Issue 98, September 1998
Smart Rockets - Data Acquisition in Model Rocketry

POWER

The power system, consisting of a battery and a voltage regulator, was one of the more difficult portions of the design because it operates under several constraints. First, the battery must source 30 mA at 3 V.

Second, the regulated 5-V rail is critical because it is the reference for the PIC’s ADC. Any change in this voltage translates into a proportional error in the recorded acceleration.

Third, the power system must be compact and lightweight because 50 g is the rocket’s maximum payload. Finally, it must be reasonably efficient because small batteries have limited energy.

We started by looking at high-energy batteries. The two obvious choices were alkaline cells and lithium primary cells. Lithium cells have, on average, three times more energy per unit mass than alkaline cells.

We settled on a CR2-style lithium battery designed for high-current applications. It provides 750 mAh at 3 V and weighs 11 g.

We selected Maxim’s MAX631A step-up switching regulator because of its low part count (one inductor and two capacitors) and small quiescent current. The selection of the inductor is critical because its resistance must be low (0.5 ? or less) and it must not saturate when the current through it peaks. The power supply is shown in Figure 2.