3-39 PULSE LOADING
3-40 The power supply will automatically cross over from constant voltage to constant current op- eration in response to an increase (over the preset limit) in the output current, Although the preset limit may be set higher than the average output current high peak currents (as occur in pulse load- ing) may exceed the preset current limit and cause crossover to occur. If this crossover limiting is
3-38
SPECIAL OPERATING CONSIDERATIONS
TM 11-6625-2965-14&P
3-35AUTO-TRACKING OPERATION (See Figure 3-10)
3-36 The Auto-Tracking configuration is used when it is necessary that several different voltages referred to a common bus, vary in proportion to the setting of a particular instrument (the control or master). A fraction of the master’s output voltage is fed to the comparison amplifier of the slave sup- ply, thus controlling the slave’s output. The mas- ter must have the largest output voltage of any power supply in the group (must be the most posi- tive supply in the example shown on Figure 3-10).
3-37 The output voltage of the slave is a percent- age of the master’s output voltage, and is deter- mined by the voltage divider consisting of RX (or Rx and RY) and the voltage control of the slave
supply, Rp , where: EM RP
=
ES R x + R p
Turn-on and turn-off of the power supplies is con- trolled by the master. Remote sensing and pro- gramming can be used; although the strapping pat- terns for these modes show only local sensing and programming. In order to maintain the temperature coefficient and stability specifications of the pow- er supply, the external resistors should be stable, low noise, low temperature (less than 30ppm per
O C) resistors.
not desired, set the preset limit for the peak requirement and not the average.
3-41 OUTPUT CAPACITANCE
3-42 An internal capacitor, acress the output terminals of the power supply, helps to supply high- current pulses of short duration during constant voltage operation. Any capacitance added externally will improve the pulse current capability, but will decrease the safety provided by the current limiting circuit. A high-current pulse may damage load components before the average output current is large enough to cause the current limiting circuit to operate.
3-43 REVERSE VOLTAGE LOADING
3-44 A diode is connected across the output ter- minals. Under normal operating conditions, the diode is reverse biased (anode connected to negative terminal). If a reverse voltage is applied to the output terminals (positive voltage applied to negative terminal), the diode will conduct, shunting current across the output terminals and limiting the voltage to the forward voltage drop of the diode. This diode protects the series transistors and the output electrolytic capacitors.
3-45 REVERSE CURRENT LOADING
3-46 Active loads connected to the power supply may actually deliver a reverse current to the power supply during a portion of its operating cycle. An external source cannot be allowed to pump current into the supply without loss of regulation and possible damage to the output capacitor. To avoid these effects, it is necessary to preload the supply with a dummy load resistor so that the power supply delivers current through the entire operating cycle of the load device.
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