Undervoltage and Overvoltage Detection Circuits

TM 11-6130-429-40

(1) U9 is an adjustable voltage reference which

(4) It should be noted that the output regulators

can only turn off the light-coupled transistor,

is activated from the +5V monitor by resistor R76.

whereas the monitor regulator turns it on to limit the

U9 is normally set at approximately 3V by front

voltage. When the power supply is on standby in a

panel voltage adjust potentiometer R8. U3 contains

four operational amplifiers designated A, B, C, and D

redundant mode, the output from the operating

respectively, and the voltage reference generated by

power supply keeps the voltage at P1 and P2 higher

U9 is applied to the negative differential terminal of

than the set voltage on the standby supply, and ac-

U3B, U3C, and U3D. For example, if the reference is

cordingly, U3C and U3D are inactive. During switch-

larger than the voltage to which it is compared, the

over to operate, the appropriate regulator loop turns

off the light-coupled transistor to assure the desired

output of the above operational amplifiers is reduced

output voltage without a "glitch" in the output.

to near zero.

(5) Capacitors C12 and C16 provide speed-up in

(2) The voltage before the redundant diodes is

the detection network for fast and stable transient

sensed by resistive divider network R63-R65-R74.

response. The feedback resistors across U3B, U3C,

If the monitor voltage is too low, the output of U3B

and U3D set the gain of the amplifier loop, and the

will be low, thereby removing drive from the base of

transistor Q15 and turning off light-coupled transis-

RC networks in the feedback loop determine the tran-

sient (high frequency) response of the regulator.

tor U6. This in turn increases the pulse width until

f. Undervoltage and Overvoltage Detection Cir-

the desired keep-alive voltage is achieved.

cuits (fig. 1-6).

(3) The voltage on P2 is monitored by resistive

(1) The undervoltage and overvoltage detection

divider R79-R80 and applied to the positive differen-

circuits monitor the voltage before the redundant

tial terminal of U3C. If the output voltage is too low,

diode. In the event of an undervoltage condition, the

the output of U3C will be reduced to near zero, hence

removing the drive of Q15 through CR21 to widen

fault lamp and a fault indicating relay are activated.

the pulse width in a regulatory fashion. In a similar

In the event of an overvoltage condition, a light-

manner, the voltage on P1 is monitored by resistive

coupled transistor is activated which permanently

divider R78-R83 and applied to the positive terminal

turns off the pulse width modulator. The circuit may

be restored after being tripped off by momentarily

of U3D. Again, if the voltage on P1 is low, U3D will

setting the ON-OFF switch on the front panel to the

turn off Q15 through CR22, thereby broadening the

OFF position.

pulse width in a regulatory fashion.

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