0
\$\begingroup\$

Why are some electronic devices rated for volts and some current? Isn't total wattage what matters? Most seem to be rated for amps, and Tesla famously ran hundreds of thousands of volts through himself, which certainly implies that current is what matters.

For instance, the Raspberry Pi's GPIO pins are 3.3V and 5V. Why are these voltage-rated and not amperage-rated?

\$\endgroup\$
1
\$\begingroup\$

The OP specifically asked about the Raspberry Pi, and why the current limits are not specified. But actually, they are. And they should be for any microprocessor or microcontroller. You just have to hunt for them sometimes.

Their are mainly two kinds of pins on the Raspberry Pi; power pins, 3.3V and 5V, and GPIO pins, which can be used as inputs or outputs. All of the GPIO pins can also be re-purposed for various peripherals, such as UART, SPI, I²C, ADC and so forth.

In addition, all of the GPIO pins on a Raspberry Pi are rated for 3.3V only -- they are not 5V tolerant.

Now to your question about current -- yes, all of the pins have a current rating. That should be specified in the datasheet for the BCM2835, the processor found on the Raspberry Pi; however Broadcom has released only a partial datasheet which does not have all the required information. Luckily someone else has made a Wiki titled "RPi Low-level peripherals" which has the information you are looking for.

For the power pins, the specifications are:

For the 3.3V power pins, the maximum current is 50 mA

For the 5V power pins, the limits are:

Model A: max current draw: 500 mA
Model B: max current draw: 300 mA

For the GPIO pins, the maximum current that the pin can either sink or source is configurable from 2 mA to 16 mA.

Sink means to draw current while the pin is at ground. For example, you could have an LED with its anode connected to +3.3V via a resistor, and the cathode connected to a GPIO pin. By making the pin an output, and setting it low, that will turn on the LED and sink current into the pin.

Source means to provide current at the positive rail (3V). You could have an LED with its anode connected to a GPIO pin via a resistor, and the cathode connected to ground. By making the pin an output, and setting it high, that will turn on the LED and source current out of the pin.

In a microprocessor's datasheet there is usually a figure for the maximum amount of current that all of the pins can draw together; i.e. you cannot expect to be able to sink or source the maximum amount of current (16 mA) from all of the 54 GPIO pins simultaneously. But this number doesn't seem to be available as yet.

\$\endgroup\$
2
  • \$\begingroup\$ I agree, the R-Pi behaviour of the pins should be clarified, otherwise the answer is incomplete, and potentially misleading. I don't understand the 'drive-by downvote', but I can balance 1. \$\endgroup\$
    – gbulmer
    Jan 1 '16 at 12:32
  • \$\begingroup\$ For some reason, the answers to this question received a whole bunch of unexplained driveby downvotes. Very odd. \$\endgroup\$ Jan 20 '16 at 14:11
-1
\$\begingroup\$

Lets say there were 3 PSU's on sale.

  1. 1kV @ 1A

  2. 1V @ 1kA

  3. 100V @ 10A

Are they all the same? Sometimes you need more amps (especially due to the complications that brings as a current source). Sometimes you need more voltage (equally complications with insulation. And sometimes you need a general purpose PSU

\$\endgroup\$
-2
\$\begingroup\$

All devices need voltage and current to function; so they are actually all rated as volt and current. You see some are rated by voltage and some by current because for that particular device, one is more important than other.

For example, a red led is rated at 2.1v, and 2.1v is its forward voltage. If you apply a 2.1v to this red led, you don't need to worry about it getting burn out or damage. but if you apply 3v to this red led, it will burn out; so you need a resistor, thus you will see the importance of the current. You can use any resistor from 10-10k, then you will get different current and brightness.

Same thing to the Raspberry Pi's GPIO pins. It has 2 tiny 3.3v and 5v voltage regulators. They can supply limited amount of current, but enough for most of the peripherals connected to it. so voltage is more important than current, but current are voltage all exist and are all important.

\$\endgroup\$
1
  • \$\begingroup\$ This really isn't the way the LEDs work. \$\endgroup\$ Oct 29 '18 at 9:25

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.