external supply you are using? The 5V pin is effectively an external supply.

UNTEngineer:
Your theory makes sense, but in practice thats not what I see is happening.

What I stated isn't a theory. It is how the board and chip works.

[quote author=James C4S link=topic=122054.msg918825#msg918825 date=1347225237]
In that case, the on board regulator has nothing to do with this. The 5V pin is connected to VUSB through a 500mA poly fuse. So you are limited to 500mA by both the USB spec and the on board fuse. [/quote]

Im drawing somewhere between 19mA and 44mA. It just occurred to me, since Im using USB, would the FTDI chip (or even my computer) be limiting the current?

[quote author=James C4S]
What is the external supply you are using? The 5V pin is effectively an external supply.[/quote]

Im using a Sparkfun 5v/3.3v Breadboard power supply. That seems to do the job just fine.

You say the Arduino isn't 'handling the current', but what symptoms make you think it isn't?

What is the voltage on between the Arduino's 5V pin and ground when the 5V is not connected?

What is the voltage when your ID-12 is connected up to the Arduino's 5V pin?

What is the voltage on the ID-12 5V supply when it is powered from your external supply?

PeterH:
You say the Arduino isn't 'handling the current', but what symptoms make you think it isn't?

Simple: the ID-12 board doesnt power on.

PeterH:
What is the voltage on between the Arduino's 5V pin and ground when the 5V is not connected?

I measured it at 5v.

Since I have Rev2 of the Arduino Uno, I figured out that it must be the Atmega8U2 chip acting as a USB-to-serial converter that is limiting the current, because when I used an 12V external power supply it worked fine. Its only when I power the Arduino via USB that the ID-12 doesnt power on.

At least now I know why I shouldnt use the USB to power it during operation. Thanks for the help.

UNTEngineer:
Im drawing somewhere between 19mA and 44mA. It just occurred to me, since Im using USB, would the FTDI chip (or even my computer) be limiting the current?

  1. There isn't a FTDI chip on the Uno
  2. No Arduino has a USB-to-Serial chip that limits the 5V rail, they are all consumers of it.
  3. For the third time, the only thing that limits the current is the computer (via Vusb) or on-board 500mA fuse

If your board works with an external supply one might consider A) the Arduino's Poly Fuse is blown, B) the computer supplying current is blown or C) the board in question is not in fact drawing than 500mA. D) Either the USB-to-Serial Chip or ATmega328 has a blown I/O and is drawing over 500mA of current.

When it comes down to it, those are the only options that make sense. (D would be obvious because when powered by USB nothing on the Arduino board should be warm to the touch if nothing is connected)

Thats probably it. While on USB power, I can use the 5v rail to power all sorts of things except the ID-12. When on external power, I can power the ID-12.

Sorry if I seem a bit thick, Im just trying to wrap my head around this problem (and Im doing it mostly late at night).