Technical specifications:
- Requires transformer : 24-18-0-18-24VAC more then 3- 4A.
- voltage output : adj. 0-50VDC (see table in figure 3)
- current maximum : 3A.
- current limit protection
- PCB dimensions : 4.53 x 1.82 inches.

How it works:
AC from the transformer rectified by 4 diodes that form a bridge rectifier, (D1-D4) changes AC to DC. C1 filters the DC power before sending to regulator circuit.
Parallel R value (R1,R2) will be decreased to 2 ohms to detect voltage.
If there is over 3A. voltage, voltage at the base is 0.6 volt, making TR1 and TR2 works.
There will high voltage at the emitter of TR2 and sending to pin 2 of IC1 through LED1.
At this time, the circuit is stop working.
SW1 is used for reset the circuit when current limitation mode is working for start again.

Figure 2. FK816 SCHEMATIC

Figure 3. FK816 Circuit Layout