Help understanding electric fence charger circuit?

Thread Starter

abdalton

Joined Apr 11, 2020
3
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.

schematic.png

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.
Whole System.JPG

Let me know if I need to provide more information. Any help is greatly appreciated!
 
Last edited:

ericgibbs

Joined Jan 29, 2010
10,054
hi ab,
Welcome to AAC.
However, I don't understand why a dual timer is being used.

The first Timer is the fence energise interval, usually 0.5sec to 1sec, which triggers the second timer.
The second timer generates the actual pulse for the 3 transistor drivers for the transformer and Fence.

The varistor protects the final drive transistor from any over voltages due switching the inductive transformer load.

E
 

Thread Starter

abdalton

Joined Apr 11, 2020
3
hi ab,
Welcome to AAC.
However, I don't understand why a dual timer is being used.

The first Timer is the fence energise interval, usually 0.5sec to 1sec, which triggers the second timer.
The second timer generates the actual pulse for the 3 transistor drivers for the transformer and Fence.

The varistor protects the final drive transistor from any over voltages due switching the inductive transformer load.

E
Okay, that makes sense for the dual timer then. The transistors are still a little mysterious for me though. To me, who doesn't know a lot, it looks like all 3 BJTs are connected in common-emitter. I'm used to seeing this when BJTs are used as amplifiers. That doesn't really make sense here though. If the timer provides the pulse, why do we need any transistors at all?

Also, thanks for the welcome.
 

AlbertHall

Joined Jun 4, 2014
9,875
The varistor protects the final drive transistor from any over voltages due switching the inductive transformer load.
And you might otherwise use a simple diode to stop the back emf from the transformer but the varistor allows a higher voltage across the primary which is then 'transformed' to a much higher voltage.
 

Dodgydave

Joined Jun 22, 2012
9,168
Q1 is driven hard on by the 10 ohms resistor by Q2, you could replace all 3 transistors with a Mosfet connected to R8, ,I would improve the output with a 1uF capacitor replacing the Varistor.
 

Thread Starter

abdalton

Joined Apr 11, 2020
3
Q1 is driven hard on by the 10 ohms resistor by Q2, you could replace all 3 transistors with a Mosfet connected to R8, ,I would improve the output with a 1uF capacitor replacing the Varistor.
Yea, a lot of circuit's I've seen operating on a similar principle (like ignition coils) use a capacitor to protect the circuit from the back EMF when the switching device is in the off state. I wasn't really sure why this one used a varistor. About replacing with a Mosfet, would I just need to make sure it has a drain current rating similar to MJE13005's 4A collector current rating?
 

crutschow

Joined Mar 14, 2008
24,980
you might otherwise use a simple diode to stop the back emf from the transformer
I would think this is a flyback circuit, so the output voltage depends upon the value of the back emf.
A simple diode would likely reduce the output to a few votts.
The varistor (or often a Zener diode) is there so that the back emf doesn't exceed the transistors rating.
,I would improve the output with a 1uF capacitor replacing the Varistor.
A capacitor is used when mechanical contacts switch the primary in a flyback circuit, but is not generally used for a solid-state switch, unless its value is specifically selected to insure the maximum switch voltage is not exceeded.
A 1uF capacitor is so large that it would likely reduce the output to a low voltage.
And a capacitor is used with a mechanical contact, not to reduce the maximum voltage, but to give time for the contacts to open and suppress the arc across the contacts that would otherwise absorb the inductive energy intended for the output spark.
 

col_panek

Joined Oct 30, 2015
11
Varistors are not good replacements for zeners if they operate every pulse. They will short out in a short period of time. I saw one design that used them in a 60 Hz power supply, and they burned out in weeks.
 
Top