5-33 Temperature Coefficient.
Definition: The change in output voltage per degree Centigrade change in the ambient temperature under conditions of constant input ac line voltage, output voltage setting, and load re- sistance.
5-34 The temperature coefficient of a power supply is measured by placing the power supply in an oven and varying it over any temperature span within its rating. (Most HP power supplies are rated for operation from 0°C to 55°C.) The power supply must be allowed to thermally stabilize for a sufficient period of time at each measurement temperature.
5-35 The temperature coefficient given in the specifications is the maximum temperature-dependent output voltage change which will result over any one degree Centigrade interval. The differential voltmeter or digital voltmeter used to measure the output voltage change of the supply should be placed outside the oven and should have a long term stability adequate to insure that its drift will not affect the overall measurement accuracy.
5-36 To check the temperature coefficient, proceed as follows:
a. Connect load resistance and differential voltmeter as illustrated in Figure 5-2.
b. Turn CURRENT controls fully clockwise. c. Adjust front panel VOLTAGE controls until
front panel voltmeter indicates maximum rated output voltage.
d.Place power supply in temperature-con- trolled oven (differential voltmeter remains outside oven). Set temperature to 30°C and allow 30 minutes warm-up.
e.Record differential voltmeter reading.
f.Raise temperature to 40°C and allow 30 minutes warm-up.
g.Observe differential voltmeter reading. Difference in voltage reading between Step (e) and
(g)should be less than the following:
62599,62600 12mV
6261B 22mV
6268B, 6269B 42mV
5-37 Qutput Stability.
Definition: The change in output voltage for the first eight hours following a 30- minute warm-up period. During the interval of measurement all parameters, such as load resistance, ambient temp- erature, and input line voltage are held constant.
5-38 This measurement is made by monitoring the output of the power supply on a differential voltmeter or digital voltmeter over the stated measurement interval; a strip chart recorder can be used to
TM 11-6625-2958-14&P provide a permanent record. A thermometer should be placed near the supply to verify that the ambient temperature remains constant during the period of measurement. The supply should be put in a location immune from stray air currents (open doors or windows, air conditioning vents); if possible, the supply should be placed in an oven which is held at a constant temperature. Care must be taken that the measuring instrument has a stability over the eight hour interval which is at least an order of magnitude better than the stability specification of the power supply being measured. Typically, a supply may drift Iess over the eight hour measurement interval than during the half-hour warm-up.
5-39 To check the output stability, proceed as follows :
a. Connect load resistance and differential voltmeter as illustrated in Figure 5-2.
b. Turn CURRENT controls fully clockwise. c. Adjust front panel VOLTAGE controls until
differential voltmeter indicates maximum rated output voltage.
d.Allow 30 minutes warm-up, then record differential voltmeter reading.
e, After 8 hours, differential voltmeter should change from reading recorded in Step (d) by less then the following:
6259B, 62600 6261B, 6268B 6269B
5-40CONSTANT CURRENT TESTS
5-41 The instruments, methods, and precautions for the proper measurement of constant current power supply characteristics are for the most part identical to those already described for the measurement of constant voltage power supplies. There are, however, two main differences: first, the power supply performance will be checked between short circuit and full load rather than open circuit and full load. Second, a current monitoring resistor is inserted between the output of the power supply and the load.
5-42 For all output current measurements the current sampling resistor must be treated as a four terminal device. In the manner of a meter shunt, the load current is fed to the extremes of the wire leading to the resistor while the sampling terminals are located as close as possible to the resistance portion itself (see Figure 5-7).Generally, any current sampling resistor should be of the low noise, low temperature coefficient (Iess then 30ppm/°C) type and should be used at no more than 5% of its rated power so that its temperature rise will be minimized, If difficulty is experienced in obtaining a low resistance, high current resistor suitable for current sampling, a duplicate of the sampling resistor used in this unit (A4R123 or A4R123A-A4R123B)
