Universal power supplies must operate from AC line voltage, whose value can range from 90 to 264 V, 50 or 60 Hz. The rectified input voltage from the diode bridge filter capacitor charges up to 120 … 370 VDC. Voltages of this level is a serious threat to personnel conducting the layout or repair of the power supply. It is desirable to discharge the filter capacitor after the power and thus secure the work of serving the power supply. Intuitive solution – you need to use alternating current relay. However, these relays can not operate in a wide range of input voltages, they also consume significant power and are rather cumbersome, moreover, have a limited number of cycles on-off switch.
On the figure shows an alternative circuit supply J. Dimitrova (Tradeport Electronics, Canada), in which you can use almost any filter capacitor value. The scheme uses the MOS transistor Q 1 and current limiting resistor R D to discharge high voltage filter capacitor C F to within one second after power is turned off. The originality of the solution is to use a monostable multivibrator to control the MOS transistor. At a time when power is on, optocoupler IC 1 and passive components continue to generate binding symmetrical square pulses that are input multivibrator IC 2 . Each pulse triggers circuit, setting the Q output low. Multivibrator generates a pulse duration of 100 ms, negative polarity, then the Q output should be set high. However, due to the fact that the trigger pulses arrive before the pulse is formed multivibrator, the output Q will never be set high. MOSFET is always off, and the rectifier works as usual. After power off, 100 ms, the output Q is set high. MOS transistor is opened and quickly discharges the output capacitor to a safe level.
The scheme was tested on both within the range of input voltage 90 … 264 V. The capacitor filter has an acceptable value of 100 uF, the maximum discharge current of O, 06 … 0.18 A. Peak current MOSFET 8 A, therefore, the scheme can operate successfully with much larger capacities, ie capacitors can be used with large denominations. If the current – still not enough, you need to use the MOSFET with a higher peak current. Need to change the resistor RD, to find the desired discharge time T D = 3xR D xC F .
Thus guaranteed output voltage drop of 95% of its initial value, which is significantly below the safety limit for any value of the output voltage.