Voltage Reference LED indicator

  
A keyed power input connector, series rectifier and a shunt rectifier, both 1N4007, prevent reverse voltage from being applied to the power input. A 27 volt metal oxide varistor clamps the voltage to the 78L05 that follow it, to less than 30 volts, thus protecting the 78L05. An input voltage of between +9.0 volts and about +24 Volts is needed to power the 78L05. The 78L05 in turn, provides 5.0 ± 0.25 volts for the LM4140 precision reference. This has two important benefits: The 78L05 keeps the input to the LM4140 from exceeding the maximum rated input voltage of 5.6 volts.
Voltage Reference LED indicator - schematic

It also keeps the input to the LM4140 constant, thereby removing up to 300 parts per million of output voltage per volt of input voltage of input voltage sensitivity. A 3.3 volt zener diode across the LM4140's 1.024 volt output limits the maximum voltage that can be applied to the LM4140's output by an accidental contact. This circuit would work well with other voltage reference chips as long as the input voltage for the reference chip is compatible with the output of the three terminal regulator and of course if the reference voltage was significantly lower than the output protection zener diode's zener voltage. Varistors are uncommon parts in some labs. The varistor on the input can be omitted, or a zener diode of the appropriate rating could be substituted. The LED and its associated 2.2 k resistor are only there to indicate that the circuit is on. It too can be eliminated without affecting the performance of the rest of the circuit. Construction The circuit was hand wired on a piece of phenolic board with one pad per hole. The parts were arranged so only one jumper, which was run across the component side of the board, was needed. The most difficult part of the construction was the mounting of the small 8 lead surface-mount LM4140. It turned out that I could place the package on the pre-etched pads on the phenolic board, but I had to split the pad that leads 6 and 7 attached to because they aren't...



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