Glowing led light without power have 2 reasons : a wire loop catches a field OR capacitance between nearby conductors causes power transfer between these nearby conductors (that are not connected). Electroboom made a video : why cheap led lights keep glowing.
Years ago this exact same thing hurt my head. After i fixed and everything working normally, i scoped everything and this is what Ive found (plus assumption) Output of both side (actual signal) is actually 50hz that is line freq here. So the input (what the other side outputs) is driven to N or L (it dont care) for the period whenever it wants to send signal. And leave it alone when it doesnt want to. And it can handle L or N shorting to signal, but there will be no communication. And if shorted long enough those resistor burns. Why can LED lights up? Stray induction and capacitance is enough for that, only one side pull N or L is all thats needed. Those capacitor and diode is very important, weak stray captured are stored in 470pf to make it useful for half cycle of 50hz. Mine is IFB 1.5ton
A puzzle wrapped in a conundrum! As I think others have suggested I think some kind of capacitive coupling is in play here. It looks like the signal input is intended to receive a high voltage high impedence drive of some kind. Think of the signal you can see on a 'scope probe if you leave the ground connection floating and touch the tip of the probe. I would be inclined to use a battery powered scope to monitor the voltage at the offending opto-isolator led cathode (the mystery connection) when you apply your LED tester voltage. Might also be interesting to see the behaviour if you just touch the signal input with your finger. However it works it is clearly a strange design. Look forward to the solution to the puzzle.
A proper comment now. 😅That is a great mix of working through the comments for alternative views and being able to compare the effective same circuit board in a different design. Very useful. Plus a legendry Rich drawing. Could the one lead from your led tester be acting as an aerial to either dissipate energy or receive it as an aerial, hence affecting the optic isolator ?
Hi Richard, The signal is going to at supply voltage (240v) or neutral, I would put a resistor of say 1m ohm between live and signal this will give you an AC voltage on the signal line with low current, this may provide enough voltage to test the working of the opto isolator.
Touching that positive lead to the circuit would raise capacitance in that part of the circuit. Try just holding a bare wire and touching that same place and see if that triggers the LED.
From what I can see the LED is not actually lighting as there is no current passing through it but then it must be because this is an optoisolator so there should be no electrical connection to the other side of the circuit.
That's a great idea to use a bare wire. Also with either the tester or the wire, see how long it is still registering on the multimeter. Any residual energy or capacitor charge won't last forever.
I don't know about the voodoo, but this seems like a two-way digital signal. If so, the other end should go to the micro. The diode in the opto most likely has a low Vf at low current, i.e multimeter supply.
The led tester probably has a smps - so generates rf noise and this is then coupled capacitively back to mains neutral through the environment in the workshop. Looks like your “signal” line is high voltage - probably just 240VAC
Yes but this is an optoisolator so there is no electrical connection between the high voltage and low voltage side of the circuits so it should not behave this way. Also the PCB is on my bench and has no connection to live neutral or anything else
@ It doesn’t need a connection if the circuit is high enough impedance. It will radiate the return signal. But I might be talking nonsense? Try connecting the led tester to a small led and connect the other side of the led to a crock lead just lying on the bench. See if it lights dimly? I hope I’m not leading you up a snicket :)
There might be a capacitive coupling. LEDs in optocouplers just need micro-amps to see the phototransistor changing resistance. Some people even see their 220V LEDs at home glowing, even after turning off the switch.
Interesting autopsy.. Maybe your LED tester has some leakage from mains which is big enough to light the LED in the OC (but very strange, I believe your PCB is not connected to anything)?
Hi Richard, have you measured the floating-end of the photocoupler to that GDT? It might be possible that they use the GDT or something like DIAC as a high-voltage zener-diode as part of its driving circuitry.
I think capacitors are doing some kind of oscillation and turnning the the test probe into some kind of antenna probably, if that makes sense!? because this is weird and has to be explained !
It certainly is weird. Also as this is an optoisolator there should be no electrical connection between the high voltage (LED) side and the low voltage opto-transistor side of the circuit. That is the whole point of the isolation
@@LearnElectronicsRepair I'm so sorry Sir I thought the probes are the multimeter's, maybe as others said it has to do with some ground loops, but if only when you connect the positive line of the LED tester the reading apears at the meter then this has to mean that somehow the tester has a comon ground with the board which makes it even more weird because no connection is seen here between the board and the tester?!!please let us know when you figure it out. Regards Mr. Richard and thank you for the useful content on this channel it is always great to wacth your channel👍
Could the 2x back to back opto's be getting a signal from the outside unit that is something akin to RS485 or vehicle CANBUS, with the point where the opto couplers are connected together creating a centre tap, effective zero? The large capacitor could block the mains leaving the signal ''floating'' say +ve to this unit and -ve to the outside unit or vice versa. Granny teaching egg sucking possibly, but CANBUS has centre point with something like + and - 2.5 volts on 2x wires giving duplex comms. I have seen the same type of arrangement years ago on a printer where the power lead was used for the comms. I think similar was also used on grid across pylons, but with a bit more isolation I suspect.
What voltage is the 3 phase output? You could connect 3 incandescent bulbs in Y config to substitute a motor load .. if too dim, you could connect them in delta config.
The outdoor pcb would send an error code when the fan capacitor is broken. Sometimes the fan cap is om the pcb. I am talking about the outdoor fan. I am not sure if it is a DC or AC fan. A fan failure would cause the airco to switch off after a short time.
When I fixed mine I discovered that the signal cable between the units ran at 300VDC. I thought this was just ridiculous until I realised you could accidentally connect the signal to Live and nothing would be damaged.
@LearnElectronicsRepair the chassis ground on the internal unit, which is bonded to earth, and the ground wire in the 4 core linking inside to outside units (L N E signal).
no LED can light up with only one connection current needs to flow, so if you don't provide a return path it will try and find it's own. I expect that you LED backlight tester came from China, and hence has very bad leakage from the output voltage back through the mains. so you LED tester output is floating above mains earth by some amount. so with one lead connected the leakage current is trying to find a return path. i expect it's flowing through the led in the isolator to the 0V or ground and this ground is finding a connection to earth. if the leakage is AC then it will only need a capacitive return path and a few mA's could be enough to light the diode up. replace the led tester with a battery and a resistor so that it's completely isolated from everything and this issue will go away. this is why you need to have better equipment with much less or no psu leakage current. for a test measure one output with reference to earth from the LED tester and see what the current. first check that the 0V output is not tied to earth.
Glowing led light without power have 2 reasons : a wire loop catches a field OR capacitance between nearby conductors causes power transfer between these nearby conductors (that are not connected). Electroboom made a video : why cheap led lights keep glowing.
Years ago this exact same thing hurt my head. After i fixed and everything working normally, i scoped everything and this is what Ive found (plus assumption) Output of both side (actual signal) is actually 50hz that is line freq here. So the input (what the other side outputs) is driven to N or L (it dont care) for the period whenever it wants to send signal. And leave it alone when it doesnt want to. And it can handle L or N shorting to signal, but there will be no communication. And if shorted long enough those resistor burns. Why can LED lights up? Stray induction and capacitance is enough for that, only one side pull N or L is all thats needed. Those capacitor and diode is very important, weak stray captured are stored in 470pf to make it useful for half cycle of 50hz. Mine is IFB 1.5ton
I fixed my minisplit that had a shorted shotkey diode for £0.70 thanks to your troubleshooting tips.
*schottky
A puzzle wrapped in a conundrum! As I think others have suggested I think some kind of capacitive coupling is in play here. It looks like the signal input is intended to receive a high voltage high impedence drive of some kind. Think of the signal you can see on a 'scope probe if you leave the ground connection floating and touch the tip of the probe. I would be inclined to use a battery powered scope to monitor the voltage at the offending opto-isolator led cathode (the mystery connection) when you apply your LED tester voltage. Might also be interesting to see the behaviour if you just touch the signal input with your finger. However it works it is clearly a strange design. Look forward to the solution to the puzzle.
The marking on the resistor is probably 18B which translates to... 1.5k
Yes. It uses the EIA marking code. That's caught me out before. The B looks very much like an 8.
Great lesson for us. I like the details and explanations. Thank you.
Stray capacitance just like a neon tester will work with one lead touched the other wire in your hand.
A proper comment now. 😅That is a great mix of working through the comments for alternative views and being able to compare the effective same circuit board in a different design. Very useful. Plus a legendry Rich drawing.
Could the one lead from your led tester be acting as an aerial to either dissipate energy or receive it as an aerial, hence affecting the optic isolator ?
Hi Richard, The signal is going to at supply voltage (240v) or neutral, I would put a resistor of say 1m ohm between live and signal this will give you an AC voltage on the signal line with low current, this may provide enough voltage to test the working of the opto isolator.
Great stuff
Nice,very helpful video for HVAC
Touching that positive lead to the circuit would raise capacitance in that part of the circuit. Try just holding a bare wire and touching that same place and see if that triggers the LED.
From what I can see the LED is not actually lighting as there is no current passing through it but then it must be because this is an optoisolator so there should be no electrical connection to the other side of the circuit.
That's a great idea to use a bare wire.
Also with either the tester or the wire, see how long it is still registering on the multimeter. Any residual energy or capacitor charge won't last forever.
The 3 phase drive is a 3 phase H bridge.
I don't know about the voodoo, but this seems like a two-way digital signal. If so, the other end should go to the micro.
The diode in the opto most likely has a low Vf at low current, i.e multimeter supply.
The led tester probably has a smps - so generates rf noise and this is then coupled capacitively back to mains neutral through the environment in the workshop. Looks like your “signal” line is high voltage - probably just 240VAC
Yes but this is an optoisolator so there is no electrical connection between the high voltage and low voltage side of the circuits so it should not behave this way. Also the PCB is on my bench and has no connection to live neutral or anything else
@ It doesn’t need a connection if the circuit is high enough impedance. It will radiate the return signal. But I might be talking nonsense? Try connecting the led tester to a small led and connect the other side of the led to a crock lead just lying on the bench. See if it lights dimly? I hope I’m not leading you up a snicket :)
There might be a capacitive coupling. LEDs in optocouplers just need micro-amps to see the phototransistor changing resistance. Some people even see their 220V LEDs at home glowing, even after turning off the switch.
Interesting autopsy.. Maybe your LED tester has some leakage from mains which is big enough to light the LED in the OC (but very strange, I believe your PCB is not connected to anything)?
I'm discovering more and more situation where a broken led takes down the entire circuit. Use freeze spray to check questionable led's.
Hi Richard, have you measured the floating-end of the photocoupler to that GDT? It might be possible that they use the GDT or something like DIAC as a high-voltage zener-diode as part of its driving circuitry.
I think capacitors are doing some kind of oscillation and turnning the the test probe into some kind of antenna probably, if that makes sense!? because this is weird and has to be explained !
It certainly is weird. Also as this is an optoisolator there should be no electrical connection between the high voltage (LED) side and the low voltage opto-transistor side of the circuit. That is the whole point of the isolation
@@LearnElectronicsRepair I'm so sorry Sir I thought the probes are the multimeter's, maybe as others said it has to do with some ground loops, but if only when you connect the positive line of the LED tester the reading apears at the meter then this has to mean that somehow the tester has a comon ground with the board which makes it even more weird because no connection is seen here between the board and the tester?!!please let us know when you figure it out.
Regards Mr. Richard and thank you for the useful content on this channel it is always great to wacth your channel👍
Could the 2x back to back opto's be getting a signal from the outside unit that is something akin to RS485 or vehicle CANBUS, with the point where the opto couplers are connected together creating a centre tap, effective zero? The large capacitor could block the mains leaving the signal ''floating'' say +ve to this unit and -ve to the outside unit or vice versa.
Granny teaching egg sucking possibly, but CANBUS has centre point with something like + and - 2.5 volts on 2x wires giving duplex comms. I have seen the same type of arrangement years ago on a printer where the power lead was used for the comms. I think similar was also used on grid across pylons, but with a bit more isolation I suspect.
What voltage is the 3 phase output? You could connect 3 incandescent bulbs in Y config to substitute a motor load .. if too dim, you could connect them in delta config.
Did you check the (AC) capacitor of fan?
I don't have the fan, an aircone engineer went out and said he tested everything and therefore diagnosed a pcb fault. of course he could be wrong
The outdoor pcb would send an error code when the fan capacitor is broken. Sometimes the fan cap is om the pcb. I am talking about the outdoor fan. I am not sure if it is a DC or AC fan. A fan failure would cause the airco to switch off after a short time.
When I fixed mine I discovered that the signal cable between the units ran at 300VDC. I thought this was just ridiculous until I realised you could accidentally connect the signal to Live and nothing would be damaged.
300VDC with reference to what?
@LearnElectronicsRepair the chassis ground on the internal unit, which is bonded to earth, and the ground wire in the 4 core linking inside to outside units (L N E signal).
no LED can light up with only one connection current needs to flow, so if you don't provide a return path it will try and find it's own. I expect that you LED backlight tester came from China, and hence has very bad leakage from the output voltage back through the mains. so you LED tester output is floating above mains earth by some amount. so with one lead connected the leakage current is trying to find a return path. i expect it's flowing through the led in the isolator to the 0V or ground and this ground is finding a connection to earth. if the leakage is AC then it will only need a capacitive return path and a few mA's could be enough to light the diode up.
replace the led tester with a battery and a resistor so that it's completely isolated from everything and this issue will go away. this is why you need to have better equipment with much less or no psu leakage current. for a test measure one output with reference to earth from the LED tester and see what the current. first check that the 0V output is not tied to earth.
ua-cam.com/video/7op_71YCnfM/v-deo.html
1st comment !!
I haven't finished watching it yet and it's only 4 minutes old 😅