Design Ideas: April 11, 1996
Testing battery-charging circuits on an actual battery can be difficult, because battery voltage changes during the charging cycle. The constant-voltage "load box" circuit in Figure 1 provides a constant-voltage load for a battery-charger circuit independent of the amount of input current. A constant-voltage load means that the circuit must have a low impedance at all frequencies. The input impedance of this circuit is approximately 0 ohms at dc and 0.5 ohms at all reasonable input frequencies for charger applications.
The circuit uses a high-speed, single-supply op amp to drive the base of a high-gain pnp transition stage, which acts as the load. The op amp derives power through a diode, so that load transients don't interrupt power to the op amp and reference. The op-amp stage has a dc gain of four so that the voltage on the op amp's noninverting input is one-fourth of the voltage of the load box. With S1 open, the load-voltage adjust range is 10 to 20V; with S1 closed, the adjust range is approximately 3.5 to 10V. You can improve the low-voltage operation by using an LT1004-1.2V reference, reducing the 10-kohm reference-bias resistor to 2 kohm, and replacing D1 with a Schottky diode. The 510-ohm and 1.1-kohm resistors provide high-frequency stability by suppressing a 1-MHz oscillation. The 1N5400 diode and 4A fuse protect the circuit against reverse voltage.
Tests show that this circuit absorbs currents from 30 mA to 3A while the output voltage remains essentially unchanged. When simulating a battery, you can increase the voltage adjust until the charger thinks the battery is full and the current into the battery simulator reduces. Conversely, adjusting the voltage down simulates battery discharge, which may cause the charger to increase the current. (DI #1854)