Page 307 - Op Amps Design, Applications, and Troubleshooting

Power-Fail Sensing 285

fall below a minimum value, called holding current. In the circuit shown in Figure
6.21, the main power source must be momentarily turned off to reset the SCR.
Capacitor Q is a transient suppressor and prevents accidental firing of the
SCR during initial turn-on of the regulator or as a result of a noise pulse.
Some power supply designs return the anode of the SCR directly to the
unregulated DC input with no limiting resistance. If an over-voltage condition
occurs and the SCR fires, the main supply is essentially shorted to ground via the
SCR. This activates the current limiting features of the main supply (often a fuse in
the primary of the supply transformer). When the SCR is connected in this way,
the circuit is called a "crowbar" because it essentially throws a short circuit (like a
steel crowbar) directly across the power supply

6.7 POWER-FAIL SENSING

An op amp can be configured as a voltage comparator circuit and used to sense an
impending power failure. This is commonly used in computer systems to protect
the computer from erratic operation caused by power loss. If an impending power
failure is detected, the computer quickly transfers all of the critical data to a per-
manent storage area that does not require power. Once power has been restored,
the computer retrieves the stored data from the permanent memory and resumes
normal operation. Figure 6.22 shows how an op amp can be used to detect an
impending power failure and send a signal to a computer in time to save the criti-
cal data before the power actually goes away.
Under normal conditions, the inverting (-) input of the voltage comparator
is more positive than the noninverting input. This is true even under conditions of
minimum unregulated voltage. If a primary power loss occurs, the unregulated
DC voltage will, of course, drop to 0; however, the filter capacitors (usually quite
large) in the power supply will prevent the unregulated DC supply from decaying
instantaneously. The regulator will continue to supply a constant voltage until the
unregulated input voltage has decayed past a certain minimum point. Thus, up to

FIGURE 6.22 An op amp voltage comparator can be used to detect an impending
power loss.