Nuts and Volts - June 2014

have one available. A linear

12V supply would probably also work, but I don’t have one that can supply 1A to test that assertion. The main requirement is that the supply is powerful enough so that its output voltage doesn’t decrease much below 12V when it’s supplying power to the Pi. If it did, the batteries would kick in and be unnecessarily depleted.

Alkaline Batteries

I chose to use alkaline batteries because they are reasonably inexpensive with a relatively high energy density. However, two questions remain: Why D cells, and why seven of them? D cells may be larger than necessary, but many retailers sell them at about the same price as C cells, so why not?

Why seven of them? As you know, all alkaline batteries are rated at 1.5V. However, their initial voltage is actually somewhat higher. I measured the voltage of each battery in a fresh set of alkaline D cells, and found that they averaged 1.625V each. Seven times 1.625V = 11.375V; with that in mind, refer to the schematic in Figure 1.

You can see that diodes D1 and D2 effectively isolate the two sources of power (12V main power and battery power). In normal operation, the 12V from the main power supply is greater than the 11.375V provided by a fresh set of seven D cells.

As a result, the battery voltage is effectively blocked by the higher 12V supply. Therefore, the main 12V supply reaches the voltage regulator, and the batteries are not drained at all.

If the battery pack contained eight cells, it would initially output

13V ( 8 1.625V = 13.0V) which would block the main supply and quickly drain the batteries down below 12V. At that point, the main supply would again block the battery supply and power the circuit. The net result is that eight D cells would cause the battery pack to be unnecessarily depleted; using seven D cells avoids this problem.

Before we move on to the next feature in the schematic, there’s one more point about alkaline batteries that needs to be made clear. Diode D2 is especially important in the circuit because attempting to recharge an alkaline battery can cause the battery to explode! If you don’t believe me, read the warning label on every alkaline battery.

LM2940 Voltage Regulator

All linear voltage regulators have a characteristic known as the “dropout” voltage (VDO). This term refers to the minimum difference between Vin and Vout that’s required for reliable regulation. For example, the LM7805 (which we’ve used in many of our projects) has a VDO of

2.0V, which means that Vin must be greater than 7V in order to maintain a regulated 5V output.

The LM2940 is what’s called a low dropout (LDO) regulator because its VDO is relatively small; it’s usually listed as 0.55V. However, using a fixed value for VDO can be somewhat misleading because a regulator’s VDO can vary significantly with temperature and with the amount of current flow. In order to see how large the variance can be, refer to Figure 2 which presents the VDO graph that’s included in the LM2940 datasheet.

First of all, it’s obvious that current flow makes a considerable difference in the At 1A of current, VDO varies approximately between 0.45V and 0.55V, so we can use 0.6V as a generous estimate of the maximum VDO for the LM2940. One final point about the VDO graph presented in Figure 2: The lines terminate at 125°C because that’s the maximum operating temperature of the LM2940.

If you look back at the schematic presented in Figure 1, you’ll see that I listed 6.3V as the minimum voltage for the battery pack. A single-cell alkaline battery is fully depleted (“dead”) at 0.9V, so our seven-cell battery pack will be depleted at 6.3V ( 7 0.9V). However, whenever the battery pack is powering our PICAXE-