Fixed Dual Regulators

When building a dual power supply with both a positive and a negative regulator, extra precautions should be taken. If there is a common load between the two supplies, latchup may occur. Latch-up occurs because a three-terminal regulator does not tolerate a reverse voltage of more than one diode drop. To prevent this latch-up problem, it is good design practice to place reversed-biased diodes across each output of a dual supply. While the diodes should not be necessary if the dual regulator outputs are referenced to

ground, latch-up may occur at the instant power is turned on, especially if the input voltage to one regulator rises faster than the other. This latch-up condition usually affects the positive regulator rather than the negative regulator. These diodes prevent reverse voltage to the regulator and prevent parasitic action from taking place when the power is turned on. The diodes should have a current rating of at least half the output current. A recommended circuit for a dual 15V regulated supply is illustrated in Figure 5-38.

Figure 5-38. Regulated Dual Supply

In Figure 5-38, 1N4001 diodes are placed directly across the regulators, input to output. When a capacitor is connected to the regulator output, if the input is shorted to ground, the only path for discharging the capacitor normally is back through the regulator. This could be (and usually is) destructive to the regulator. The diodes across the regulator divert any discharge current, thus protecting the regulator.