The author encountered a problem with a microcontroller system in which the +5-V supply voltage did not decay to 0 V sufficiently quickly after being switched off. A certain residual voltage remained, and it declined only very slowly. As a result, certain system components could not perform a clean reset if the power was quickly switched on again. To remedy this problem, a very simple circuit was used to discharge the +5-V supply. It consists of two resistors and a type Si9945 dual MOSFET from Vishay Siliconix (www.vishay.com/mosfets).
These MOSFETs switch fully on at a threshold gate voltage between +1 V and +3 V. MOSFET T2 connects discharge resistor R2 for the +5-V supply line to ground if the voltage on its gate exceeds the threshold voltage. When the +5-V supply is switched off, the first MOSFET (T1), whose gate is connected to the +5-V supply voltage, no longer connects pull-up resistor R1 to ground, so the standby voltage is applied to the gate of T2 via R1.
This requires the standby voltage to remain available for at least as long as it takes to discharge the +5-V supply, even when the system is switched off. R2 is dimensioned to avoid exceeding the 0.25-W continuous power rating of a type 1206 SMD resistor. It may be necessary to change the component value for use in other applications. The circuit can be constructed very compactly, since the dual MOSFET is housed in an SO8 SMD package, but it can also be built using ‘ordinary’ individual FETs,such as the BS170.