Powering External Devices

I'm confused by volts and amps, what is the difference between these two things?

Voltage, Current, Resistance and Power are so bound together you can't affect one without affecting one or more of the other. Power is just a simple term for Voltage multiplied by Current, so that one is easy.  However, the first three, voltage current and resistance form a trinity that are completely interdependent.

The equation “voltage = current X resistance” or V=I x R relates these variables but let's keep things simple and disregard the equations. The bottom line is when adding a module to the flight controller we must consider (1) what power the module will take (require) and (2) what the power supply can give (source). The power supply will give a set voltage to the module, but the module will pull a set current from the power supply. So again, the power supply gives voltage, the module takes current! When matching the power supply with the module, we must ensure that the voltages match and that the module will not take more current then the power supply is capable of handing out.

When powering a module the voltage between the power supply and the module will stay constant (at least, we hope so!). The module, however, may alter its current demand based on what it is doing at that point in time. For example, while searching for satellites a GPS module may increase power in order to better it's chances of finding a weak signal but once the it has found enough satellites it may reduce power to a more reasonable level. 

Requiring a 3.3V or 5V regulator

If your module is 3.3volts you may need to add a voltage regulator inline if it’s current demand will reach more than 35 milliamps (mA), which is not a lot of current. For example, you cannot use the 3.3v power from the 4pin sensor bus (I2C bus) header to power a GPS module. A GPS module can reach more than 100mA during startup and typically 60mA when locked. The Quadrino GPS module pulls 5 volts from the main board and has an onboard 3.3 volt regulator, so it is fine.

You can purchase the voltage regulator from Digikey below. You can solder wires to the regulator, but the easiest way to use this regulator is to use a 3pin RC cable extension just like a servo has. This 3pin header can be plugged right into the regulator's 3 pins. Another convenient method is to use single jumper wires like those found on sparkfun or adafruit. I prefer the jumper wires because they are more versatile and usable for other projects and I can choose my own color-coding. Now with the wires attached shrink wrap the whole thing or wrap in electrical tape to so as to secure it and to prevent the metal tab on the regulator from shorting anything else out inside your quadcopter.

The color-coding on the 3pin header is not going to be correct. If you are looking at the voltage regulator from the front (where the part text is), then the left pin will be the voltage input (5v), the center pin is GROUND, and the right pin is voltage out (3.3v). You can either pop out the wires from the 3pin header and move them to match, or simply accept the incorrect color coding. Popping out the wires of the 3pin header is usually pretty simple and we can then make the black wire match GROUND, red wire match 5V (left pin) and brown or yellow match 3.3V (right pin).

We will require 5V and GROUND pins from any of the ESCs. One ESC already powers the Quadrino board, so any of the others will do. If you use the EZ connect cable, just remove the pins on the servo extension cable from the black plastic housing and pop into the 3pin housings of one of the other ESC connections on the EZ connect cable. Now we have one ESC powering the FC board, and another powering our 3.3volt regulator and its all still part of the EZ connect cable and we did it without the need for soldering.

The 3.3v regulator has 3 pins, we’ve taken the 5V and GROUND from an ESC and connected it to pins 1 and 2 in the figure (IN and GROUND). We now only have 1 pin left and it is our positive 3.3 volts (OUT). However, to power our device we need to supply it with the 3.3 volts and a GROUND. We must either split the middle pin on the regulator into 2 wires, both GROUND, or we can pull the GROUND from anywhere else, including off the Quadrino flight controller. Since we are already wiring into the sensor bus (2 wires, clock and data) we might as well take GROUND from there too. So then, 3 wires (clock, data and ground) will come from the flight controller and the 1 power wire (3.3v) will come from the external regulator.

What regulator can I use to provide 3.3volt power?

One option is the 3.3volt 500milli-amp (mA max) voltage regulator in a TO220 physical package (part# UA78M33CKCS) from Digikey. This will provide the necessary current for the GPS module and probably any other modules you need in the future. It's pins are also the right size and spacing to fit a standard RC 3pin connector. You can plugin the connector and heat-shrink it to keep it in place. This means no soldering necessary!

The 500mA is a current limit maximum but it does not consider the heat that may be generated by the device and must be dissipated somehow. The more current the modules suck from the regulator, the more heat it will generate. If you approach 350mA you may notice the device heat up considerably and a heat sink will be needed. Luckily our modern micro GPS devices and most i2c sensors do not take more than 100mA in normal operation so a heat sink won’t be needed. You can test the temperature with your finger; if you can’t keep your finger on it then it's too hot!

The above diagram is the technical circuit wiring diagram for a voltage regulator. You can see how the GROUND signal is carried from the input directly to the output but is also tied to the regulator GND pin. This diagram also shows an input and an output filter capacitor. These help filter regulator noise and help maintain a constant voltage during times the regulator or input voltage may drop slightly which is possible during fast throttle up. Most ESCs already have the input capacitor so this one is optional. The output capacitor is recommended but most devices also contain one of these and may not be necessary thus making our wiring easier. If you do elect to add a capacitor you can stick the legs of the capacitor into the wiring sockets already plugged into the regulator and shrink wrap all of this together to secure it. Most power filter capacitors have a polarity so make sure the (+) on the capacitor is connected to the (+) pin, either the input or output positive pin. The values of the capacitor are not critical, use 47uF to 400uF on the input, and 1uF to 10uF on the output.


Voltage regulator tutorial & USB gadget charger circuit by Afrotechmods