The overload indicator consists of a window comparator that measures the magnitude of an a.f. signal. Two of the opamps contained in an TL072 are supplied with a reference voltage by potential divider R1-R2-R3-P1. The outputs of the opamps drive T1 via diodes D1 end D2 (that function as half-wave rectifier), which in turn actuates D3. Network R5-R6-C2 ensures that the LED lights even during short signal peaks. Capacitor C2 is charged fairly rapidly via D1 (or D2)and R5, after which it discharges slowly via R6, R9 and the base-emitter junction of T1. Capacitor C1 also contributes to the longer lighting of the LED.
.
When the level of the signal at the input is high enough, IC1a, is toggled by the positive half periods of the signal and IC1b by the negative halves. In this way, a peak above the maximum level will be indicated even when the signal is asymmetrical. Because of the symmetrical power supply and design of the indicator, the reference voltage for both opamps can be set with one potentiometer. The circuit draws a current of 5-6 mA when the LED is off. When an overload peak is indicated, the LED draws an additional 20 mA. With values as shown, the reference voltage can be set roughly between 0.9 V and 5.5 V.
The circuit can be connected to the output of a power amplifier, but potential divider R7-R8 then needs to be adapted and protected by diodes to the supply lines.
Comments
Post a Comment