| Electronics > Repair |
| Electric fence energizer repair |
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| fzabkar:
I wonder if the dual multivibrator works this way. One half of the multivibrator generates fixed width pulses which drive the MOSFET, and this then charges up the boost capacitor in discrete steps. When the boost voltage reaches its preset level, the comparator stops the multivibrator. The other half of the multivibrator is then triggered by the MCU, and this then sends a single pulse to the thyristor which energises the fence. Then the cycle repeats. The MCU senses the boost voltage via a separate potential divider on the main board. Would it make sense to disable the thyristor part of the circuit and then see if the boost voltage comes up? If so, then this could point to some fault protection issue. Another thread with a similar device: https://www.elektronik.si/phpBB2/viewtopic.php?p=566285 |
| Bubaaak:
Great find of the similar thread, although he had it way easier with the component being visually burned :D I will however try to measure/test this opto-triac to rule it out. I am for a few days out now, but my next step will be drawing a schematic of the daughter board and at least part of the boost converter to better understand its function. I will report soon :) |
| Bubaaak:
So I was able to create a crude schematic of the daughter board and part of the HV supply, but it didnt really help me with understanding the circuit so far. Only a part of this board is duplicated, and that is the comparator part with HV sensing divider. Second input of the comparator is another voltage divider of 6k8 and 10k resistors, creating almost exactly 2 volts out of the 3.37V supply. This should mean the comparator will turn off the output once there is about 620V at the capacitor. This part of circuit doesnt have any connection to the rest of the board, or atleast i wasnt able to find any. The comparators and AND gates does have some conections to the MCU, but it looks like that part is driving only the output thyristor. While measuring I found out the 10k resistor of the divider at main board was meauring about 5k, so i tried to desolder it, but apparently the double diode nearby was skewing the measurement. Then I was unable to solder it back and lost it later, so i have to find another one somewhere first :D The double diode is marked LD3, so it should be BAT54S. It measures 300mV drop / 5,5kohm in forward , 1,2V / 8,2k in reverse. This looks kinda weird to me, but all the other LD3s on the board measure similiar values, so i guess it is a feature? In same way the 10k resistors of the low voltage comparators measures 4,7k, but once again thats due to the comparators connected and since both measure the same, it is probably not a fault. Edit: I have just noticed that those comparator parts are not exactly the same either, one comparator have HV sense at pin 1, the other at pin 3. Therefore when one comparator outputs high, the second is low and vice versa, however both outputs are just connected together. |
| Bubaaak:
Took a beter look and I got that wrong during creating the schematic. The comparator parts are the same, with 2V from the 3,3V reference connected to the non-inverting input and the HV-sensing divider connected to the inverting input. Also found out the pin1 of mosfet driver is also connected to MCU via 1k resistor, so thats where PWM for driving the mosfet comes from, comparators are probably just a fail-safe? I will try to solder back the resistor and look around with a scope to see if there is any activity on the MCU Pin 41 of mcu, which should control the opto-triac is 0.1v (floating?) and gets pulled to zero as I bridge the SW contacts. Could this be sign of shorted diode inside the opto triac? |
| fzabkar:
--- Quote from: Bubaaak on August 04, 2024, 02:33:49 pm ---Pin 41 of mcu, which should control the opto-triac is 0.1v (floating?) and gets pulled to zero as I bridge the SW contacts. Could this be sign of shorted diode inside the opto triac? --- End quote --- I would think that the 270 ohm resistor would prevent the MCU pin from being pulled down to ground. In any case, if the output is being pulsed, you won't see this on a multimeter. |
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