Some time ago my CEM seemed to have bad function of the triggar i.e. the fan is running but there is no feed of wire (but ok gas flow) when the trigger is pressed. Investigation shows that the tiny transformer and rectifyer on the controller pcb gives 17VDC, the wiring from the trigger is ok (the two inputs to the pcb from the trigger is shortened when the trigger is pressed) but there is no voltage to the wire feed motor and no voltage (+current) for the arcing...Does anyone have a circuit diagram or a tip regarding which component(s) that might cause this malfuncion? As both power to motor and arc is missing, some common part for both these functions is failing I guess..
- Thread starter Putte62
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There are some previous threads about CEM ( Cemont ) 140 DualMig welders, but no specific data.
The reason for both arc and wirefeed motor failing is most likely because the DC welding output ( typically 15 to 30 volts ) is also used to run the wirefeed motor, via the speed controller.
The small transformer and rectifier on the PCB provide the auxiliary or "standby" power supply, as soon as mains power is applied. The torch trigger switch then feeds this 17v DC to a switch which enables AC power to the primary of the main welding transformer. The switch may be a relay, mounted on the PCB or off-board, or an electronic AC switching component - a TRIAC. The contacts on the relay may have stuck or burned. If the PCB has a 6-pin integrated circuit ( an optoisolator ), the switching is probably done by a TRIAC.
There are some previous threads about CEM ( Cemont ) 140 DualMig welders, but no specific data.
The reason for both arc and wirefeed motor failing is most likely because the DC welding output ( typically 15 to 30 volts ) is also used to run the wirefeed motor, via the speed controller.
The small transformer and rectifier on the PCB provide the auxiliary or "standby" power supply, as soon as mains power is applied. The torch trigger switch then feeds this 17v DC to a switch which enables AC power to the primary of the main welding transformer. The switch may be a relay, mounted on the PCB or off-board, or an electronic AC switching component - a TRIAC. The contacts on the relay may have stuck or burned. If the PCB has a 6-pin integrated circuit ( an optoisolator ), the switching is probably done by a TRIAC.
Thanks for your answer, much appreceated! There is a white 6-pin component, which I assume is an optoisolator. I will check if it operates as it should (next week), and which component(s) it controlls. The attached picture shows a similar but not identical board, I assume the black and ALU heatsinks might handle two parallel tasks, i.e. motor speed and arc current on/off? I think as both failing, the input to the optoisolator, or the optoisolator itself is failing?
The shiny aluminium heatsink mounts a TO-3 power transistor, type 2N3055, which drives the wirefeed motor when enabled with pulse-width modulated signals from the logic on the PCB. The DC power for this circuit comes from the output of the main welding transformer.
The black finned heatsink mounts a TRIAC, which is ( should be ! ) triggered by the white TLP3061 zero-voltage crossing-point/optoisolator when the torch trigger is pressed. The TRIAC conducts one side of the AC mains through wires connected to the two push-on tags. You could pull those two wires off and temporarily join them with a mains-voltage-capable switch, to confirm that the welding output works and can produce an arc.
There are details about this PCB in an earlier thread:
The trigger voltage may not be reaching the LED in the optoisolator, the isolator may not be switching on, or the TRIAC may not be conducting when triggered.
The black finned heatsink mounts a TRIAC, which is ( should be ! ) triggered by the white TLP3061 zero-voltage crossing-point/optoisolator when the torch trigger is pressed. The TRIAC conducts one side of the AC mains through wires connected to the two push-on tags. You could pull those two wires off and temporarily join them with a mains-voltage-capable switch, to confirm that the welding output works and can produce an arc.
There are details about this PCB in an earlier thread:
Wire Feed Upgrade
Hi guys, I have a relatively old basic mig (Kestrel Multimig 100 - no I've never heard of it since I bought it either!!). it hasn't had much service although I did use it about 25 years ago to weld up the front of my wife's mini metro quite successfully. Recently I have started to use it again...
www.mig-welding.co.uk
The trigger voltage does not be reach the LED in the optoisolator!!
Still not found the reason for this...
The 8-pin comparator has about 7V difference over its input pins 2&3 and this just changes some mV when the trigger is pressed, I removed the comparator it and it seems to work i.e. the pin7 (COL OUT) goes low when change of input.
If 17V via 800Ohms is connected to the White 6-pin optocuppler LED the uot put reacts* .
Also the black 6-pin Optocopler seems to react* on the output if LED current is applied..
*i.e. there is a "change of resistance" on the output.
I changed the two electrolytes to new ones.
All diods seems to isolate backwards and have voltage drop forward as they should,
The 15V Zener is removed and checked OK
All transistors shows also N-P-diodvoltage drops..(not checked their real function).
The primary transformer windings shows 1.7, 1.5, 1.3, and 1.1 Ohms(DC) for current selector in pos 1,2,3 and 4, i.e. the windings and the mechanical switch seems to be OK.
So it seems that the problem is that the trigger doesn't make the correct voltage change to the comparator input (which is as mentioned about 7V delta with almost no change when trigger pressed.).
The triac conducts when gate get voltage, but as the white opto isulator doesn't give it gate signal its function doesn'n matter in this case?? (and the triac isn't involved in the motor control circutry?.)?
Perhaps some resistor is failing or the function of transistors (even if they show reasonable PN voltage drops)..?
Still not found the reason for this...
The 8-pin comparator has about 7V difference over its input pins 2&3 and this just changes some mV when the trigger is pressed, I removed the comparator it and it seems to work i.e. the pin7 (COL OUT) goes low when change of input.
If 17V via 800Ohms is connected to the White 6-pin optocuppler LED the uot put reacts* .
Also the black 6-pin Optocopler seems to react* on the output if LED current is applied..
*i.e. there is a "change of resistance" on the output.
I changed the two electrolytes to new ones.
All diods seems to isolate backwards and have voltage drop forward as they should,
The 15V Zener is removed and checked OK
All transistors shows also N-P-diodvoltage drops..(not checked their real function).
The primary transformer windings shows 1.7, 1.5, 1.3, and 1.1 Ohms(DC) for current selector in pos 1,2,3 and 4, i.e. the windings and the mechanical switch seems to be OK.
So it seems that the problem is that the trigger doesn't make the correct voltage change to the comparator input (which is as mentioned about 7V delta with almost no change when trigger pressed.).
The triac conducts when gate get voltage, but as the white opto isulator doesn't give it gate signal its function doesn'n matter in this case?? (and the triac isn't involved in the motor control circutry?.)?
Perhaps some resistor is failing or the function of transistors (even if they show reasonable PN voltage drops)..?
The LM311 Op Amp/Voltage comparator is usually used as part of a pulse generator circuit for PWM motor speed control, so I would not expect it to have any role in the torch trigger function. The trigger switch is supposed to activate the white TLP3061 optocoupler, which then enables the TRIAC and powers-up the main welding transformer.
I do not have a full diagram for the board, but from the previous thread, the trigger switch joins Pin 5 and Pin 3, and Pin 5 has 1.8 volts relative to Pin 1 ( the negative ) before pressing the trigger. I think this 1.8 volts is used by the LED in the optocoupler.
The welding voltage also runs the wirefeed motor, via the speed control circuit with the 2N3055 power transistor. As mentioned, you could bypass the TRIAC with a simple On/Off switch to confirm that the main transformer works.
[ Compared to the PCB in the previous thread that I mentioned in my post #4, your version has an extra optocoupler and another TO-220 type component in the bottom left corner. Maybe this is a second TRIAC to control the gas valve? ]
I do not have a full diagram for the board, but from the previous thread, the trigger switch joins Pin 5 and Pin 3, and Pin 5 has 1.8 volts relative to Pin 1 ( the negative ) before pressing the trigger. I think this 1.8 volts is used by the LED in the optocoupler.
The welding voltage also runs the wirefeed motor, via the speed control circuit with the 2N3055 power transistor. As mentioned, you could bypass the TRIAC with a simple On/Off switch to confirm that the main transformer works.
[ Compared to the PCB in the previous thread that I mentioned in my post #4, your version has an extra optocoupler and another TO-220 type component in the bottom left corner. Maybe this is a second TRIAC to control the gas valve? ]
With all board connectors disconnected, I get welding voltage when connecting the two wires with flat connectors together (simulate the triac operation). Separate powering if the motor proves that the motor also works.
I checked the LM311, and its output (which is missing) is only connected to the OI controlling the Triac.. The gas doesn’t seem to need any electronic components, its valve is in the handle and operates directly via the “mechanical” button (which also close/connects the trigger wire to the welding wire potential).
Thanks for the information regarding “The welding voltage also runs the wirefeed motor”, which mean that it's hard to trouble shoot the board standalone with only the 17VDC on the board rectifier..
Two (for me, among many other) strange things are that:
1.the connector pin 1(8) seems to be positive wirefeed motor feed (via pow transistor 3055), and pin 6(8) the negative motor power, but this is also connected to between the transformer high amp output and the “? Choke-coil?” is this to get a (low) voltage AC-component to make the PWM function possible? If so, how does’t the “of-board” 0-2k potentiometer (between connector-pins 2&4 (8)) relates to this voltage (note the LM311 output only connects to the OI controlling the “welding current” triac).
2.The “trigger” operates by closing/connecting the trigger wire (board-connector-pin 5(8)) to the “welding wire” potential which is connected to board-connector-pin 3(8), but in “no gas” mode the “welding wire” potential will be reversed, meaning that the trigger wire (board-connector-pin 5(8)) will be connected to another potential (difference = welding voltage) which also requires some circuitry to handle?
I checked the LM311, and its output (which is missing) is only connected to the OI controlling the Triac.. The gas doesn’t seem to need any electronic components, its valve is in the handle and operates directly via the “mechanical” button (which also close/connects the trigger wire to the welding wire potential).
Thanks for the information regarding “The welding voltage also runs the wirefeed motor”, which mean that it's hard to trouble shoot the board standalone with only the 17VDC on the board rectifier..
Two (for me, among many other) strange things are that:
1.the connector pin 1(8) seems to be positive wirefeed motor feed (via pow transistor 3055), and pin 6(8) the negative motor power, but this is also connected to between the transformer high amp output and the “? Choke-coil?” is this to get a (low) voltage AC-component to make the PWM function possible? If so, how does’t the “of-board” 0-2k potentiometer (between connector-pins 2&4 (8)) relates to this voltage (note the LM311 output only connects to the OI controlling the “welding current” triac).
2.The “trigger” operates by closing/connecting the trigger wire (board-connector-pin 5(8)) to the “welding wire” potential which is connected to board-connector-pin 3(8), but in “no gas” mode the “welding wire” potential will be reversed, meaning that the trigger wire (board-connector-pin 5(8)) will be connected to another potential (difference = welding voltage) which also requires some circuitry to handle?
It is good that you get welding power when bypassing the TRIAC with a direct link, so we know the welder works, we just have to succeed in switching it on.
The low-power logic on the board is supplied by the 17v DC, we only need the welding voltage to send power to the wirefeed motor via the 2N3055. If the TRIAC could be triggered, the welding voltage would have the opportunity to run the wirefeed, so the first step is to get the trigger circuit working. This would need at least a rough circuit diagram, drawn by tracing the PCB tracks and getting the pinouts of the integrated circuits from datasheets.
The PWM circuit needs pulses, which drive the base of the 2N3055, with the speed-control pot varying the "on-time" width of those DC pulses.
The trigger switch connections "borrow" the high-current welding power wire in the torch cable as one of the two switch wires. It is normally the positive torch feed, from the transformer and rectifier. However, when the trigger is first going to be activated, that wire will not have any welding power on it, because the main transformer has not yet been fed via the TRIAC. As mentioned in post #6, that wire ( Pin 5 ) is supposed to sit at 1.8 volts before the trigger is pressed. In "no-gas" mode, that wire would be linked to the negative side of the transformer/rectifier, but again, we are just using it as a conductor in the initial power-up circuit.
The low-power logic on the board is supplied by the 17v DC, we only need the welding voltage to send power to the wirefeed motor via the 2N3055. If the TRIAC could be triggered, the welding voltage would have the opportunity to run the wirefeed, so the first step is to get the trigger circuit working. This would need at least a rough circuit diagram, drawn by tracing the PCB tracks and getting the pinouts of the integrated circuits from datasheets.
The PWM circuit needs pulses, which drive the base of the 2N3055, with the speed-control pot varying the "on-time" width of those DC pulses.
The trigger switch connections "borrow" the high-current welding power wire in the torch cable as one of the two switch wires. It is normally the positive torch feed, from the transformer and rectifier. However, when the trigger is first going to be activated, that wire will not have any welding power on it, because the main transformer has not yet been fed via the TRIAC. As mentioned in post #6, that wire ( Pin 5 ) is supposed to sit at 1.8 volts before the trigger is pressed. In "no-gas" mode, that wire would be linked to the negative side of the transformer/rectifier, but again, we are just using it as a conductor in the initial power-up circuit.
I thaugt that I had tested all diods (verifyed that they on board had expected vooltage drop) but it seems that I missed that one was shorted (0 ohm both directions removed from board). It is Marked 1N 5355B C810 (Zener 18V?).
The trigger connects to a transistor-collector and via 2.2k to the transistor-base, and the emitter to the Optoisolator-pin1 (OI-pin2 to via 820Ohm to ground (low potential of the on board rectifier (0-17V) output)).
The 1N5355BC810 connects the transistor-base directly to the ground. With the 1N5355BC810 shorted (0=ohms), the trigger signal wont reach the Optoisolator.
I wonder why the body of 1N5355BC810 is quite big (d/l = 3.5/9mm) i.e. does it have some protection purpose? Is it supposed to withstand e.g. transients, and is the "short" a way to protect something..
IF the reason for the failing 1N5355BC810 is e.g. metal-dust (from metal grinding) i supposed that with the board (and welder) cleaned, it should just be replaced (if I can find another..)..
Regarding the picture of the board, that isn't mine, i foundit on the web.. 1N5355BC810 (if (still) present) can't be seen because it is hidden by the transistor (TIP 41C) at the lower left.. The "added" resistor (close to the mounting hole) might be some kind of solution (long lasting??) for a similar problem?.
The trigger connects to a transistor-collector and via 2.2k to the transistor-base, and the emitter to the Optoisolator-pin1 (OI-pin2 to via 820Ohm to ground (low potential of the on board rectifier (0-17V) output)).
The 1N5355BC810 connects the transistor-base directly to the ground. With the 1N5355BC810 shorted (0=ohms), the trigger signal wont reach the Optoisolator.
I wonder why the body of 1N5355BC810 is quite big (d/l = 3.5/9mm) i.e. does it have some protection purpose? Is it supposed to withstand e.g. transients, and is the "short" a way to protect something..
IF the reason for the failing 1N5355BC810 is e.g. metal-dust (from metal grinding) i supposed that with the board (and welder) cleaned, it should just be replaced (if I can find another..)..
Regarding the picture of the board, that isn't mine, i foundit on the web.. 1N5355BC810 (if (still) present) can't be seen because it is hidden by the transistor (TIP 41C) at the lower left.. The "added" resistor (close to the mounting hole) might be some kind of solution (long lasting??) for a similar problem?.
External DC (17V) powering the board on bench via the rec-bridge-output, makes both the "triac control OI" and the "motor control OI" to operate when trigger pressed with the faulty 1N5355BC810 removed, but I'm really confused of the purpose of this component.. As the two Opto-isolators galvanically isolates both the arc and the motor control circutry I can't see why its needed, so i'm a little reluctant to put the board into operation without this component.. Could it be that it is a remain from previous designs, not doing any harm in this version.. still it is confusing why it got damaged (shortened)....
It's good that you have found the shorted diode, and now the main transformer switches on and the wirefeed motor speed controller works. Have you tried welding?
As an 18 volt Zener, it may be there to stabilise or protect the 17 volt on-board DC power supply. The 1N5355B component is rated at 5 Watts, quite a high value, so that is why it looks larger than an average low-power axial-lead diode.
A Zener will act like any diode when tested with a digital meter set to the Diode range. However, in-circuit it will normally be wired 'backwards", i.e. reverse-biased. No current will flow, until the Zener voltage is reached, which is 18 volts in this case. Then the Zener will break down and conduct in the reverse direction, which will cause the applied voltage to drop. Unlike ordinary diodes ( which would be destroyed if the Peak Inverse Voltage is exceeded ), a Zener is not damaged by breaking down in the reverse direction.
To understand the exact purpose of the Zener would require a full circuit diagram of your board. This can be reverse-engineered from clear photographs of both sides of the PCB. At a guess, that component is a stabiliser or over-voltage protector for the 17v supply. It should be fairly easy to obtain, e.g.:
www.cricklewoodelectronics.com
As an 18 volt Zener, it may be there to stabilise or protect the 17 volt on-board DC power supply. The 1N5355B component is rated at 5 Watts, quite a high value, so that is why it looks larger than an average low-power axial-lead diode.
A Zener will act like any diode when tested with a digital meter set to the Diode range. However, in-circuit it will normally be wired 'backwards", i.e. reverse-biased. No current will flow, until the Zener voltage is reached, which is 18 volts in this case. Then the Zener will break down and conduct in the reverse direction, which will cause the applied voltage to drop. Unlike ordinary diodes ( which would be destroyed if the Peak Inverse Voltage is exceeded ), a Zener is not damaged by breaking down in the reverse direction.
To understand the exact purpose of the Zener would require a full circuit diagram of your board. This can be reverse-engineered from clear photographs of both sides of the PCB. At a guess, that component is a stabiliser or over-voltage protector for the 17v supply. It should be fairly easy to obtain, e.g.:
Zener Diodes, 5 Watt Wire Ended | Cricklewood Electronics
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