A Useful Tool for Work Safety in SMPS Repair

Switched power supply, SMPS circuits can take a long time to repair, continuous measurements are made, electricity is supplied, re-checking, etc. When the input voltage 220VAC is cut off, there will be a voltage between 300VDC….320VDC on the DC high voltage line (in active PFC circuits, it can be 380V or higher) because the filter capacitor used after 220VAC DC rectification is slowly discharged.

Although precautions are taken to discharge the high voltage capacitor more quickly in the SMPS Circuit (generally parallel connected resistor), these cannot ensure that the capacitor discharges very quickly. In addition, the discharge time of the capacitor voltage can be very long according to its capacity. Since there is continuous on-off control in repair work, the risk of shock is very high. In order to accelerate the capacitor discharge, I simply connected a load resistor, I used a 100-ohm 10W resistor for years. (I just realized it was 17W 🙂 )

I am thinking of continuing with a slightly more advanced circuit right now, I saw that it worked very well in the tests, setting up a circuit according to the resistance method is tedious, costly but I think it turned out well. This circuit is also sold as a ready-made kit (Capacitor Discharge Pen) and more advanced models also have LED display voltage indicators and probes.

The circuit discharges the capacitor voltage via a 56K 2W resistor and high current LEDs, I had 10ma SMD LEDs, I used 1 parallel to the display LED and 3 to the load LED. You can use 20..30ma DIP LEDs instead of SMD.

PCB dimensions 42x27mm Printed circuit drawing was done with Sprint layout 6 PCB drawing program

Capacitor Discharge Pen Circuit Diagram

Capacitor Discharge PCB Drawing

CAUTION: The circuit works with high voltage, be careful, pay attention to the capacitor connections, if you connect the + – poles in reverse, there may be big explosions at high voltage, use a Fused Electric Line, protective glasses before operating the circuit.

source: Радиоконструктор Бутов А.Л.