TM 11-6625-2958-14&P

switch in series with the load is not adequate, since the resulting one-shot displays are difficult to observe on most oscilloscopes, and the arc energy occurring during switching action completely masks the display with a noise burst. Transistor load switching devices are expensive if reasonably rapid load current changes are to be achieved.

5-30 A mercury-wetted relay, as connected in the load switching circuit of Figure 5-5should be used for loading and unloading the supply. When this load switch is connected to a 60Hz ac input, the mercury-wetted relay will open and close 60 times per second. Adjustment of the 25K control permits adjustment of the duty cycle of the load current switching and reduction in jitter of the oscilloscope display.

5-31 The load resistances shown in Figure 5-5are the minimum resistances that must be used in order to preserve the mercury-wetted relay contacts.

Switching of larger load currents can be accomplished with mercury pool relays; with this technique fast rise times can still be obtained, but the large inertia of mercury pool relays limits the maximum repetition rate of load switching and makes the clear display of the transient recovery characteristic on oscilloscope more difficult.

5-32To check the transient recovery time, proceed as follows:

a.Connect test setup shown in Figure 5-5.

b.Turn CURRENT controls fully clockwise.

c.Turn on supply and adjust VOLTAGE con- trols until front panel ammeter indicates 5 amps output current.

d.Close line switch on repetitive load switch setup.

e.Set oscilloscope for internal sync and lock on either positive or negative load transient spike.

f.Set vertical input of oscilloscope for ac coupling so that small dc level changes in power supply output voltage will not cause display to shift.

g.Adjust the vertical centering on the scope so that the tail ends of the no load and full load waveforms are symmetrically displayed about the horizontal center line of the oscilloscope. This center line now represents the nominal output volt- age defined in the specification.

h. Adjust the horizontal positioning control so that the trace starts at a point coincident with a

major graticule division. This point is then repre-

sentative

of time zero.

i. Increase the sweep rate so that a single

transient

spike can be examined in detail.

j.

Adjust the. sync controls separately for

the positive and negative going transients so that not only the recovery waveshape but also as much as possible of the rise time of the transient is dis- played.

k. Starting from the major graticule division representative of time zero, count to the right 50µ- sec and vertically 10mV. Recovery should be within these tolerances as illustrated in Figure 5-6.

Figure 5-5.Transient Recovery Time Test Setup

Figure 5-6.Transient Recovery Time Waveforms