SIP 130 wire-feed power supply - alternative approach

After reading all the threads about upgrading the SIP wire-feed power supply, I finally got round to modifying mine. I took a slightly different approach from the previous threads.

I didn't have a 24V transformer to hand that was meaty enough to drive the motor (we're talking about an amp, I think, but certainly less than two amps) but I did have an old laptop switching supply, 16VDC at 3.6A, which I thought might just be enough, voltage-wise (more than enough current capability).

Now, as pointed out by AdrianH, a switching PSU is likely to suffer if its mains supply is switched on and off repeatedly. I have seen some industrial switching power supplies disappear in a cloud of smoke under these conditions. Also, its output is likely to ramp down slowly when the mains is disconnected, meaning that the motor will run on unacceptably and give more and more "stick-out" after each run. The SIP is bad enough for this as standard without aggravating the problem.

My solution, given the available parts, was to keep the laptop PSU permanently connected to the mains supply (as long as the welder is switched on) and to switch the DC output instead. This also gave the opportunity to make another improvement. More on that later.

Having looked at various schematics here and elsewhere, I realise that the wiring of the modern SIPs (with the troublesome wire-feed) varies from one model to the next and over time, even for the same model. Therefore I'm not going to attempt to describe a procedure of the form "Cut the blue wire..." (as someone said in another thread) because it might not be applicable to all models and someone might end up frying themselves by putting 240VAC onto the torch output or something. Instead, I'll give circuit diagrams showing the modifications. If you can't follow them, find someone who can to help you transfer them to your own welder and do the modification safely.

Oh, and I didn't fit a separate fuse for the power supply for the following reasons:

1. No other part of the internals of the welder has a separate fuse (where's the fuse for the control board with its piddly little transformer?)

2. The 13A plug fitted to the laptop PSU I used was found to be fitted with a 13A fuse - on what can't have been more than a 5A cable

Whether or not you decide to fit a separate fuse for the PSU is entirely up to your judgement and at your own risk.

Anyway, here you go:

This is the original schematic from the manual for my welder, a SIP 130 turbo.



And the modified schematic (modifications in blue).



Running through the modifications, what I have done is:

1. Connected the AC input for the PSU to the switched mains supply, just after the on-off switch. Not shown is the earth connection. I connected this to the main earth point in the welder case.
2. Cut the connection from the welding voltage DC output to the motor controller at CN1 (the wire from CN5 to CN1) and connected the "+" output from the PSU instead.
3. Connected the "-" output from the PSU to the negative connection from the welding transformer, which is also the negative terminal of the motor. This and the preceding step effectively replace the welding voltage with the laptop PSU voltage as the motor power supply.
4. Now, the switching. I used a relay with a 240V AC coil and connected the coil between live (L1) and the switched neutral from the control board to the primary of the welding transformer. This takes the place of the 24V transformer used on other versions of the wire-feed mod and causes the relay to be energised whenever the torch trigger is pressed (and thereby the welding transformer switched on).
5. For the contacts of the relay, I connected the common terminal to the motor positive terminal and moved the output wire of the motor controller from the motor positive terminal to the normally-closed terminal on the relay. In this way, the output of the motor controller is only connected to the motor when the torch trigger is pressed. Clever, eh?
6. But there's more! Having a spare normally-closed contact on the relay (a terminal that is now connected to the motor positive when the torch trigger is not pressed), I connected this to the negative side of the motor. So when the torch trigger is released, not only is power removed from the motor, the two terminals of the motor are shorted together. This acts as a very effective brake on the motor and practically eliminates run-on of the wire-feed when the trigger is released.

So, how well does it work?
I have to say straight away that 16VDC isn't really enough. I had to adjust the preset pot on the board all the way to get the wire to crawl (most of the time) with the speed set to minimum. I have to turn the knob about two-thirds of the way up to get an acceptable speed for welding on low-to-medium power. That's just lack of voltage. I have my eye on an industrial 24V switching supply (at 10A ) to replace it and get the speed up in order to weld at higher power settings.

However, it welds so much more smoothly and controllably than it did before (as you'd expect from previous reports about similar mods). Also, wire run-on is a thing of the past. Experimenting with and without the "braking" wire connected, I can even hear the difference. Without it, the motor audibly spins down after releasing the trigger. With it, the motor just stops dead

I'll replace the supply with a higher-voltage one but I'm pleased to have proven the principle and shown that if you do only have a switching PSU to hand, there is a way to use it in this application.

This would also work, though, if you replaced the switching PSU with a 24V transformer and a bridge rectifier. Additionally, it means that should you wish, you could add a smoothing capacitor to the output of the rectifier without worrying about the motor running on as the capacitor discharged.

thanks for that was planing to use a 15V with speed control with a variable speed controller in a wire feed unit I'm makeing but was concerned speed wouldn't be good enough i now know.

Originally Posted by madkayaker

thanks for that was planing to use a 15V with speed control with a variable speed controller in a wire feed unit I'm makeing but was concerned speed wouldn't be good enough i now know.

It does depend on the motor, of course, and the gearing on the wire-feed. If you were planning to use the wire-feed from a SIP I would say you definitely won't get enough speed from 15V as that's what I see across my motor now at max speed.
I would also say you were barking mad

Presumably, you're using some other wire-feed mechanism?

If you feed a known voltage to the wire feed motor and measure the meters of wire in 10 seconds, you can find the volts needed.
A PM motors speed is proportional to voltage, the torque is proportional to amps.
The speed regulation is best at mid speed.
Robert

Right - I've upgraded to a 24V PSU and can now get a decent turn of speed out of it. I've measured the maximum wire feed speed at 13.2m/min (43 ft/min).
Is this likely to be all I need to weld up to 130A with 0.8mm mild or stainless wire?

Hi Bill

I searched out this thread after reading Malcolm’s comments about you’re wire feed in the other thread. Was going to post earlier but got side tracked.

How does your mod differ from the usual transformer and bridge mod? It says it can be done using a transformer and bridge rectifier but maybe a bit more of an explanation of the difference would be useful?

Have you now replaced the psu with a transformer similar to the one others have used? I know Bchild is keen to know more as I think will a lot of others.

Originally Posted by WindWalker

Hi Bill

I searched out this thread after reading Malcolm’s comments about you’re wire feed in the other thread. Was going to post earlier but got side tracked.

How does your mod differ from the usual transformer and bridge mod? It says it can be done using a transformer and bridge rectifier but maybe a bit more of an explanation of the difference would be useful?

Have you now replaced the psu with a transformer similar to the one others have used? I know Bchild is keen to know more as I think will a lot of others.

Good find

Bookmarked

Originally Posted by WindWalker

How does your mod differ from the usual transformer and bridge mod? It says it can be done using a transformer and bridge rectifier but maybe a bit more of an explanation of the difference would be useful?

There are two main differences. The first is the use of a regulated power supply instead of the simple transformer/bridge arrangement. This is simply a metal box with 240V input and 24VDC output so is rather simpler to wire in. It is also lighter and probably smaller than the transformer most people use for this mod.

A regulated power supply will, within its operating limits, maintain a constant voltage output regardless of the current drawn from its output. A transformer's voltage output will "droop" when current is drawn from it. (In general, a bigger transformer will "droop" less for a given output current)

The second difference is that while the simple mod switches the 240VAC supply to the transformer whenever you press the trigger, my version has the power supply connected permanently to the 240V supply so it is powered up all the time the welder is switched on. This is because switch-mode power supplies such as I used don't like their input voltage being switched on and off frequently.

Therefore, I had to add a relay to switch the output voltage instead. A similar arrangement would work for a transformer/bridge supply and would allow for use of a smoothing capacitor which might give many of the advantages of the regulated supply. On the simple form of the mod, this would be unusable since the capacitor would continue to discharge through the motor when the trigger was released, leading to a huge amount of run-on.

As a side-effect, using this relay also gave me the flexibility to use the unused contact as a brake.

Originally Posted by WindWalker

Have you now replaced the psu with a transformer similar to the one others have used?

No. When the original laptop power supply proved unable to work at higher motor speeds due to lack of output voltage, I replaced it with a 24V unit. I can't remember the spec. at the moment but it wasn't the big 10A supply I mentioned above. It is probably nearer 3A but I'll try to get some photos this evening.

OK so I could use my current transformer bridge and power it up from the permenant Live (Rather than the switched live) - I could then use teh relay to switch the output

sounds perfect unless I have gone way off base and again failed at electrickery!!

I’m with you on that one Bchild, a further mod on what we have would be ideal but I guess it’s not the end of the world if a power supply is required instead of the transformer.

The key would seem to be the relay and how it’s wired to allow permanent live supply to the PS/Transformer. I believe Bchild already has a relay that is used for braking but I don’t yet so any information on relays would be welcome.

Many thanks for taking the time to explain Bill, much appreciated