Replace Amperage Control

Hello to the welders:
I have recently acquired a small stick welder that operates on 110 volt supply. The existing amperage control is a two way switch that selects either 50 amps, or 90 amps. I do not wish to use this welder in 90 amp mode as the duty cycle is ridiculous at this setting. I also want to be able to control the current in smaller steps and be able to run the welder as low as 30 amps for some sheet metal purposes.
Is it reasonable to swap out the coarse, two position output amperage switch with a type of rheostat or potentiometer that could deliver lower amperage and smaller increments of adjustment?
I am not a complete knucklehead with electricity, basic circuitry, and wiring in general. I have not however worked with a welder assembly. What would the preconditions and precautions that should be observed when planning such a modification? Has anyone been involved in such a modification to a welder's circuitry?
Thanks, Skeeter.

On Mon, 30 Jul 2007 08:31:57 -0700, snipped-for-privacy@sympatico.ca wrote:
  Quoted Text Here  
No. Those switches simply turn on or off taps on the transformer. You need more than those to vari the amperage.
Gunner

  Quoted Text Here  
welder
amperage
and
assembly.
a
VERY much depends on what sort of welder. If it is a simple transformer buzz box, the amperage control could either switch between tappings on the transformer (which actually changes the voltage, and the current follows) or it could switch in varying amounts of inductance from a choke. For this type of welder forget rheostats or potentiometers - the power you are handling is far too much. (Pure inductors are effectively a wattless resistance in an ac circuit)
If it is an inverter type then it very much depends on what the two stage control actually does in the circuit.
AWEM

On Jul 30, 11:31 am, snipped-for-privacy@sympatico.ca wrote: Hello to the welders:
<snip>
Here is an amendment to my previous post.
The stick welder in question is rated for 50 and 70 amps output. My caveat regarding the higher setting's duty cycle remains. I cannot draw much of a bead at the 70 amp setting. Perhaps 3 to 4 inches. The arc stability at this higher setting is not very good either. Would only be good for tacking.
The welder is house branded "Mastercraft", but after some research it appears to be manufactured by Campbell Hausfeld. I hope this additional information may help with figuring whether a different amperage control may be adapted to this welder. It definitely needs to get down to 30-40 amps to serve well in light sheet metal work.
Thanks, Skeeter.

wrote:
  Quoted Text Here  
it
But you haven't answered the basic question: Is it a simple transformer 'buzz box' or is it an inverter type. Is it heavy or is it light - the answer to that would go a long way to making a correct guess <G>
AWEM

Hello Andrew:
On Jul 30, 2:05 pm, "Andrew Mawson"
  Quoted Text Here  
Sorry about omitting whether this welder is a "buzz box" or an inverter type. Having no real world experience with welder circuitry, I cannot offer a valid response to this question at this moment. My welding experience consists of a basic 4 position provincial ticket... and just a modest amount of welding and some fitting of weldment assemblies.
I will attempt to find a schematic, or failing that, I will open the welder and examine the circuit and attempt to describe the arrangement that I find.
Thank you for your willingness to help. I will be able to offer further information tomorrow.
Cheers, Skeeter.

snipped-for-privacy@sympatico.ca wrote:
  Quoted Text Here  
If it's a "buzz box" welder, there will be nothing inside except a large transformer and maybe a fan. If it's an inverter welder, you will see a smaller transformer, and a circuit board with various electronics on it.
All the inverters I know of will have a variable current control. The fact that you have a two position current switch makes me believe it's just a buzz box with multiple taps on the transformer.
An inverter welder in that power range will weigh about 17 lbs. A buzz box welder like that will be more like 30 to 50 lbs. The Lincoln Tombstone welder is a "buzz box" welder which is about 100 lbs (it's got a larger transformer which gives it a higher duty cycle).
Inverters cost more so the cheapest I would expect an inverter of that size to cost knew (even for a low end cheap unit) would be around $200. The better quality ones are more like $1500.

with welder circuitry,
  Quoted Text Here  
^^^^^^^^^^^^^^^^^ I thought about replacing the step switch on my Lincoln 120v MIG welder with a continuously variable voltage control, thereby upgrading it to the next higher level in the catalog. One look inside the box put that idea to rest. The stepswitch went to transformer taps. Obviously, the more expensive welder uses a different transformer, and solid state control. The step switch cannot be replaced with a potentiometer without adding circuitry and changing the transformer.
I think you are facing the same situation.

Hello to all once more:
Here is a further amendment to my original post...
I removed the sheet metal cover of the little welder. This is what I found.
* Large transformer with a couple of taps * 3 position switch to select either OFF, 50 AMP, & 70 AMP * Thermostat (I think) on the side of the transformer * Indicator light wired to the switch and thermostat
So I believe this device falls into the "buzz box" category... no? Really simple circuit. Right at home in Fred Flintstone's garage. I believe the thermostat monitors the transformer to shut it down before heat failure. (thus establishing the duty cycle)
Perhaps a simple method to lower the power ranges would be to change the input voltage to the little welder with an outboard transformer of sufficient current supply capability. Being that I would only use this outboard power transformer when welding with less than 40 amps, this may be possible... no?
I noticed that a responder named Dan was suggesting along this line of thought. If I am wrong, please do not hesitate to correct me or guide me in any way.
Thanks to all, Skeeter.

snipped-for-privacy@sympatico.ca wrote:
  Quoted Text Here  
Yes, with the correct transformer, it should be possible. But, if you had the correct transformer in the first place, you could just weld with it instead of getting another transformer to fix the transformer which isn't correct. Getting the correct welding transformer might be easier and cheaper than trying to find a transformer of just the right design to offset what your current welding transformer is doing.
The fact that you only want it to work for about 40 amps of welding current should make it a little more likely to find for cheap but you are still talking a high power transformer and for the most part, they aren't cheap. Most the cost of a transformer based welder is the transformer.
  Quoted Text Here  
Yes, Dan thinks you might be able to use a transformer from an old junked microwave. I don't know what type of transformer they typically include, but it sounds like a place to start. Even a broken microwave should be good because it's unlikely the transformer in it is broken.
If you measure the output voltage of your current transformer on the two power settings like Dan asked it would help him suggest what you need to do.
I'm not sure what he's thinking of doing with the microwave transformer.
Dan - what type of transformers do microwaves include? Step up? Step down? How are you thinking it should be wired with this welder to get the results he is looking for?
I don't understand the full details of how a welder limits the current flow while welding - but I assume it's mostly limited by the reactance of transformer windings - mostly the primary winding. So, it seems to me, that the best way to reduce the output current (without re-winding the transformer) is to simply increase the primary inductance, which it seems to me could be done simply by wiring the correct large transformer winding in series with the welding transformer primary. Is this what you are thinking to do with the microwave transformer?

Most welders of this class limit the amperage by saturating the core. Output amperage is limited by the lines of flux available. Essentially, you run too small a core for the transformer. This would be poor design for a regular transformer.
  Quoted Text Here  

On Jul 31, 9:35 pm, snipped-for-privacy@kcwc.com (Curt Welch) wrote:
  Quoted Text Here  
Microwave ovens have a big step up transformer. Which is not what we need, but they are designed to be constant current transformers. This means that the primary is not tightly coupled to the secondary. They do this by separating the windings and putting in laminations to shunt some of the flux so it does not couple the two windings.
Since the windings are separated, it is relatively easy to remove the secondary with damaging the primary. And then wind a few turns of much bigger diameter wire to make a low voltage transformer.
One problem with inexpensive AC welders is that the open circuit voltage is low. So I am thinking the first thing to do is see what the secondary voltage is. My guess would be about 60 volts. And it would be better if it were about 80 volts. Microwave oven transformers have about one volt per turn. So rewinding a MOT with about 20 turns and putting the transformer in series with the welder output would be about right. The secondary wire needs to be at least as big in diameter as the secondary wire in the welder.
The next thing to do is try it out. With those laminations shunting the flux in the MOT the current may or may not be more than it was. With a little luck I think it might be less. If it is too high adding some more laminations in the MOT might be enough. So when looking for junk microwaves, look for two with identical sized transformers.
Dan

  Quoted Text Here  
I
I
before
of
of
guide
Skeeter,
For the welder to remain usable you need to keep the open circuit voltage (ocv) somewhere between 50 and 80 volts in order to start the arc reasonably easily. Once struck the voltage across the work and electrode will fall to about 18-20 volts, with the remaining 30-60 volts being dropped across the internal resistance / inductance of the transformer and leads. If you reduce the input volts to the welder using an external transformer, this will reduce the arc current but you end up dropping the ocv as well. As previously mentioned by another poster, the transformer in a buzz box is designed to have a loose coupled magnetic circuit to enhance the drooping voltage / constant current characteristic. To reduce the current further you need to increase the external losses. The conventional method is to introduce a choke, as the choke impedance limits the currents but retains the same ocv, but only the low winding resistance of the choke contributes to heat losses. Most buzz boxes with a variable output merely have a mechanical means of varying the choke magnetic gap to alter its inductance. A more technically advanced method is to use a very high powered Triac in series with the output, and vary the trigger point in the mains cycle where the triac conducts. This gives you a very wide range of control, and is a very simple circuit to implement. I have an ancient BOC Autolynx MIG welder that uses this method to very good effect. A triac can conduct on both the positive and negative halves of the mains cycle, and it is important that it does otherwise the transformer will saturate.
A few basics here: http://www.st.com/stonline/products/literature/an/3566.pdf
AWEM

snipped-for-privacy@sympatico.ca wrote:
  Quoted Text Here  
Is it an AC only welder?
If so, it's probably nothing more than a small transformer with with a couple of different taps which are selected by the two position current switch. The only way to get a different current from it is to either rewire the transformer to add more taps (not a very easy or cheap option), or create some type of voltage control going into the transformer - which is likely to be nearly as expensive as just buying a new cheap welder designed to do what you want to do.
The classic Lincoln tombstone welder (the AC-225) goes down to 40 amps and up to 225 amps and sounds like more of what you need. But it's a 220 V welder and only costs about $350 new. They are for sell on ebay all the time in the $150 range and if you look around between ebay and craigslist you can probably pick one up for a less than you can do something to your cheap 110 volt unit.
Or, if you want to go cheap, Harbor Freight I see has a 110 (and 220) AC welder with variable adjustment amperage for only $120:
http://www.harborfreight.com/cpi/ctaf/Displayitem.taf?itemnumber@388
Again, it's nothing more than a transformer in a box, but it works as a variable inductor by sliding a movable core in and out of the transformer to reduce the current. The documentation doesn't say how low the current will go on this unit but I suspect it will go down to what you are looking for.
The currents these machines draw are fairly high, even the cheap 110 volt units draw something like 15 to 20 amps from the line - which means you need a very high power current control (2000 watt?) which just don't come cheap. It's not just a matter of buying a $5 rheostat and hooking it up because they can only handle power in the 1/4 watt range. These low end welders are nothing more than high power current control devices and if they don't do what you want, you basically have to add a high power current control device to it in order to make it do what you want - but that's just the same cost as buying the correct high power current control device (aka welder) in the first place.
If you want to experiment with something cheap and tacky and dangerous just to try and make it work, you might for example try something like getting an old electric heater, and turning it into a high power rheostat by hooking up the heating element in series with the input power to the welder. A trick like that will reduce your current, but whether it will give you enough voltage to make a decent arc is another question. Or, if you have a large transformer laying around, you could hook a winding from that that in series with the input power to your welder to try and reduce the output current. But these are just desperate attempts to try and do what you want without spending money (if you have either of those laying around for free). Otherwise, spend a couple of hundred bucks and get the welder you need.

On Jul 30, 4:31 pm, snipped-for-privacy@sympatico.ca wrote:
  Quoted Text Here  
a tapped primary or secondary. There is likely a fan to cool the transformer. The transformer is constructed so that the primary is not tightly coupled to the secondary. So when welding the voltage drops and the current remains more or less the same. You should be able to verify this by removing some of the sheet metal around the transformer and looking. The only precaution needed would be be unplug the welder from the mains.
I think you can modify the welder so that it will work better at the lower current setting and have some more lower settings. If I am right about your welder, the low current setting has a higher output voltage. You can verify this with a AC voltmeter. My guess would be the open circuit voltage is about 60 volts on the low current range and maybe 45 volts on the high current range.
The way I would do this is to add another transformer to your welder. You probably don't have enough room inside the case to do this, so this would be either external to the existing welder or a new case. The cheapest additional transformer you can get would be from a junked microwave oven. If you are still gung ho about doing this, why don't you look inside your welder and verify how it is made. And measure the voltage coming out of the welder at both current settings while no welding is being done. If it is all as I said, then start looking for a junk microwave oven to salvage a transformer. The higher wattage rated the oven is the better. More when you provide more information.
Dan