Arduino supply voltage management

Table of Contents  [ Hide]

Overview
Arduino Uno R3 schematic rearranged
Notes on each input
Some salient component properties
Main questions of interest
USBVCC switching
All combinations of two power sources examined
Particular challenges
Alternative strategy for battery power

Overview

The somewhat canonical Arduino Uno R3, and the many other variants of Arduino, have multiple supply voltage input and voltage regulator paths. This article explores them, and identifies some hazard situations.

Arduino Uno R3 schematic rearranged

We can rearrange the Arduino Uno R3 schematic to make clearer the relationship between the various supply voltage inputs and regulators. Note that different Arduino models have different power input arrangements, as do partial clones and related boards from other vendors. So the discussion here is only representative.

The schematic below shows only the power inputs, voltage regulators, and the main regulated voltage destinations. (Click to expand).

The next schematic focuses just on the inputs and regulators, and shows the four plausible supply voltage inputs.

Notes on each input

Some salient component properties

Main questions of interest

1. Voltages required at VIN or PWRIN (DC jack) to properly operate U1?

• VIN > 5V + 1.1V (dropout) = 6.1V    Increase to 6.2V for loads closer to 500mA
• PWRIN > VIN + 0.8 (diode forward drop) = 6.9 to 7V.

BUT see also the "Two supplies" question below!

2. Insufficient input voltage: What happens with insufficient input voltage for U1's drop-out voltage spec? This happens with depleting batteries, for example.

Does U1 output just decrease proportionately? With less regulation capability? Or does U1 shut down?

Not characterized on Onsemi's datasheet. To be determined by experiment.

3. Two supplies: What happens when two different supplies are connected? This happens when alternating between programming (using USB) and running from battery or other supply. 

• U5A along with T1 is intended to arbitrate between power supplied by USB and power supplied via VIN or PWRIN.
  • But this doesn't cover all situations. Elaborated below.

USBVCC switching

Op amp U5A, configured as a comparator, switches GATE_CMD low when input CMP < 3.3V. That condition turns on T1 (P-channel MOSFET FDN340P), essentially shorting USBVCC and +5V nodes, permitting current to flow in either direction.

Conditions that disable T1:

• CMP > 3.3V, which happens when:
• VIN > 6.6V
• PWRIN (DC jack) > 7.4V

Hazard: If power is supplied by USB and also one of VIN or PWRIN, then there are ranges in which USBVCC remains enabled by T1, and both supply voltages are trying to drive the +5V node:

• VIN between 6.1V and 6.6V, or equivalently
  
• PWRIN between 6.9V and 7.4V

The main concern here is current flowing back through the USB cable to the attached computer, with unknown consequences for the computer's USB power supply.

All combinations of two power sources examined

Here are all combinations of power supplied to two Arduino Uno R3 inputs.

Particular challenges

The preceding table identifies a number of hazard situations, each of which might crop up in some scenario or another.

Of particular interest are the challenges of using batteries, while developing code over USB.

• Running an Arduino from batteries at all, given that battery voltages don't relate nicely to the thresholds noted, especially bearing in mind the battery voltage change during its discharge life.
  
• For example, a 4  x Alkaline "1.5V" battery pack varies from about 5.6V down to 4.4V. So it can't supply 5V per se, and isn't a high enough voltage to connect to a regulated input.
• A "6 x 1.5V Alkaline battery pack" varies from 8.4V down to 6.6V. Would need to be connected to VIn (not DC Jack) in order to consistently power the Arduino, and avoid conflict with the USB-supplied power (when battery voltage < 7.4V)
  
• So far as I can see, there is no scenario where the USB input takes over supply of power to the Arduino (relieving the load on the battery).
• This is bad from the point-of-view of developing code on a host PC while attached to the Arduino. (Batteries drain.)
  
• Might be good where the batteries are called upon to supply additional attached devices (such as servos) that might overload the USB power source.
  

Alternative strategy for battery power

Instead of connecting batteries directly to the Arduino, one can instead use the batteries to power a small DC inverter boost/regulator module. This way, A 4 x "1.5V" AA battery pack, which has plenty of capacity (mAH) for many applications, can be used to produce a consistent 6.8V to apply to VIn, (or 7.6V to apply to the DC Jack). This will keep the Arduino's input voltage in a range which avoids conflicting with USB power when USB is plugged in. The battery can be switched on and off to avoid draining the power when USB power is available.

A further refinement would add wiring direct from the batteries to significant loads (such as servos or motors) that draw considerable current, but are not too fussy about voltage.