Programmable Voltage Source

EE JULY/AUGUST 1988 A number of appliances, such as an EPROM programmer, require a supply voltage that can be switched to a variety of levels. The proposed circuit enables the user to do so between 5 V and 21 V. As soon as the switching transistor con- ducts, R3 is connected in parallel with R2. This lowers the total resistance be- tween the adr pin of the LM317 and earth, and consequently the output voltage. It is possible to add a number of switching transistors and associated resistors and capacitor to the circuit to increase the number of available output voltage levels. The level of the output voltage depends on the ratio between Ri and the resulting value of R2 in parallel with R3. The p.d. across 111 is always 1.2 V. Thus, Uo = [1+ R2/R3 I volts R Capacitors Ci and C3 serve to optimize the switching behaviour of the circuit. The value of these components has to be established with the aid of a square- wave generator and an oscilloscope. The effect of these capacitors on the...