LEDs Deliver Voltage Indication And Adjustable Regulation

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Fig 1. This voltage regulator circuit (a) uses green LEDs as both the stabilization elements and as the voltage indicators at the front-panel switch (b).

Fig 2. The addition of resistor R4 to the original circuit ensures that all LEDs can light simultaneously and still maintain adequate brightness for the front panel.

Fig 3. The original circuit’s control resolution can be enhanced by the inclusion of additional resistors between the LEDs (a). The values chosen for these resistors will determine the actual values of the regulated voltages (c).

The circuit in Figure 1 employs LEDs in a series-connected chain as the stabilization elements in a voltage regulator. It makes use of the fact that a green LED’s current-voltage characteristic provides a drop of about 2 V. As a result, the user can pick from the voltages shown by changing the position of switch S1 to select which of the diodes D2-D5 are used. Transistor VT2 provides the regulator short-circuit protection.

One disadvantage with this circuit is that increasing the number of LEDs in operation decreases the devices’ light output. You can avoid this problem with the addition of R4, which allows all the diodes to glow simultaneously so they can provide bias lighting of the switch’s front-panel scale (Fig. 2).

Another improvement allows greater resolution of the regulator’s output voltage. Figure 3 shows the approximate voltage levels at the switch scale and actual values, which are determined by the magnitude of the load resistance as well as the individual properties of the device elements in the figure.