Electronics > Repair
miller dynasty what is precharge relay
coppercone2:
I am not 100% sure but my inverter welder broke again, possibly having something to do with a beetle that got on the circuit.
On of the symptoms is a relay clicking on and off repeatedly. It is called a "precharge relay"
They don't go into detail about what it does.
Does this mean a inrush current limiter circuit that uses a relay and resistor? Is that industry slang for it?
What the hell kind of logic could result in a precharge relay deciding to oscillate, it keeps clicking every 2 seconds or so. Is the machine trying to clean a contact? I.e. I don't know how those circuits work. Does it actuate a relay and check for a voltage drop maybe? Then if there is no voltage drop it keeps trying?
If I made it, it would have a simple timer, thats it. But I guess you can make a more advanced one with fault diagnostics that make sure its acutally open. Is that common at all? Would someone make it to try to maybe like clean a relay contact by repeatedly smacking it? I.e. comparator to measure voltage drop across the limiting resistor. If it does not go away, try again, and again?
floobydust:
There's no PCB pics or repair info, but which model #?
I would guess it's an inrush limiting relay for the DC bus capacitors, so they get a pre-charge. Some soft-start circuits are dumb - a timer or voltage sense, others "smart" using MCU control.
The pre-charge resistors will cook and burn if they see mains for more than a few seconds. It might use a NTC or PTC (bank) as well.
When it switches in full mains it might reboot if there is a short? It is drawing high current from mains when the relay clicks in?
Worst case you might have a bagged IGBT preventing the bus voltage from coming up. Take it apart and a pic will tell how they did it (pre-charge).
coppercone2:
Well I bought a LEM module and a replacement relay because if I take that board its not worth saving 35$ on the LEM module to put it back in. because its a big fat hard to remove piece of garbage. I dont like the part# prefix "HY" , I am just gonna replace it. I did not bat an eye gambling on $80 of parts because I really hate that mother board because it means I need to deal with the SKIP MODULE IGBT RECTIFIER and that makes me really angry. I am not taking that board out until I get those 2 replacement parts its like climbing everest. Well I can take it out but I am not gonna put it back in until I change something even if its fine because i want to smash it into the concrete wall like a old carpet :rant:
I did all the diode tests for the IGBT they look OK.
What its doing is...
1) turn on
2a) operate
2b) operate with the inrush relay clicking. The boost board is a transistor that sinks the relay to turn it on. I measure this control pin, it just pulses every second or so.
2c) operate without turning on display
3a) sparks arc HV pilot when I press the foot pedal if display is on
3b) eventually or first time shuts down display when I press the foot pedal
4) shuts down display regardless if I did anything. I got flashed with HELP 8 before.
I put the service manual and this has pictures but does not post schematics
https://community.element14.com/technologies/experts/f/forum/38156/miller-boost-circuit-board-assembly-no-200841
The only thing connected to the machine is the foot pedal. It was doing the same thing without foot pedal. I take a good look at everything I can see and there is no problems. I don't think its related to user input because it will turn off even if I just turn it on and wait a minute or so.
Also, if it turns off, I need to wait a while before I can turn it back on, its almost guaranteed for the screen not to turn on if I turn it off and turn it back on with a wait period of less then like 3 minutes.
The Boost board in that picture is the PCB that is connected to the relay, and I assume the HY-25P module (not 100% sure)
I thought maybe it somehow uses the HY25P module to verify the inrush relay, so if the HY25P is giving erronous signal, maybe the relay go brezerk like that. While relay is going brezerk, the voltage is like ~700V with alot of ripple because it never fully charges or discharges with the relay clicking on and off. If the relay is clicking and I press the foot pedal it turns off.
It stopped working when I left it on and went to go into the house for a minute to go to the bathroom before starting a practice weld on coupons. It was set to ~35 amps in DC mode when it stop working.
I bought a UC3854, n26 optocouplers, IRF9250 transistor. I have all the other active on this PCB in my parts storage. I am just gonna do all the silicon on the booster card because its only my life force and $20. There is some forum post about this happening where it makes noise but its turned off that its the boost card. Maybe the reason why its so loud is because the boost / PFC board was degraded. I think I read some where that pFC can sometimes make inverters sound obnoxious before they break. I think I can do all the ceramic capacitors too because there was a bad ceramic cap on the main board a long time ago and I have several cubic inches of 100nF beads. I did have to replace the other card (which had arc damage) and I did replace all the parts on the motherboard and the IGBT snubber board and whatever else. I figure its natural to assume whatever happened damaged this card too and it just took a dozen feet of high current weld to make it show up. I would just replace it, but since the boost card looks pristine, and I never replaced parts on it, maybe its that. :-//
no free lunch, I guess this card did had to get damaged when the other one got totaled.
coppercone2:
What I am curious about is why the failure mode is so dynamic. Normally it fails 1 way, this one can do a whole bunch of outcomes from a power up.
It makes me think of leakage or contacts, maybe something with capacitors. Why is it being random? This is exactly what I expect if something got corrosion damage, that is, random behavior based on whatever governs the impedance of the corrosion at the time, but this board looks pristine.
I wonder if there is something up with the UC chip or the HY current chip, because those are complicated. Or if one of those CD switches is misbehaving and playing busdriver with me. All that CD stuff is high impedance FET IIRC? it reminds me of mashing buttons
I wonder if its a over heating bond wire that is playing whack-a-mole with me. I see what hot springs do, they behave randomly (contacts), sometimes retracting like a turtle head.
I want to keep smacking it until it finally breaks into a particular failure mode lol (I know you cant and its high voltage but the thought makes me amused)
Maybe I can strip the conformal coat to see (even though it looks great). I need to do it anyway for the parts replacement.
DisasterNow:
Really a cockroach? If the circuit board was drowned in lacquer like mine, this would not have happened. Have you checked the relay activation signal if it is stable? Mine attacks when it gets 12dc, this happens at about 110ac of the network.
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