I've been attempting to reverse engineer a circuit board from an old electric fence charger designed for cattle. It's powered by 6 VDC and then outputs to a transformer. As the pictures (shown below) indicate, it has a timer (NE556N) that I believe is providing the pulse for the transformer. However, I don't understand why a dual timer is being used. My biggest question is what exactly are the 3 transistors doing, and also what does the varistor accomplish?
The schematic I've drawn could be wrong, but I've checked it many times. I'm about 80% sure this matches the circuit on the board. The transformer's two leads to the primary coil connect to pins 1 & 5 of the screw terminal. DC power comes to the circuit from the pin 2 connection on the screw terminal. DC power from the battery comes in to pin 4 and there is an unseen switch between pin 2 and 3. The two cap_conn pads represent a .01u ceramic capacitor. There is also a pad (H1) that represents an LED. This LED would blink every time the fence charger pulses.
Here is an image of the board when I removed it from the case. Note: schematic above features a screw terminal where this old board does not.
Let me know if I need to provide more information. Any help is greatly appreciated!
The schematic I've drawn could be wrong, but I've checked it many times. I'm about 80% sure this matches the circuit on the board. The transformer's two leads to the primary coil connect to pins 1 & 5 of the screw terminal. DC power comes to the circuit from the pin 2 connection on the screw terminal. DC power from the battery comes in to pin 4 and there is an unseen switch between pin 2 and 3. The two cap_conn pads represent a .01u ceramic capacitor. There is also a pad (H1) that represents an LED. This LED would blink every time the fence charger pulses.
Here is an image of the board when I removed it from the case. Note: schematic above features a screw terminal where this old board does not.
Let me know if I need to provide more information. Any help is greatly appreciated!
Last edited: