Power control unit for testing



Element functions:

  • J1 โ€” AC grid input
  • J2 โ€” oscilloscope connection, current monitoring
  • J3 โ€” output for power supply under test
  • SW1 โ€” circuit sensitivity switch for measuring Inrush current
  • SW2 โ€” zero-cross switching on and off
  • SW3 โ€” instant switching on and off for inrush current measurements
  • T1 โ€” current transformer, 10A, 1:1000
  • U1 โ€” Solid State Relay, zero-cross, controlling voltage 90โ€“250 VAC

AC-synchronized switching on and off

If the device requires zero-cross on-off switching, the operator should use SW2, while SW3 should be open. In this case, the SW1 should be switched to the lower position according to the diagram. Then, 10A of the grid current will correspond to the connector J2 voltage equal to 1V.
In synchronous mode, the on state of SW2 corresponds to a continuous mode; the value of the source current can be easily measured by briefly connecting the ammeter to the output terminals of SSR U1 without changing the position of the switches. With this measurement method, the ammeter does not risk being overloaded by the shock current of turning on the device.
Asynchronous switching on and off
When measuring INRUSH CURRENT, the operator should reduce the sensitivity of the current measurement circuit (upper position for SW1 on the diagram), open SW2, and use SW3 to turn on the device. In this case, 10A of grid current will correspond to a voltage at connector J2 equal to 0.1V.
Since powering on will be accidental relative to the source phase, the measurement procedure should be repeated several times (at least ten). Only then can the maximum and average values for the INRUSH CURRENT be reliably determined.


The described power control unit was assembled on a breadboard with a 0.1-inch pitch; the look of the unit is shown in the photos below:

On view 1 of the power control unit in the foreground, one can find the terminals for connecting the source and the device under test, switch SW2, and current transformer T1:

Power control unit view 2:

Power control unit view 3 shows the current sensitivity switch SW1 and connector J2:

Usage example

For example, if one uses the device to test a power supply, then with an oscilloscope, one can determine the following characteristics of the unit:

    Peak input current at full load
    Power factor of AC mains draw (usually listed if there is a PFC in the supply) https://en.wikipedia.org/wiki/Power_factor
    Time to set up from the moment of applying the input voltage until the output voltage reaches 90% of the rated level at 100% load.
  • โ€RISE TIME
    Time for the output voltage to rise from 10% to 90% of the nominal level.
    Time to keep operating at 100% load from the moment the input voltage is turned off until the output voltage drops to 90% of the rated level.
  • โ€FALL TIME
    The output voltage decay time is 90% to 10% of the nominal level.


Best regards.