Wire Feed Upgrade

[eddie49]

Thank you for the update, and I am glad that you persevered and have found the modifications to be an improvement!

Something to consider in the future:
- adding a bank of high-grade screw-terminal capacitors across the DC rectifier output ( before the inductor ), up to 50,000uF at 50v or more. For example:

NOS SIEMENS Capacitors 10000uF 100V 4 PCS Screw Mount 63x112mm NOS Quality | eBay

Find many great new & used options and get the best deals for NOS SIEMENS Capacitors 10000uF 100V 4 PCS Screw Mount 63x112mm NOS Quality at the best online prices at eBay! Free delivery for many products!

www.ebay.co.uk

- using a "light dimmer" on the transformer primary to fine-tune the power output "sweet spot" between 28 and 40 Amps:

5000W AC Motor Controller Power Controller Stable Durable for Controller | eBay

Find many great new & used options and get the best deals for 5000W AC Motor Controller Power Controller Stable Durable for Controller at the best online prices at eBay! Free delivery for many products.

www.ebay.co.uk

 

[Nige2108]

Thank you for the update, and I am glad that you persevered and have found the modifications to be an improvement!

Something to consider in the future:
- adding a bank of high-grade screw-terminal capacitors across the DC rectifier output ( before the inductor ), up to 50,000uF at 50v or more. For example:

NOS SIEMENS Capacitors 10000uF 100V 4 PCS Screw Mount 63x112mm NOS Quality | eBay

Find many great new & used options and get the best deals for NOS SIEMENS Capacitors 10000uF 100V 4 PCS Screw Mount 63x112mm NOS Quality at the best online prices at eBay! Free delivery for many products!

www.ebay.co.uk

- using a "light dimmer" on the transformer primary to fine-tune the power output "sweet spot" between 28 and 40 Amps:

5000W AC Motor Controller Power Controller Stable Durable for Controller | eBay

Find many great new & used options and get the best deals for 5000W AC Motor Controller Power Controller Stable Durable for Controller at the best online prices at eBay! Free delivery for many products.

www.ebay.co.uk

Thanks Eddie for this. Would you use both of these together or do they both do the same thing and you choose between one or the other? I was thinking about adding a cooling fan as I notice the rectifier plate and the large transformer generate quite a lot of heat with continual use. I do like the idea of being able to control the power more finely though. I feel with the 1 mm plate that the lower setting is a bit too low and the higher setting (40 amps) is a little bit too much and I have to move fairly quickly to avoid burn through. I would be interested to understand how these further mods would work and how easy or difficult they would be to wire in.

 

[eddie49]

The basic reason to add capacitors to the DC output of a MIG welder is to get a smoother, more stable arc.

After full-wave rectification, the negative-going lower part of an AC sine wave will be inverted, so the output is now uni-directional DC, but it will still be a varying curve, fluctuating from 0v to the peak, which, depending on the power step switch setting, will be from 15v to 30v multiplied by the square root of 2 ( which is 1.414 ), at a frequency of 100 Hz ( twice mains frequency ). Adding capacitors will "smooth out" these peaks by charging from the supply to near the peak value, and discharging to the load during the gaps between the half-cycles. The resulting output waveform will be much smoother, with just a slight ripple at the top.

There are several threads in the Forum about "adding capacitors to a MIG". For example:

Adding a smoothing capacitor to a Clarke mig

Would anyone know what spec capacitor/s would be needed for a Clarke 160en and what value bleed down resistor?

www.mig-welding.co.uk

They need to be high-grade electrolytic capacitors, typically with screw terminals rather than solder tags, rated at 5,000 to 10,000 microFarads ( uF ), and 50 to 100 volts. Rather than one large one, a bank of several medium-sized capacitors is normally used, totalling anywhere from 20,000 to 100,000uF. Good ones are not cheap, costing £5 to £10 each.

BHC 4700uF 250VDC ALS31A472NF250 Aluminium Electroyltic Capacitor | eBay

This is standard practice for electrolytic capacitors that have been in storage Surge-proof capacitor in aluminium can with insulation sleeve. During re-aging a maximum charging current of twice the specified leakage current or 5 mA (whichever is greater) is suggested.

www.ebay.co.uk

You could start with one or two capacitors, and then add more if you see an improvement. Capacitors in parallel are cumulative, so 5,000 added to 8,000 will total 13,000.

You would need to find space inside the welder near the rectifier, and wire the capacitors to each other in parallel and across the rectifier output before the inductor, observing polarity, with short heavy cable.

Capacitors hold their charge even after the supply has dropped, so in-between welding operations there will be a small voltage on the MIG wire tip, which will create an annoying spark if touched against the workpiece. To prevent this, a discharge resistor is either wired permanently across the capacitors, or switched in by Normally-Closed relay contacts when the weld output is off. Maybe the 24v relay that you fitted across the original motor output from the PCB has a "spare" set of Normally-Closed contacts?

Adding capacitors will raise the measured ( RMS ) output voltage by 1.414, so all the power settings of your welder will increase. The present "weak" lowest setting may become perfect for thin metal.

Regarding a fan: it is quite common that a cooling fan is needed, and today most welders do have one, with "Turbo" being added to the model name. The easiest method is to wire a 240v fan across the output terminals of the main On/Off switch.

 

[eddie49]

In this post I will describe the option of using a high-wattage AC power regulator device ( "light dimmer" ) to allow infinitely-variable control of the welding power output. This is separate and different from adding smoothing capacitors to the DC welding output. However, since capacitors do increase the average output voltage, adding them may make a variable input control more necessary.
This modification allows the flexibility of stepless power control for a traditional transformer-based machine, going right down to a very low power level for thin sheet, without the complexity and potential unreliability of a low-end inverter machine.

However, there are caveats: a simple "light dimmer" is typically designed for a resistive load, not the large inductive load of a welding transformer. At initial switch-on, and when switching off, inductive spikes may damage a cheap imported device. Due to the inductive current lagging the voltage, when each half-cycle of the AC waveform ends, the Triac in the dimmer may not switch off, so it would run continuously at full power. This modification is therefore a bit "left-field", and may not work. In the worst case, it will be the dimmer that fails.
This earlier thread has some details:

tm 150 mig mods

hello everyone, some years ago i've bought at 100€ a mig welding machine, tigmig tm150mig, the machine runs in ac but it have a gas connection, i'd like to do some car bodywork with it and i wanted to connect a rectifier and two diodes, but the minimum amerage is 50A and if i try to weld some...

www.mig-welding.co.uk

There is at least one commercial product that uses something similar - the Lidl "Parkside PFDS 120 A2", as discussed in this thread:

Welder recommendation

Hi, I'm looking for a cheap mig welder, maybe with gas, for making small scrap art figures. My limit is around 300$, and it has to within EU. I've been browsing through ebay, but it's been a bit overwhelming with so many products available. So far I've come across 3 welders that caught my eye...

www.mig-welding.co.uk

It is an £80 gasless transformer MIG that provides a fully-variable 25 to 120 Amp output using a Triac controller on the input, instead of the usual simple "MIN/MAX" and "1/2" switches. They were recently on sale again, and I bought one and plan to reverse-engineer the circuit diagram.
Additional info:

Flux core welder modification plus a big hello

So this is a little bit of two post's into one, I'm new to the forum so a big hello to every one, quite new to welding done some in the past but is been a few years. I'm a sparky by trade, was in the game for over 10 years but been in the building trade all my life, but after the loss of my...

www.mig-welding.co.uk

In this thread

Electronics type advice again please

Hope I'm not being a pain but am hoping eddie49 or mike109444 or one of the other electronics wizz's on here could answer a few questions about transformers for me. I know MIG is a CV process and that a mig transformer uses a tapped primary to adjust power settings (voltage) and I know a stick...

www.mig-welding.co.uk

Forum member @johnrev used a 10kW dimmer to help run an old arc-welding transformer as a MIG.

The dimmer that I linked to in post #21 has only 2 terminals, so it is simply wired in series with the load, which in this case should be the wire that goes from the PCB to the "1/2" switch. If you try this idea, I would suggest limiting it's use to the MIN and 1/2 ranges, not the high-power MAX setting.

 

[Seadog]

There's an interesting video that had recently been posted by The Post Apocalyptic Inventor. Modifying an MMA to infinitely variable output using electronic control rather than the traditional mechanical shunt -

 

[Nige2108]

The basic reason to add capacitors to the DC output of a MIG welder is to get a smoother, more stable arc.

After full-wave rectification, the negative-going lower part of an AC sine wave will be inverted, so the output is now uni-directional DC, but it will still be a varying curve, fluctuating from 0v to the peak, which, depending on the power step switch setting, will be from 15v to 30v multiplied by the square root of 2 ( which is 1.414 ), at a frequency of 100 Hz ( twice mains frequency ). Adding capacitors will "smooth out" these peaks by charging from the supply to near the peak value, and discharging to the load during the gaps between the half-cycles. The resulting output waveform will be much smoother, with just a slight ripple at the top.

There are several threads in the Forum about "adding capacitors to a MIG". For example:

Adding a smoothing capacitor to a Clarke mig

Would anyone know what spec capacitor/s would be needed for a Clarke 160en and what value bleed down resistor?

www.mig-welding.co.uk

They need to be high-grade electrolytic capacitors, typically with screw terminals rather than solder tags, rated at 5,000 to 10,000 microFarads ( uF ), and 50 to 100 volts. Rather than one large one, a bank of several medium-sized capacitors is normally used, totalling anywhere from 20,000 to 100,000uF. Good ones are not cheap, costing £5 to £10 each.

BHC 4700uF 250VDC ALS31A472NF250 Aluminium Electroyltic Capacitor | eBay

This is standard practice for electrolytic capacitors that have been in storage Surge-proof capacitor in aluminium can with insulation sleeve. During re-aging a maximum charging current of twice the specified leakage current or 5 mA (whichever is greater) is suggested.

www.ebay.co.uk

You could start with one or two capacitors, and then add more if you see an improvement. Capacitors in parallel are cumulative, so 5,000 added to 8,000 will total 13,000.

You would need to find space inside the welder near the rectifier, and wire the capacitors to each other in parallel and across the rectifier output before the inductor, observing polarity, with short heavy cable.

Capacitors hold their charge even after the supply has dropped, so in-between welding operations there will be a small voltage on the MIG wire tip, which will create an annoying spark if touched against the workpiece. To prevent this, a discharge resistor is either wired permanently across the capacitors, or switched in by Normally-Closed relay contacts when the weld output is off. Maybe the 24v relay that you fitted across the original motor output from the PCB has a "spare" set of Normally-Closed contacts?

Adding capacitors will raise the measured ( RMS ) output voltage by 1.414, so all the power settings of your welder will increase. The present "weak" lowest setting may become perfect for thin metal.

Regarding a fan: it is quite common that a cooling fan is needed, and today most welders do have one, with "Turbo" being added to the model name. The easiest method is to wire a 240v fan across the output terminals of the main On/Off switch.

Thanks Eddie for the additional info. While I don't pretend to understand all the technicalities I can see the potential benefits in term of improving power control by going down this route. I've attached a wiring diagram to double check I understood your instructions correctly. In terms of limiting the dimmer usage to just the Min 1/2 ranges, does this mean I won't be able to use the high power ranges at all?

 

[Robert Weaver]

a few suggestions on the wiring...

i think the capacitors need to run to the centre tapping of the secondary winding.
where you've got the dimmer, it's common to all power settings; stick it on the other side.
the discharge resistor needs to dissipate ~36 volt (max); using 150 ohm, current will by 0.24 amp and power will be just less than 10 watts.

 

[eddie49]

As Robert Weaver wrote, the positive end of the capacitor bank needs to go to the positive output of the rectifier - where the MIG torch power cable connects, and the negative goes to the junction of the welding transformer centre-tap and the start of the inductor winding. The windings may be aluminium ( rather than copper ), and the junction may be a splice.
The bleed resistor is wired across the capacitor bank. However, I do prefer the idea of having it switched in and out by a relay.Wired permanently, it wastes welding power and gets hot. It has to be a high resistance, and so discharges the caps slowly - you may still get a spark at the tip of the wire. Using a relay, the resistor is only carrying current when the actual weld power is off. It can be lower wattage, and lower resistance, so will discharge the caps quickly. E.G. :

Tradesmig 245 mystery component

Was inside my tradesmig investigating wire feed issues which I believe is the pot and have a new one coming tomorrow but while there spotted what I think is some kind of 'add on' resistor which has at some point broken one of its legs as it was just left hanging. . Any ideas as to its...

www.mig-welding.co.uk

You could use a 30 Ohm 20 Watt resistor. The relay needs to have Normally-Closed ( NC ) contacts. It can be a 240v AC coil, across the welding transformer primary, or 24v DC across the welding output ( i.e. the same place as the capacitor bank ). Alternatively, does the 24v relay that you put across the original motor output from the PCB have any spare contacts?

Regarding the Dimmer, yes, it should go in series with the "Min" transformer tapping. That is the wire from whichever pin of the MIN/MAX switch gets mains power when the switch is moved to the Min position. In the diagram, it would in fact be the left-most transformer wire ( maximum number of turns on the transformer primary = minimum voltage output on the secondary ).

This whole idea is a bit experimental, dimmers don't like inductive loads, it may blow up at first use. It may be OK at MIN and "1" or "2". With the dimmer in series with the MIN feed to the transformer primary, you would still be able to switch to MAX and have the choice of 1 or 2, with no dimming capability.

 

[Robert Weaver]

Regarding the Dimmer, yes, it should go in series with the "Min" transformer tapping. That is the wire from whichever pin of the MIN/MAX switch gets mains power when the switch is moved to the Min position. In the diagram, it would in fact be the left-most transformer wire ( maximum number of turns on the transformer primary = minimum voltage output on the secondary ).

sorry - my error. for the record, this updated image shows the position wired to the MIN tapping.

 

[Nige2108]

Thanks Robert and Eddie, appreciate the further advice. The capacitors that Eddie linked to have now expired and all the other ones I've looked at appear quite a bit more expensive. A number of them, although the rating appears similar seem to be for audio use? Are these suitable for use in this situation? Also there don't appear to be many that have a screw type terminal connection. I think I would have to mount these outside the unit at the back inside a plywood or plastic box as there is a lack of space inside if they are 100 x 63mm diameter. I'm thinking some sort of copper strip bus type connection would be preferable with heavy gauge wire to connect up with. I've had a look at the wire feed relay and there is a spare closed contact that I believe could be utilised for the bleed resistor. Would this be suitable for the resistor ? https://www.ebay.co.uk/itm/50W-Alum...744396?hash=item3da0fa030c:g:8rQAAOSwHwVejgXQ Would it be ok to use a piggy back spade connector off the wire feed motor supply for the negative connection to the capacitors or is it essential to tap in on the transformer/inductor connection? This is connected using a large metal crimp which I would probably have to cut off and find some way of making a similar connection. Thanks again guys.

 

[eddie49]

The capacitors need to be "Computer Grade" or "Low ESR" type. The term "Audio Grade" has a similar meaning ( and often is used to justify a high price ), but they do not always the required high ripple current capability, so audio capacitors often have solder tags, but we really need screw terminals.
Copper or aluminium strip would be suitable for the bus-bars; flattened copper pipe would work. Post #10 of the Tradesmig thread that I linked in my post #28 has a photo of how a capacitor bank is built. Mounting outside the welder case would be OK, as long as the connecting cable is thick ( minimum 10mm2 ).
A push-on spade connector is not big enough for the negative, you would need to join at the metal crimp. However, do not cut it. I would suggest cleaning and filing the outer surface of the crimp, then wrapping it with a copper strip tightened by a bolt, with a ring cable lug for the new wire:

Copper Tube Terminals Terminal Battery Welding Cable Lug Eyelet Crimp PACK OF 10 | eBay

We operate a dispatch policy to ensure your parcel will be processed within 24 hours.

www.ebay.co.uk

Five of these capacitors would probably be enough capacity to make a worthwhile improvement:

BHC 4700uF 250VDC ALS31A472NF250 Aluminium Electroyltic Capacitor | eBay

This is standard practice for electrolytic capacitors that have been in storage Surge-proof capacitor in aluminium can with insulation sleeve. During re-aging a maximum charging current of twice the specified leakage current or 5 mA (whichever is greater) is suggested.

www.ebay.co.uk

The finned aluminium-cased resistor you found would be perfect, at 27 or 30 Ohms.
Regarding using the wirefeed relay to open the bleed resistor circuit when the wirefeed motor path is actuated - that would be OK, as long as there is a spare pair of contacts ( i.e. moving contact and fixed contact ) that are closed when the relay is at rest.

 

[eddie49]

These capacitors have screw terminals, are 22,000uF each, and have a very adequate 80-volt rating. Two of them at £8.98 each with free postage would give you 44,000uF:

22000uF 80V Electrolytic Screw Capacitor Audio Amp Power Supply 50x80mm 105â | eBay

Capacitance: 22000uF. High quality, general purpose, aluminium electrolytic capacitor with a radial style case suitable for a wide range of electronic applications needing a cost effective high capacitance solution.

www.ebay.co.uk

I do have some 10,000uF capacitors that I could send to you to try. However, due to their 250 volt rating, they are large and heavy - 3" diam, 6" high !

 

[Nige2108]

The capacitors need to be "Computer Grade" or "Low ESR" type. The term "Audio Grade" has a similar meaning ( and often is used to justify a high price ), but they do not always the required high ripple current capability, so audio capacitors often have solder tags, but we really need screw terminals.
Copper or aluminium strip would be suitable for the bus-bars; flattened copper pipe would work. Post #10 of the Tradesmig thread that I linked in my post #28 has a photo of how a capacitor bank is built. Mounting outside the welder case would be OK, as long as the connecting cable is thick ( minimum 10mm2 ).
A push-on spade connector is not big enough for the negative, you would need to join at the metal crimp. However, do not cut it. I would suggest cleaning and filing the outer surface of the crimp, then wrapping it with a copper strip tightened by a bolt, with a ring cable lug for the new wire:

Copper Tube Terminals Terminal Battery Welding Cable Lug Eyelet Crimp PACK OF 10 | eBay

We operate a dispatch policy to ensure your parcel will be processed within 24 hours.

www.ebay.co.uk

Five of these capacitors would probably be enough capacity to make a worthwhile improvement:

BHC 4700uF 250VDC ALS31A472NF250 Aluminium Electroyltic Capacitor | eBay

This is standard practice for electrolytic capacitors that have been in storage Surge-proof capacitor in aluminium can with insulation sleeve. During re-aging a maximum charging current of twice the specified leakage current or 5 mA (whichever is greater) is suggested.

www.ebay.co.uk

The finned aluminium-cased resistor you found would be perfect, at 27 or 30 Ohms.
Regarding using the wirefeed relay to open the bleed resistor circuit when the wirefeed motor path is actuated - that would be OK, as long as there is a spare pair of contacts ( i.e. moving contact and fixed contact ) that are closed when the relay is at rest.

Hi Eddie, thanks for the further advice. There is a note in the description of the capacitors recommending they are put through a re-use cycle before use as they have been in storage:-

Note: As these capacitors have been sat in stock for a few years it is recommended that they are put through a re-age cycle prior to use as described below. This is standard practice for electrolytic capacitors that have been in storage:

Re-age (Reforming) Procedure Apply the rated voltage to the capacitor at room temperature for a period of one hour, or until the leakage current has fallen to a steady value below the specified limit. During re-aging a maximum charging current of twice the specified leakage current or 5 mA (whichever is greater) is suggested.

Not sure I fully understand this procedure. Any thoughts on this? Also these are rated quite a bit lower than the 10000uF of the original ones you identified. Are you thinking five of these are roughly equivalent to two of the 10000uF ones?

Sorry I've subsequently seen your earlier reply and answered on that so this is probably irrelevant now.

 

[Nige2108]

These capacitors have screw terminals, are 22,000uF each, and have a very adequate 80-volt rating. Two of them at £8.98 each with free postage would give you 44,000uF:

22000uF 80V Electrolytic Screw Capacitor Audio Amp Power Supply 50x80mm 105â | eBay

Capacitance: 22000uF. High quality, general purpose, aluminium electrolytic capacitor with a radial style case suitable for a wide range of electronic applications needing a cost effective high capacitance solution.

www.ebay.co.uk

I do have some 10,000uF capacitors that I could send to you to try. However, due to their 250 volt rating, they are large and heavy - 3" diam, 6" high !

That's very kind of you Eddie, thanks. These 22000uF ones seem quite good value so perhaps I should go for them. The delivery lead time is quite long but sometimes I've found goods from China arrive earlier than the estimate and in any case there's no panic. OK I'll go ahead and order all the bits and pieces. I think I have some old jump leads with thinner than normal cables but a good 3-4mm diameter so I'm thinking this might be good enough, alternatively I'll order some 10mm2 cable.

These capacitors have screw terminals, are 22,000uF each, and have a very adequate 80-volt rating. Two of them at £8.98 each with free postage would give you 44,000uF:

22000uF 80V Electrolytic Screw Capacitor Audio Amp Power Supply 50x80mm 105â | eBay

Capacitance: 22000uF. High quality, general purpose, aluminium electrolytic capacitor with a radial style case suitable for a wide range of electronic applications needing a cost effective high capacitance solution.

www.ebay.co.uk

I do have some 10,000uF capacitors that I could send to you to try. However, due to their 250 volt rating, they are large and heavy - 3" diam, 6" high !

 

[Nige2108]

The capacitors need to be "Computer Grade" or "Low ESR" type. The term "Audio Grade" has a similar meaning ( and often is used to justify a high price ), but they do not always the required high ripple current capability, so audio capacitors often have solder tags, but we really need screw terminals.
Copper or aluminium strip would be suitable for the bus-bars; flattened copper pipe would work. Post #10 of the Tradesmig thread that I linked in my post #28 has a photo of how a capacitor bank is built. Mounting outside the welder case would be OK, as long as the connecting cable is thick ( minimum 10mm2 ).
A push-on spade connector is not big enough for the negative, you would need to join at the metal crimp. However, do not cut it. I would suggest cleaning and filing the outer surface of the crimp, then wrapping it with a copper strip tightened by a bolt, with a ring cable lug for the new wire:

Copper Tube Terminals Terminal Battery Welding Cable Lug Eyelet Crimp PACK OF 10 | eBay

We operate a dispatch policy to ensure your parcel will be processed within 24 hours.

www.ebay.co.uk

Five of these capacitors would probably be enough capacity to make a worthwhile improvement:

BHC 4700uF 250VDC ALS31A472NF250 Aluminium Electroyltic Capacitor | eBay

This is standard practice for electrolytic capacitors that have been in storage Surge-proof capacitor in aluminium can with insulation sleeve. During re-aging a maximum charging current of twice the specified leakage current or 5 mA (whichever is greater) is suggested.

www.ebay.co.uk

The finned aluminium-cased resistor you found would be perfect, at 27 or 30 Ohms.
Regarding using the wirefeed relay to open the bleed resistor circuit when the wirefeed motor path is actuated - that would be OK, as long as there is a spare pair of contacts ( i.e. moving contact and fixed contact ) that are closed when the relay is at rest.

Just a quick question @eddie49 on the use of the existing relay. There is only one spare contact which is closed when the torch trigger is released. If I connect the resistor to that and its switching contact it will mean the wire feed circuit and the capacitor circuit will be connected albeit with only one pole (+ or - ) when the torch trigger is pressed. Will this be a problem? If so, its easy enough to wire in another relay.

 

[eddie49]

Regarding the relay which is being driven by the original PCB "motor" output to the 24v coil:
the recommended part would be a 2-pole 2-way ( known as double pole, double throw DPDT ) relay. The two moving poles would be wired to the two terminals of the new wirefeed motor. The two fixed contacts that are closed at rest ( normally closed, NC ) would be connected together with a wire. This acts as a brake, by shorting the motor to a dead stop and preventing run-on of the MIG wire.
The two normally-open ( NO ) contacts are wired to the "motor" terminals of the new PWM speed controller, supplying speed-controlled power from the Laptop PS to the motor.
If you have used a DPDT relay, there should not be any spare contacts. Also, linking the motor and the capacitor discharge circuitry is probably not a good idea. If you therefore decide to add another relay, it can either be 24v in parallel with the existing one, or 240v across Neutral and the switched Live output from the original PCB going to the "1/2" switch.
If the relay has more than one set of NC contacts, they can be paralleled to share the capacitor discharge current.

 

[Nige2108]

Regarding the relay which is being driven by the original PCB "motor" output to the 24v coil:
the recommended part would be a 2-pole 2-way ( known as double pole, double throw DPDT ) relay. The two moving poles would be wired to the two terminals of the new wirefeed motor. The two fixed contacts that are closed at rest ( normally closed, NC ) would be connected together with a wire. This acts as a brake, by shorting the motor to a dead stop and preventing run-on of the MIG wire.
The two normally-open ( NO ) contacts are wired to the "motor" terminals of the new PWM speed controller, supplying speed-controlled power from the Laptop PS to the motor.
If you have used a DPDT relay, there should not be any spare contacts. Also, linking the motor and the capacitor discharge circuitry is probably not a good idea. If you therefore decide to add another relay, it can either be 24v in parallel with the existing one, or 240v across Neutral and the switched Live output from the original PCB going to the "1/2" switch.
If the relay has more than one set of NC contacts, they can be paralleled to share the capacitor discharge current.

Hi Eddie, could you point me in the direction of a suitable relay for the resistor? I'm mindful of the potential current it may have to carry. I could order another similar to the Omron I used for the wire feed although spade connectors are preferable. Also with regards the cooling fan (240v) I was considering using a manual switch which I've now purchased, however I was wondering if a relay switch from the main power switch might be a better option. Any thoughts on this?

 

[eddie49]

I think selecting a 240v coil for the bleed resistor relay would be the simplest method. I looked for a New-Old-Stock relay on eBay and found this one:

RS DPDT Relay 240V AC 10mA (coil) 349-456 | eBay

RS 349-456 DPDT 240V AC 10mA (coil) relay with tab connections.

www.ebay.co.uk

It is a RadioSpares ( RS ) product, and larger than the PCB-mount types. The terminals may accept push-on spade tags, but they might be a bit thin for that. There are two sets of contacts, probably rated at 10A each, which can be paralleled-up to share the current, wiring one end of the resistor across the pins for the two moving contacts and the positive of the capacitors across the two fixed NC contacts.

A 240v cooling fan probably draws 25 Watts or less; I would just wire it in parallel with the rest of the machine, at the output of the main on/off switch, so it runs all the time the machine is on - rather than only when the trigger is pressed.
However, on looking at the circuit diagram and the picture of the front panel, I see that there isn't really an on/off switch as such! Instead, the Min/Max switch has a central "Off" position. The fan could be wired with one end to the moving pole of that switch ( left-hand in the diagram ) - marked 7 at the switch and going to pin 7 on the PCB. The other fan wire would go to the junction of the two fixed contacts for the right-hand ( in the diagram ) section of the switch - the point where the two indicator bulbs and the thermostat meet, and the thick line goes to the PCB.

 

[Nige2108]

Just a quick update. I have now added the dimmer control to the Min transformer tapping. I have only tried it to any extent on the 1/2 setting. The 1/1 setting (marked as 28 amps on the machine) appears not quite powerful enough for welding even 1mm thick steel. On the 1/2 setting the dimmer control has to be turned right up to probably 2/3 way around the dial before the wire feed is triggered and then the usable range is really in the last quarter of the range. It does seem to be having some effect but only within a limited range. I have only welded a couple of bits of steel so I will need to continue experimenting and adjusting with the wire speed. It will be interesting to see the effect when the capacitors are added to it which I believe will increase the power that little bit more. Unfortunately due to space restrictions I will have to mount the dimmer on the side of the case towards the back rather than the front panel. It is quite a bit larger than I was anticipating. I've also had to mount the fan on the outside of the case which is not ideal but again due to lack of space.

 

[Nige2108]

Finally I have installed the capacitors and thought I would give @eddie49 and @Robert Weaver an update on the results. Here are a few pictures of the final installation.

Following installation of the caps and the dimmer I noticed a significant increase in power on the lower settings particularly. I then decided to trial fit the case cover to check make sure it all worked inside the case. Follow this when I pressed the torch button the wire feed triggered and there was a buzzing sound which didn't sound right so I took it apart again thinking the dimmer might be earthing out somewhere. It wasn't and I tried it again with the dimmer separated from the case and the wire feed failed to trigger and the 1-1 / 1-2 setting appeared to be dead. I checked for continuity across the two dimmer contacts and there was nothing. I reconnected the transformer to the switch by-passing the dimmer (as per standard setup) and everything worked as normal so I'm thinking the dimmer has given up the ghost which is a pity because as you suggested Eddie it just helped to to take down the power at the bottom end to work more comfortably on thinner metal.

Now I am making some test welds with the new capacitor setup (without the dimmer) and immediately the additional power is evident. I am only using the 1-1 setting on the 1mm sheet and about 3.5 / 4 on the wire speed and as you can see from the above on a lap weld the penetration (on the right hand side) is quite heavy and the edge on the lapped piece is quite easily burned away if you don't move fairly quickly. The arc is also a little more spity than fried bacon I would say, so it just feels as though it needs tweaking down a little. Subsequently I tried it on thicker material making a tap spanner welding a 2.5mm walled tube to the side of a spanner (4/5mm thick) and that seemed to work quite well, no doubt the additional power and smoother delivery all helped in that regard.

Overall I'm pleased with the improvements and together with all the additional practice I'm getting I feel my welds are getting a bit more consistent. I really appreciate all the advice and help you guys have provided. If you have any alternative suggestions as to how I might be able to fine tune the power I would be interested. I saw this voltage regulator on ebay which I wondered if it might be worth a try or would this suffer the same fate as the dimmer? https://www.ebay.co.uk/itm/4000W-SCR-Voltage-Regulator-Motor-Speed-Controllor-Thermostat-Dimmer-AC-220V-UK/402619083488?_trkparms=aid=111001&algo=REC.SEED&ao=1&asc=20160811114145&meid=cf55e1294d82427aa510764d9704bec0&pid=100667&rk=1&rkt=8&mehot=none&sd=402619083488&itm=402619083488&pmt=1&noa=1&pg=2334524&brand=Unbranded&_trksid=p2334524.c100667.m2042