Regulated vs Unregulated Supply

Did You Know? Regulated vs Unregulated

When a USB is connected to the micro:bit, the Edge Connector connects the USB supply’s GND voltage to the left bus strip’s (-) rail.  It also connects the regulated 3.3 V from the micro:bit to the 3.3 V pin.  When the battery is connected its (-) terminal is also connected to the left bus strip’s (-) rail.  This is called a “common ground”.  In this case, ground is common to three systems: the USB 5 V supply, the micro:bit’s 3.3 V supply, and the battery pack’s 3 V supply.

THE DIFFERENT POSITIVE SUPPLY VOLTAGES SHOULD NOT BE CONNECTED IN COMMON.  That is why the battery pack’s Vbat supply is only available from the left bus strip’s (+) rail.  The micro:bit’s 3.3 V supply is available from the center and right bus strip’s (+) rails, which are connected to the Edge I/O Adapter’s 3V connection with jumper wires you added in the Set Power for Circuits activity’s Parts & Circuit page.   Since the battery pack’s voltage might be 3.2 V when the batteries are new and 2.8 V when they are almost dead, it is called an unregulated supply.  In contrast, the micro:bit module’s 3.3 V supply is considered a regulated supply.  For example, with the USB cable connected to the micro:bit, it might supply something in the 4.5 to 5.5 V range to the micro:bit.  But, the micro:bit’s built-in voltage regulator would still supply 3.3 V at the 3V pin.  The job of a voltage regulator is to supply unchanging or “regulated” voltage.

Your Turn

Let’s compare the unregulated battery voltage to the regulated micro:bit voltage. To do this you will, probe the center bus strip’s (+) and (-) rails.  In other words:

• Leave the alligator clip probes connected to the 3-pin headers by P0 and P2.
• Unplug the alligator clip probes from sockets in the left bus strip’s (+) and (-) rails.
• Plug the red probe into a socket next to the red (+) stripe in the center bus strip.  
• Plug the black probe into a socket next to the blue (-) stripe in the center bus strip.
• Measure the voltage.  

The voltage you just measured is the voltage regulated by the micro:bit, so your voltage measurement should be closer to 3.3 V.   Again, while the unregulated battery voltage will probably be somewhere in the 2.8 to 3.2 V range, when the micro:bit is connected to USB, the supply from the Edge Adapter’s 3V pin should be very close to 3.3 V.

• Compare the unregulated battery voltages you measured earlier to this regulated voltage measurement.  
• Click the CYBERscope’s Disconnect button.