This little circuit enables you to measure the so-called dynamic response of a DC power supply. A power MOSFET, T1, is used to switch the supply load on and off at a user selectable rate. The response of the supply to these fast load variations is displayed on an oscilloscope.
The switching rate is selected with the aid of a rotary switch, S1, which also serves as -the on/off switch. The available switching frequencies are: 10 Hz, 100 Hz, 1 kHz and 10 kHz. The well-known 555 timer IC is used to supply the switching signal. Diodes D3 and D4 cause the astable multivibrator to supply an output signal with a duty factor of about 0.5. The switching transistor, T1, is protected against too high currents by a fast 10-A fuse inserted in the drain line. The tester may be powered by any regulated DC supply with an output voltage between 6 V and 15 V. However, this must not be the supply under test! Given the low current consumption of the tester (40 mA max.), a 9-V battery is an excellent power source.
The tester is extremely simple to use. First, select, the load resistance of the supply you wish to test; say, 12 Ω/15 W for a 12-V, 1-A PSU. This resistor is connected between output of the tester, and the output of the PSU. The ‘0’ output of the tester goes to the ‘-‘ (or ‘0’) terminal of the PSU. Next, connect the scope input to the PSU outputs, and the trigger input to K3 Of the tester. Switch on the scope, the PSU and the tester. The scope will now display the dynamic regulation characteristic of the PSU at the given output current (1 A) and. the selected switching rate (initially 10 Hz). Construction of the tester is straightforward on the small printed circuit board shown here. The power MOSFET is bolted on to a small PCB-mount heat sink, and will not run very hot even when the maximum permissible drain current (about 10 A) is approached.