Nuts and Volts - June 2014

Pi circuitry, its voltage also passes through diode D2 which has a forward voltage drop (Vf) of 0.55V at

1A. Therefore, in the worst-case scenario (i.e., a dead battery pack), V ( 6.30V – 0.55V) or less, so the circuit should function correctly until the battery pack is fully depleted at 6.3V.

Of course, we’re going to use a PICAXE processor to monitor the functioning of our UPS. We’ll display the real time data for two variables: the voltage of our main power supply; and the voltage of our backup battery pack. So, let’s again turn our attention to the two voltage-

Figure 1.

Voltage-Sensing Circuit (R1 and R2) for the Main Supply

We don’t need to know the actual voltage of the main power supply. We’re only interested in whether it’s good or if it has failed.

Therefore, we don’t need an ADC (analog-to-digital converter) input for this purpose; we just need to choose R1 and R2 to produce a digital high or low voltage at an input pin on the PICAXE. In order to do that, we need to know the minimum voltage level that results in a high input when using a 3.3V supply.

There are some differences between PICAXE processors, and even between specific pins on any given processor. The details are presented in the documentation for the inputtype command in Section 2 of the PICAXE manual. However, rather than bore you with those details, we’ll simply use a value of 2.7V which exceeds the minimum voltage level necessary to produce a high level on all input pins of all current PICAXE processors.

With a 12V supply and a 0.55V voltage drop across the 1N5818 diode, if we choose a 68K resistor for R1 and a 22K resistor for R2, the standard voltage divider rule yields the following result: Therefore, if the line that’s labeled “Power” in the schematic is connected to a PICAXE input pin and the 12V main power supply is functioning correctly, the input pin will be in a high state. On the other hand, if the main power supply goes down, R2 will pull the Power input down to ground, so the input pin will be in a low state. This gives us a simple way to know whenever the main power goes down. We could also set up a PICAXE interrupt routine, so that the program could immediately respond to the main power outage.

There is an important caution about the main supply that I need to mention. Typical wall wart power supplies frequently output a voltage that’s less than or greater than the stated value. If the voltage is much below 12V, the input pin may not be in a high state. More importantly, if the power supply’s actual voltage is 14V or higher, the input pin will be at a level that’s higher than 3.3V which could damage the pin. Therefore, it’s important to test the specific 12V supply you intend to use before connecting the Power line to an input pin on the PICAXE processor. This needs to be done when the Pi is being powered by the main supply because the voltage level can change significantly, depending on whether or not it’s actually providing power.

To check the voltage of your main supply, power the Pi from it and measure the voltage at the power connector on the UPS. The pin at the back end of the connector is +V and the shell of the USB connector is grounded, so it’s easy to do with the probes of a multimeter.

Voltage-Sensing Circuit (R3 and R4) for the Battery Pack

For the backup battery supply, we’re definitely interested in monitoring the real time analog voltage level, so we need to connect the UPS “Batt” line to an ADC input on the PICAXE processor. We also need to choose appropriate values for R3 and R4. When I first built my UPS, I included all four resistors on the stripboard circuit. However, as I was testing the supply, I realized that I needed to experiment further with different values for R3 and R4.

To do so, I removed R3 and R4 from the UPS, and modified the stripboard circuit so that the Batt line was directly connected to the positive terminal on the battery pack. (After we’ve constructed the UPS, we’ll discuss this issue further.)