Figure 55. Remote Programming Connections
The wire size of the programming leads must be adequate to withstand any programming surges (consider effects of any large storage capacitors which have to be charged or discharged through the programming leads). The temperature coefficient of a very long programming leads may degrade power supply temperature coefficient and drift specifications. This is particularly true if the power supply is exceptionally well regulated, or the programming leads are subjected to considerable ambient temperature changes, or when programming is done with low resistance values.
Protecting Against Momentary Programming Errors
Using remote programming, several different values of fixed output voltage are obtainable with resistors and a switch, so that the output voltage of the supply can be switched to any pre-established value with a high degree of reproducibility. Figure 56 illustrates several switching techniques that can be used in conjunction with resistance programming.
Suppose it is desired to program a supply having a programming coefficient KP of 200/ohms volt to any of three values--5 volts, 10 volts, and 15 volts; the circuit of Figure 56A is a typical configuration. However, if a break-before-make switch is used in the configuration of Figure 56A, there will occur for a short interval during the switching action, a very high resistance between the two programming terminals, and the power supply during that interval will raise its output voltage in response to this high resistance input.
To eliminate this output overshoot corresponding to an infinite programming resistance, a make-before-break switch should be employed. However, this solution has the disadvantage that during the short interval when the swinger of the switch is contacting two switch terminals, two programming resistors will momentarily be paralleled across the power supply programming terminals, and the supply will for this short interval seek an output voltage which is lower than either the initial or the final value being programmed. This output undershoot increases the time required for the supply to settle to its new value.
The switching circuit of Figure 56B, using a make-before-break switch, eliminates both the overshoot and the undershoot problems associated with Figure 56B. When the switch is rotated clockwise the resistance value between the two programming terminals will go directly from 1000 to 2000 ohms, and then from 2000 to 3000
