SIP / Cosmo Mods

[silverfox]

Aaron,

I've sent you a PM re the Euro Torch upgrade

 

[bump]

Nice to see that someone has found this of use.
Get a thread going for the euro mod in the sip section.

 

[Elnino]

So, looking at the design of the original unit, If i perform this mod, what regulates the wire feed speed when the power is changed?

i.e before, the voltage to the motor would have been governed by whichever tap was used from the main transformer but now we are just using a straight 24v...

I can't see anything on the PCB that regulates the speed once it is tuned on.

 

[malcolm]

I'm still confused guys. I can figure out roughly what type of bit needs to be where but the vast range of electrical components to choose from is baffling to me.

I think I can cope with the transformer spec - if you want to drive a maybe 2 amp 24V motor you'd be looking at a 100VA transformer - and seems the 2*12V thing can be wired for 24V output.

How do you spec a bridge rectifier though? All sorts of massive currents and voltages in the specs - are those maximums and all of them will work perfectly well at the low volts and currents that interest me, or should I be more picky?

Finally, for a bonus point, is there anything available off the shelf that would smooth the output voltage from the transformer? ie give you a constant 15V when the output is varying eiether side of 15V all the time? I guess a big capacitor would be a starting point, but I'm sure there must be ready built things that are cleverer than I am.

 

[rtbcomp]

So, looking at the design of the original unit, If i perform this mod, what regulates the wire feed speed when the power is changed?

i.e before, the voltage to the motor would have been governed by whichever tap was used from the main transformer but now we are just using a straight 24v...

I can't see anything on the PCB that regulates the speed once it is tuned on.

The "power" switch supplies a voltage to the motor circuit depending on the tap selected. In addition the speed knob controls the current running through the motor. I.E. the switch sets the speed in the ball-park, the speed knob fine tunes it.

The straight 24v supplied by the mod. is switched on and off by the PWM circuit, at zero speed it is off all the time, at max speed it is on all the time, and half speed it switches on and off in equal parts, many times a second, and so on for all speeds.

 

[rtbcomp]

I think I can cope with the transformer spec - if you want to drive a maybe 2 amp 24V motor you'd be looking at a 100VA transformer - and seems the 2*12V thing can be wired for 24V output.

How do you spec a bridge rectifier though? All sorts of massive currents and voltages in the specs - are those maximums and all of them will work perfectly well at the low volts and currents that interest me, or should I be more picky?

Finally, for a bonus point, is there anything available off the shelf that would smooth the output voltage from the transformer? ie give you a constant 15V when the output is varying around 15V in time? I guess a big capacitor would be a starting point, but I'm sure there must be ready built things that are cleverer than I am.

2 amps at 24v volts is 2*24 = 48 va (I think this only approximate, due to AC having a mind of its own)

Yes, connect two 12 volts in series to get 24v, BUT connect them in phase, otherwise you get 0 volts.

Rectifier ratings are maximum. Bridge rectifiers are cheap enough, go for one twice the rating you need.

You can get voltage regulators, I'm sure there is a fixed 15v one available, just make sure it can handle the current, and that you mount it correctly.

For a motor drive I guess a decent electrolytic capacitor would do the job, 47000 uF 50v perhaps?

 

[malcolm]

Rectifier ratings are maximum. Bridge rectifiers are cheap enough, go for one twice the rating you need.

Excellent - should be able to choose between a wide range then. Tried restricting a search to low range ones and they were more expensive than the high range ones. Why are there so many?

You can get voltage regulators, I'm sure there is a fixed 15v one available, just make sure it can handle the current, and that you mount it correctly.

I was thinking a smooth 15V when the output was operating at a nominal 15V, or a smooth 12V when it's operating around there. I remember stuff about capacitors and inductors being used to smooth a supply but don't fancy trying to design anything with my very limited knowlege. Sure there must be something ready made but I'm not sure what to look for.

 

[rtbcomp]

Sure there must be something ready made but I'm not sure what to look for.

Not quite sure what you mean - are you looking for something that takes mains and gives a smooth DC output?

 

[malcolm]

No, something that plugs into the output of the transformer that takes in that really spiky stuff at anywhere between 12V and 14V that goes to the torch while you are welding, and outputs the same voltage only smoothed.

Just thinking about something that was mentioned elsewhere about losing the change in wire speed with power setting - wondering if there was some other method of smoothing the motor supply while retaialing that characteristic.

 

[rtbcomp]

No, something that plugs into the output of the transformer that takes in that really spiky stuff at anywhere between 12V and 14V that goes to the torch while you are welding, and outputs the same voltage only smoothed.

Just thinking about something that was mentioned elsewhere about losing the change in wire speed with power setting - wondering if there was some other method of smoothing the motor supply while retaialing that characteristic.

I'm with you now. You're talking about smoothing the output of the welding transformer, rather than one dedicated to the motor circuit.

Tricky - because you're dealing with a lot of power, you'd need a v.big capacitor accross the output of the rectifier to do that.

Does the DC motor supply come from the output of the welding rectifier? If so I'd try providing a seperate bridge rectifier, fed from the AC output of the welding transformer, and feed that to the motor circuit through a choke and capacitor arrangement.

 

[malcolm]

It appears so far that the power supply to the motor does come from the torch (+) and the other output from the main transformer before it gets to the choke (-). That's how they seem to vary the wire speed with voltage setting. Can see why it might end up a bit spiky.

I've not drawn out the circuit in full detail yet so I might be totally wrong. Though there aren't any other DC inputs to the circuit board, and the control works on voltage drop so it needs DC. Circuit board pics here

Would a separate rectifier help? I'd assumed the spiky draw from the torch would effect the voltage on the secondary coil of the transformer so a secondary regulator would give the same rubbish out. This is where I was thinking capacitors or my off the shelf magic smoothing device would help. Other manufacturers (and I'm not naming names) seem to manage this trick with extra stuff on the board (or bigger transformers maybe).

I don't see any capacitors on the circuit board or elsewhere in the welder - I think this is where the bump mod wins - the supply to the wire feed control must be very erratic as you can see the wire moving up and down while you weld.

Big capacitors to do what I want can't be expensive. You mention an inductor (choke) as well. I'm certain an arrangement of the two was educated to me, but electrics went over my head - no application for me at the time.

 

[rtbcomp]

If you put capacitors accross the output of the rectifier they will have to smooth out the voltage drops when you strike & stop the arc.

I was reckoning that another rectifier would decouple most of this from the motor circuit.

You take one end of the choke to DC+ve and the other end to the load (motor circuit). The capacitor goes from the load to DC-ve.

There are formulae to determine the inductance & capacitance needed, but I've forgotton all that years ago!

The de-coupling effect of the 2nd rectifier should mean you don't need a huge capacitor.

 

[Elnino]

The "power" switch supplies a voltage to the motor circuit depending on the tap selected. In addition the speed knob controls the current running through the motor. I.E. the switch sets the speed in the ball-park, the speed knob fine tunes it..

Yes, i see that in the original design

The straight 24v supplied by the mod. is switched on and off by the PWM circuit, at zero speed it is off all the time, at max speed it is on all the time, and half speed it switches on and off in equal parts, many times a second, and so on for all speeds

Err, i don't see how it is doing PWM all of a sudden when the original was not. It's just a power transistor...

 

[rtbcomp]

Err, i don't see how it is doing PWM all of a sudden when the original was not. It's just a power transistor...

My mistake - I was confusing this thread with another one suggesting a PWM alternative for the existing linear one!

 

[Elnino]

My mistake - I was confusing this thread with another one suggesting a PWM alternative for the existing linear one!

No probs... You had me second guessing myself then tho...

Although, after using it on the weekend over varying powers, I seem to be able to select wire speeds without problem. It'd be nice to have but i can get a full range of control from the dial so i'm happy....

Circuit board pics here

Malcolm, while this may be picky, i think you need to make sure that you're not giving mis-information. This is 240v stuff we're talking here, not playing with LED's. Make sure you're terminoligy is right and that the information you give is not going to get someone hurt.

For example, in that post linked to, you state that they need to connect to the relay and the neutral on the switch but infact the relay itself switches the neutral and not the active.

Don't take this as a personal attack, it's not. It's just that what we're dealing with here is high voltage, high current and extreme potential for someone to get seriously hurt. Completeness and as much accuracy as possible is important.

 

[malcolm]

Last time I asked a welder supplier to advise me on where to put the live and neutral he said it didn't matter. I made my own mind up. On that basis I don't think you could argue a difference.

Though which side of a 240V AC circuit is more deadly when swallowed and why? (Serious question here - I'm coming from the starting point of one being as bad as the other).

 

[Elnino]

Yeah agreed, when it comes to wiring a transformer/coil to AC, it does not matter which is on the live and which is on neutral. But, what i was saying is that you were suggesting that the relayed 'output' there is the live connection when it infact is neutral.

Generally, if you isolate a live wire then the neutral can be safe but this is not always the case. There is still a PD between say the neutral and an earth.

We all have a learning curve and that's why i do not want you to take this personally. It's just the danger involved in this sort of thing that makes me worry about someone reading here that does not really understand what they are doing.

Me, i make sure i understand something fully before doing stuff like this. I am not one to just follow a 'cut the blue wire' type instruction - but many are....

 

[rtbcomp]

Last time I asked a welder supplier to advise me on where to put the live and neutral he said it didn't matter. I made my own mind up. On that basis I don't think you could argue a difference.

Though which side of a 240V AC circuit is more deadly when swallowed and why? (Serious question here - I'm coming from the starting point of one being as bad as the other).

Neutral is effectively connected to earth, usually at the substation, so live is at 240 volts above earth and neutral. You are connected to earth, either directly or via resistance (damp atmosphere) and/or capacitance between your body and the planet.

Thus grabbing a live gives you a good chance of getting a shock.

AC circuits are symetrical, it doesn't matter from a functional point of view which way live & neutral are connected. But from a safety point of view it does, the most obvious reason being that fuses are placed in the live lead. If a fuse in the neutral side blew, the whole appartus would be live. In the old days single pole switches were used, again these must be in the live side.

 

[malcolm]

Good feedback there - So is there a convention for L1 and L2 in the way of connecting one to live and
the other to neutral?

I may have mislabeled my diagram based on poor advice about the two - I'll check where the blue and brown wires go on the SIP, meanwile I'll edit that post ans say it might be either way.

 

[Elnino]

On mine, L1 is live, L2 is neutral.

When i open the case, Neutral (blue) is the closest to me on the switch.