Power Supplies (5V)

The Basics

The Arduino Nano Carrier has two 5V supplies, each regulated down from the 7-12V input. The signal 5V supply provides up to 500mA for the Arduino Nano itself, as well as low-power components such as a radio receiver. The power 5V supply provides up to 3A for servos.

Though they are nominally the same voltage, the two supplies are kept separate with the exception of a single common ground connection. This allows the signal 5V supply to remain clean and stable even if the power 5V supply is loaded heavily by servos. The power 5V supply is only available through the DxO (output) pins and the PWR5V headers above the breadboard. The signal 5V supply is available through the DxI (input) pins and the SIG5V headers below the breadboard. See the pin diagram for more details.

The following table summarizes important aspects of both power supplies:

Note: If your system used only the power available from the Arduino Nano Carrier for driving servos, you would have an upper limit of 15W (5V x 3A) available for mechanical work. After accounting for system efficiency, the actual mechanical output will be lower still. Keep this in mind when designing your servo-driven mechanisms!

Signal 5V Supply

The signal 5V supply is created by a linear voltage regulator on board the Arduino Nano. "Linear" implies that the regulator is functioning like a simple variable resistance in series with the 7-12V input. The value of the variable resistor is controlled to keep the output voltage at steady at 5V. The following figure shows this representation of a linear regulator:

No matter what the input (7-12V) or load current (0-500mA), the value of the variable resistor is changed such that the output voltage is 5V. This is done by a feedback mechanism in the linear regulator itself. Linear regulators are inefficient compared to switching regulators; all the power dissipated by the variable resistance is lost as heat. This dissipated power depends both on the input voltage and the load current.

In general, it is easy to calculate the efficiency (power out divided by power in) of the linear regulator: it's just the ratio of the output voltage to the input voltage. (Can you see why?) Thus, a higher input voltage will make the linear regulator less efficient and possibly unable to achieve it's maximum output current of 500mA.

The following excerpts from the Arduino Nano and Arduino Nano Carrier schematics show the electrical connections related to the signal 5V regulator:

The net BAT+ is the unregulated battery input of 7-12V. (After the switch and reverse polarity protection discussed in the power input section. Diode D2 is not necessary for reverse polarity protection, but it does prevent the signal 5V supply from feeding current back into the main power input. The scenario where this is useful is if the servos or other external load draw so much current that the battery voltage drops significantly. In this case, capacitor C3 will buffer the signal 5V supply so it can "ride out the storm." A blue LED on the Arduino Nano indicates that the signal 5V supply is on.

The signal 5V supply is available to the user on the headers above and below the breadboard section for use in prototype circuits. It can be used to power sensors, LEDs, one or two small relay drivers, or any other components requiring 5V and a total load of less than 500mA. It is also available on the center pin of each DxI header for powering the radio receiver. See the pin diagram.

Power 5V Supply

The power 5V supply is created by a switching voltage regulator on the Arduino Nano Carrier. Unlike a linear regulator, the switching regulator rapidly turns the input voltage on and off in order to control the output voltage. In order to produce a smooth output voltage, a filter comprising a capacitor and inductor is used. The capacitor resists changes in voltage and the inductor resists changes in current; together, they convert the switching voltage into a smooth, regulated output. The figure below shows a simplified switching regulator schematic. Clicking on the figure will bring up a simulation of how a switching regulator works.

Click here for simulation.

In the simulation, a 12V supply is regulated down to 5V. The "regulator" in this simulation comprises the switch, diode, inductor, and capacitor. (There is more to the regulator, including a feedback circuit that controls the duty cycle of the switch based on the measured output voltage.) The sizes of the components are chosen based on a variety of factors, including the voltage, current, switching frequency, and allowable "ripple" in the output.

The key advantage of the switching regulator is that it can be more efficient than the linear regulator. By using energy storage elements as a buffer, it avoids the need to burn off extra power as heat to get from a higher to a lower voltage. (In the ideal circuit depicted above, all the elements except the diode are lossless. In reality, each component contributes some small inefficiency.) The efficiency is also less dependent on the difference between input and output voltages. Thus, switching regulators are often used for high power conversion between a wide range of voltages.

The switching regulator on the Arduino Nano Carrier is based on the LM2596-5.0, which switches at 150kHz. The following schematic excerpt shows the regulator, including filter elements, used to create the power 5V supply:

The components L1, C1, and C7 act as the output filter and are sized to handle 3A output at 5V. C2 is an input filter that helps buffer the input supply from switching noise. LED1 indicates when the power 5V supply is active.

Even thought it is more efficient than the linear regulator, the switching regulator handles a lot more total power. For this reason, it is normal for both the regulator and the inductor L1 to be warm or even hot to the touch when running a high load. The regulator is protected against overcurrent and will limit power to protect itself. Signs that this may be happening include:

• Constant load of more than 3A.
• Regulator or inductor is very hot.
• Flickering or dimming of LED1, the power 5V supply indicator.
• Loss of power to the servos.
• Audible noise (clicking or hissing) from the regulator. The thermal and current limiters make noise in the audible frequency range.

If you notice any of these signs, you should check for short circuits and/or cut back load on the servos to well under 3A until the regulator cools down.

Caution: these components may be warm or even hot!