Reverse "engineer" and understanding 12V fence energiser circuit. - Page 1
Forum section: electronics / beginners

author: Bendba
author_rank: Regular Contributor
timestamp: April 02, 2017, 01:16:35 pm
content: Hi,Here is the schematic if a 12V electric fence energiser that I have but is not working anymore (corrosion damage)I hope I drew the circuit right. I think I understand most of it but I wouldn't mind someone else's input.Basically it is an Armstrong oscillator (similar to the popular joule thief), charging C4 and C5. At the same time, C6 get charged (by voltage spikes when T1 cuts off? ) When voltage in C6 gets high enough, it triggers SCR1 (and discharges through SCR1 gate?)That discharges C4 and C5 through the output transformer, giving the pulse for the fence and flashes the neon indicator beside R3.R3 and R4 are simply a voltage divider to supply the neon bulb with the right voltage (C3 disable C4 and C5 from discharging through R3 and R4? )I guess D3 is there to protect T1's base from reverse polarity spikes.Are P1 and P2 there to set the duty cycle?Is P1 setting the space and P2 the mark? (P1 setting how fast C1 charges thus how long it takes for the base voltage to get high enough. And P2 sets the base current,thus how fast C1 discharges. Or is it doing something else?)Probably got the part wrong.C2 selects the frequency of the pulse. Is it simply lowering the frequency of the oscillator, thus taking longer to charge the C4 C5 capacitors?I don't really get the role of D2 and D4.What did I get wrong, or miss?ThanksBen

author: SeanB
author_rank: Super Contributor
timestamp: April 02, 2017, 01:40:05 pm
content: D2 is there so the off pulse is always going to be longer than the on pulse, as it increases the time the core of the transformer takes to get back to zero field, and during the time the transistor is saturating ( on time) D4 is preventing the capacitors C1,C2 from being reverse charged.If it is not working, but charging the capacitors and discharging them, then most commonly the fence transformer has shorted windings on the secondary, as this has a very high interwinding stress, and the wire will break down with time. The energisers i now use have this transformer as a complete block with the voltage clamp and feedback IR LED, making repair very quick, you just open it up, undo the board and unsolder 3 wires to replace it.

author: Bendba
author_rank: Regular Contributor
timestamp: April 02, 2017, 01:57:16 pm
content: Hi SeanB,Thanks a lot for the input. Did I get the rest of the function right?The output transformer was indeed shorted out in the secondary winding, so I replaced the energiser with a working one (this one I drew the circuit from cost me $5)I completely forgot about this one and yesterday I thought I'd fix it. Aside from the bad transformer, the pcb is all corroded (probably by the vapors of the lead acid battery over the years)I just wanted to test myself by actually understanding it instead of just fixing the physical symptoms.I thing I'll make a new pcb and rebuild it, apart for two electrolytic caps and the transformer, the main components are still good.Is there something that could obviously be improved with the circuit or should I just stick with it? (If it works, don't fix it, they say)

author: SeanB
author_rank: Super Contributor
timestamp: April 02, 2017, 05:34:33 pm
content: It works, and you have found the most common fault of the dead transformer, and will probably find the next one sooner or later of blowing up the 2 caps C4 and C5, best is to have them separated with a small air cored choke ( around 20 turns of 0.5mm wire on a former) to limit discharge current into the SCR circuit, or just use a thinner wire on the leads, which does about the same to limit dI/dt of the capacitor.Other than that there are no other real failure modes, just that the transformer for the high voltage would benefit from some interwinding tape and some extra sleeving over the last few turns to keep the voltage stress down, and of course you will need to vacuum impregnate it with transformer varnish as a final step.