Tester cycles system-power supplies
A simple two-relay circuit cycles system power on and off to test a design on the benchtop.
Goh Ban Hok, Infineon Technologies Asia Pacific Ltd, Singapore; Edited by Martin Rowe and Fran Granville -- EDN, July 10, 2008
Power-cycle testing is important because it tests the user environment. A poorly designed system board or chip can cause the power-cycle testing to fail, however. What’s more, the power-cycle-test setup for system-board bench testing could require the use of a bulky and expensive commercial power supply. The situation gets worse when you need to simultaneously test several system boards.
This Design Idea describes a simple and inexpensive power-cycle circuit using just a few components (Figure 1). The power-supply input voltage is a dc supply from an inexpensive switching-power-supply adapter. This type of power adapter normally provides power for the system board. The circuit uses a 12V supply. You plug the power jack of the power unit into power socket J1. The output voltage of this circuit from socket J2 then connects to the system board to perform the power cycling. The 12V supply passes through resistors R5 and R6, which limit the current flowing through relay switches S1 and S2.
During start-up, the contact of relay S2 is normally closed, allowing the 12V supply coming from R6 to pass to resistors R1 and R2 and charge up capacitor C1. Resistor R8 in series with transistor Q2 increases the charging and discharging duration of capacitor C1. Transistor Q2 turns on once capacitor C1 charges toward 2V. This action impresses approximately 0.7V across the base-emitter voltage of transistor Q2, which turns on Q2. When transistor Q2 turns on, it provides a low-resistance path for the coil of S2 and thus energizes the relay, causing S2’s contact, 2B, to close.
When this scenario occurs, the 12V power supply switches its path to contact 2B and enables the optocoupler’s diode to conduct, turning on its internal transistor. The optocoupler then drives transistor Q1. When Q1 turns on, it provides a path for the coil of S1, which energizes and thus connects the 12V supply to the output voltage. The circuit connects the output voltage to the power supply of the system board, thus powering up the board. The system board remains powered up for approximately 45 sec. During the on time, capacitor C1 discharges slowly through R2, Q2, and R8. C1 turns off transistor Q2 once the voltage across the base of the transistor is below the transistor’s turn-on voltage. Then, contact 2B connects to contact 2A, and the cycle repeats.
The off time for this circuit should be approximately 17 sec. Freewheeling diodes D1 and D2 reduce the large transient voltages that occur when the currents through the relay coils change quickly.
