Negative-voltage, source.
Pulses from free-running multivibrator IC are
inverted and smoothed by Q2, Q2, and
Q3. Negative output volt
age across Q3 is regulated
by the transistor, Which increases or de creases multivibrator frequency
to charge 03 as often as necessary. With components shown, output is -10
V. Regulation is 0.05% at 0.2 mA and 5% at 10 mA, providing good bias supply.
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555 as switching regulator supplies negative voltage by S.L. Black Western Electric
Co., Columbus, Ohio
Latest addition to the 555 IC timer's seemingly endless bag of tricks is its use to generate a negative dc biasing voltage from a positive source. A current of well over 10 milliamperes can be delivered, and a form of switching regulation is employed to assure a constant output volt age. All of this is done with little more than an npn transistor and the 555 integrated circuit. The 555 is operated in the astable mode, with the pulse width and frequency controlled by resistors R1 and R2 plus capacitor C1. These parameters can be se lected for maximum regulation at the output voltage level desired. Terminal 3 of the IC is connected to a net work consisting of C2, C3, and diodes D1 and D2. Series capacitor C2 causes the pulse train to lose its ground reference, so that D1 and D2 can rectify the signal and ca- |
pacitor C3 can filter it into a negative dc output voltage. The magnitude of this output voltage depends on the amplitude and repetition rate of the pulses coming from the IC. To regulate the output voltage, the 2N2222 transistor varies the control voltage of the 555, increasing or de creasing the pulse repetition rate. Resistor R3 acts as a collector load for the transistor; the base is driven from potentiometer R4, which compares the output voltage to the supply voltage. If the output voltage becomes less negative, the control voltage goes closer to ground, causing the repetition rate of the 555 to increase so that C3 recharges more frequently. If the output voltage be comes more negative, the control voltage goes closer to the positive supply voltage, so the repetition rate de creases, and C3 is recharged less often. The output voltage can be set to any level from 0 to -10 volts
by means of Potentiometer R4. With the com ponents shown in
the figure, this circuit supplies —10 V
from a 12-v source. Regulation is less than 5%
at a cur rent of 10 mA and less than 0.05% at 0.2 mA. C
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