Resistance Spot Welder

Why not contact the seller/supplier?

Did you pay for it? Manual??

Used, no manual. I am trying to look up the Mfg.

Thanks, I will try that in the morning.

Note that in step 3A, it says zero Ohms. In actuality, it should not really be zero, but a very low value. The exact value depends on the size of the transformer, with larger transformers generally having lower resistance.

With many ohmmeters, when you short the leads together, they will indicate a fraction of an Ohm, which may be the resistance of the leads and contacts, but also may include some value due to incorrect zero calibration. If the meter is capable of indicating to fractions of an Ohm, you should be able to see a slightly higher resistance when connected to the primary of the transformer (on cleaned contact surfaces) than when the leads are directly shorted.

The secondaries of these transformers are virtually indestructible, unless they were designed to be water cooled and were used heavily with no water.

If the unit is a capacitive-discharge type, there was a thread about repairing one a year or two ago. If you can't find it, I still have some info...

No power to the transformer from the controller?

Unless the controller has a circuit sensor to detect an open circuit,why test the transformer?

I recommend testing the controller if it is not supplying power to the transformer.

If you know the output parameters of the controller,you can substitute the same voltage and current controls from any source.

The transformer will not know the difference.

However,if you know the input /output ratio of the transformer,and want to test it anyway,apply an a/c source from another source,such as a stick welder,with amps set at lowest level.

Be certain to disconnect the transformer from the controller during the testing.

Measure the voltage across the primary,and the secondary should give a proportional output voltage.

This should give a proportional output voltage.

Another method is to place a 100(approx) watt light bulb in series with a 120 volt test lead and perform same voltage checks.

Remember, the output voltage will be very low,but you should get a measurable value.

A very low-tech method of testing but it will work.

Thanks, I will try it.

The resistance welder transformer probably has a thermal cable, which is normally closed,(0 ohms). That is the first check to make, and check the connections at the controller/power supply from the transformer. Is this unit water cooled? If so, there may be a water pressure sensor to check. Are you getting any error codes? Some welders have a memory which only lasts a few months. If it has been in a closet for awhile. you may simply need to review all of the settings. If you can't find the manufacturer's name, try Googling any numbers you find on the outside cabinet, or the number of the transformer. The second test is to remove the load, and measure the transformer at the output, to the part being welded. The transformers I've worked with have one or more diodes, and if you have 0 ohms, it probably means the diodes are shorted.

Let us know what you find.

Thanks, it will try to find addition information, such as numbers.

A note to M p's post about diode testing.

When using a meter with the diode test symbol,it uses enough voltage to overcome the junction voltage of the diode.Normally,it will be in the range of .6v to .7volts.in one polarity,and zero in the opposite polarity.Test the diode in both directions.

It is best to test the diode out of the circuit,but I have tested them in circuit simply by looking for a difference in the forward/reverse indications.

The in circuit test is not as accurate,but it will allow a go-no-go test for further testing.

To test an SCR,out of circuit,connect the leads in forward direction,and connect a jumper from the positive terminal to the gate terminal.

The SCR should conduct and stay that way after the test lead is removed.

For testing MOSFETs see link below.

https://www.youtube.com/watch?v=RkWy1EirEu8

Obviously, everyone has a different knowledge set. None of the spot welders I've worked with, that had the transformer in a separate box as indicated by the OP, had diodes. The secondary of the transformers all connect directly via weld cables to the electrodes, and do indeed measure zero Ohms (plus lead resistance) on all the Ohmmeters I've had available, either at the secondary terminals or at the electrodes. For any reasonable cable length, any measurable difference between the resistance at the secondary and the electrodes would indicate a bad connection or a defective cable.

All of these were for welding thin sheets/foils, with the transformers generally in the 4"-7" cube size range.

In my experience the item most likely to fail is the SCR or other device that connects the energy source to the transformer primary.

A photograph and/or dimensions of the unit in question could help a lot.

You need to supply us with some additional information, like a manufacturer and model #. Lacking this simple information, we are shooting randomly. Remember that spot welders don't deliver any power until discharged into a low impedance load, and then, only for a fraction of a second. They can't be tested with a conventional VOM.

Welderman, G.A. Other than determining if the welding power supply, or the transformer is bad, it may be a job for a service center. Here is what I use when repairing and testing Resistance Spot Welders, http://www.amadamiyachi.com/products/resistance-welding/rw-monitors-checkers/mm122a

Without the make, and model information, we are just guessing.

Thanks.

GA to you too!

Thanks for the link! I was not aware of that unit. Can you give a ball park figure for cost?

We do use a number of Unitek/Miyachi weld supplies, both capacitive discharge and 3Ø.

We design and construct spot welders that compete against Miyachi; especially in the larger weld arena. I can tell you that some of the earlier Unitek machines are no longer serviced by Miyachi. I have, or can obtain service manuals for many of the older machines

I am only spot welding stainless mesh. I will send a photo of the controller and transformer.

Stainless mesh could be made from material as thin as 0.02mm or as thick as 10mm, or possibly even larger for earthmoving/mining operations, so that does not tell us much. Mesh can also be woven from wires or cut from sheets. Give us some numbers!

That appears to be a Robotron MX 77. I didn't find any information to speak of, but since it has a switch for half or full cycle, it presumably uses SCRs as contactor devices. As I believe I mentioned in an earlier post, the SCRs are the most likely thing to fail.

Is that Luma photo the label of the transformer? Since it operates on 120V, this must be a very light duty welder, so the mesh you mentioned must be very light weight.

The LUMA is the transformer. The mesh is the same weight as hardware cloth, but stainless. Can the SCRs be replaced?

Interesting that the transformer label says nothing about its output. I'd expect to see an output voltage and/or current rating, or at least a turns ratio. It's likely something around 40:1, which would make it put out around 3 Volts at around 600 Amps.

It's easy to check that. Just disconnect the transformer from everything, so nothing is connected to its input or output, then connect a standard 120V power cord to the input terminals, and measure the AC voltage at the output terminals (the heavy ones that connect to the weld tweezers or electrodes). This was mentioned in an earlier post as a way to check whether the transformer is good. If you are not absolutely sure which are the input terminals to the transformer, don't do it! Connecting 120V to the output terminals would definitely blow something, most likely the breaker for that 120V circuit, and it would momentarily (until the breaker opened) produce a voltage around 4800 Volts, which could be lethal.

Again, hardware cloth is made from wire at least as small as 0.005" diameter all the way up to at least half an inch diameter, so that does not tell me anything.

Certainly, the SCRs can be replaced. here's a photo of one I replaced a while back in a similar welder.

It's on the right, and is labelled SCR1. You can easily see the large red cable coming out the top, which connects directly to the transformer. It's harder to see, but there is a second, much smaller wire that is the trigger, it is almost hidden, covered in black shrink tubing just above and to the right of the red cable There is a 1/"-20 nut on the other side of the PC Board that holds it in place.

There are other shapes and sizes of SCR, but this is the most common in my experience for similar low-power welders, and especially for older welders.

In case your SCRs don't look like this, they should be easy to identify. There are two SCRs, one for positive half-cycles and one for negative half-cycles. Each of them will have one heavy wire or circuit board trace that connects directly to one side of the 120V power input, a second heavy wire or circuit board trace that connects to one of the transformer terminals, and a third much lighter wire or circuit board trace that connects to the control circuitry.

Could the SCR be in the controller box?

Absolutely! The SCRs are the devices that connect power to the transformer. The rest of the circuitry is there to 'tell' the SCRs when to conduct, how long to conduct, and how many times to conduct for a given weld.

An SCR is a controlled diode. It never conducts in one direction, and only conducts in the opposite direction when it gets a start signal pulse at the third terminal. Once it is triggered, it continues to conduct until the power goes off. Of course with 60Hz AC, the power goes to zero every 120th of a second, so the longest time a single SCR can conduct is just under 1/120 of a second. The large knob in the center of the welder determines the point during a half-cycle that the firing signal is sent. If it is set to 50%, then it will turn the SCR on halfway through the half-cycle, etc.

The half/full cycle switch determines whether one or both SCRs are triggered. BTW, you currently have that switch in the Full Cycle position, which means that if indeed the SCRs are the problem, then both of them are blown.

The two knobs at the bottom determine how many times the SCRs fire for a single weld. They are set to 27, which means that each time you press the footpedal or squeeze the tweezers, it will send 27 pulses from each SCR, so your weld will last 54/120, or almost half of a second. The parts must be held without movement until the parts cool, so you need to hold those parts motionless for pretty close to a full second before releasing the pressure.

We most commonly weld material only a few thousandths of an inch thick, and normally have it set to send just one pulse. I can't really tell, but it looks like your heat control is set to 19%. If so, you just might get a better weld by increasing that setting and reducing the number of pulses accordingly. So for example, increasing the heat control to 80% and reducing the number of pulses to 7 would put roughly the same amount of energy into the weld, but in 1/4 of the time. This would actually create a higher weld temperature because there would be less time for the heat to spread out into the nearby metal.

Assuming that your unit was working correctly for a significant time before it failed, I can easily imagine that there came a point where you had to increase the setting to get a good weld. If so, that would have been when the first SCR failed. After that, it would have been operating in half cycle mode, even though the switch was set to full cycle. During that time, changing the switch from full to half cycle would have either had no effect, or made it stop working, depending on which of the two SCRs failed first.

Finally, I saw at least a couple of similar or identical welders on sale on eBay for around $125-$150. That's cheap enough that you ought to get another as a spare, or to use while you get this one fixed.

GA to you, Sir.

It is considered good practice to set the phase shift to about 50% and then to increase the number of cycles as required to adjust heat. This method provides the most precise heat control.

Replacement controls for your welder are available from www.isi iquidator.com for $100-125. You will find them listed under Luma, Robotron, or other brand names.

Thanks!

I presume there is an 'l' missing from the address you posted. I don't have time to look right now...

correct

The web site I intended to post is www,iseliquidator.com/4730.htm

Thanks. That's http://iseliquidator.com/4730.htm.

Nice to know about.

In our most common case, we are tack welding two very thin (usually 0.002-0.006" thick) layers of material that are together moving past a rotating weld head, so we want the weld to be as fast as possible. Thus we set it to a single pulse and adjust the phase to get the proper heat for the material. Fortunately, a single pulse weld does the job. It only needs to hold the parts in position until braze material wicks into the gaps and makes the permanent bond during a second process.

Thanks

Looking at The site Welderman provided, I see that Luma makes weld controllers; that's not a brand I was familiar with. I assume the Luma box you have is just a transformer. Have you opened it to see/find out?

It is just the transformer.

Would you tell me the names on ebay, not sure what I am looking for. Thanks

Just do a Google search for "Robotron Welder"

Here's the main result I found: http://www.ebay.com/itm/MX-77-ROBOTRON-MODEL-01015-01-WELDING-CONTROLLER-/350752692065

In the upper left corner of that page is another similar one, but the main one of this link looks identical to your current one.

Here is the inside of the controller, which is the part I need to replace and is it worth it, your opinion is valued.

I suggest you test the fuse. If it is no good, replace it and follow my previous suggestion about using 50% of the phase shift. If the fuse is good, it will be a closer call. I'd buy a used, but good replacement and save your old one for a rainy day.

Thanks

Here is what I am welding, do you have a suggestion for a welder?

Stainless Steel Woven Wire Mesh is a strong, durable, and corrosion resistant material that is extremely popular among industrial users. Darby proudly maintains one of the most extensive inventories of T-304 Stainless Steel Woven Wire Mesh, to include a wide range of mesh counts to meet our customers' specific requirements. T-304 Stainless Steel Mesh is available in woven and welded constructions.
T-304 Stainless Steel Wire Mesh is the most common stainless alloy in the wire mesh industry. T-304 Stainless Steel is often referenced as "18-8" due to 18% chromium and 8% nickel chemical composition, as indicated in the chart below. The inclusion of chromium and nickel allows steel to become resistant to staining and corrosion. Further, differing amounts of carbon, chromium and nickel affect the secondary characteristics, such as welding and forming. T-304 Stainless Steel Wire Mesh is non-magnetic in the annealed condition. According to industrial standards, T-304SS melts at 2550 ºF, but the maximum recommended temperature for continuous service is largely recognized at 1450 ºF.
T-304 STAINLESS STEEL ALLOYS- Standard Chemical Composition

You STILL haven't said anything about how heavy this mesh is! Find out the wire diameter and wire-to-wire spacing, and let us know. Also, are you welding mesh-to-mesh, or mesh-to-solid? Finally, if the latter, what material and thickness is the solid?

It is mesh to mesh. This is what I order:

Specifications

Alloy 304 Stainless Steel Weave Type Plain Mesh 4 Wire Diameter .105 in Width of Opening 0.145 in % of Open Area 33.6 Weight 3.138 lb/ft² Density 0.290 lb/in³

I am welding mesh to mesh. This is the ordering information, by the way, the mesh is being used for bird feeders. I have a webpage at www.qdloon.com.

Interesting project you have there!

There is NO WAY those specs are for the mesh you are using for your feeder! The 4 mesh (1/4" wire spacing) sounds about right, but 0.105" wire diameter is way too big. Here's a section of one of your photos, scaled so the mesh fits the (barely visible) fine one inch yellow square. I've superimposed what it would look like with 0.105", 0.030", and 0.025" wire diameters.

The 0.025" wire looks about right to me. The birds would never be able to remove a seed through the 0.105" mesh openings. Also, you would need a fairly heavy machine to roll 0.105" mesh into a 3" cylinder, whereas 0.025" mesh could be rolled by hand around a pipe.

Back to the welder! Since I now know you are not part of an industrial organization, it becomes more important to try to save some money, if possible. I'm going to try that SCR test procedure myself, right now. I'll get back here in a while.

This is my retirement job, sorry if I miss led you. We sell at craft fairs, coops, and a few small stores.

Thanks

Not to worry!

I just finished doing the testing, and the procedure does indeed work. I was, however mistaken at how high the resistor value could be, at least for the size/kind of SCR you have, and for a 12V supply. I didn't have a charged 12V battery on hand, so I used a 12VDC power supply rated at 1.5 Amps (originally powered some computer device), and I didn't find a 12V lamp that would run on that power supply, so I used a 12VDC fan as a test load.

Using an old but good 1/2" stud type SCR, very similar to yours, it would trigger reliably using a 390 Ohm or lower value resistor, but only some of the time using the next common size of 470 Ohms. Higher values would not trigger it at all. I did also try some newer but much smaller SCRs, and they would trigger with a resistance as high as 3000 Ohms. I suspect they would all trigger with higher values of resistance when operated from higher source voltage, but using 12V is much safer for the experimenter.

As I was doing the testing, I was reminded of a very important detail. They make stud-type SCRs that look identical , but have opposite polarities. The two in your unit are probably identical, but they could conceivably be one of each.

Here's a photograph of the one i just tested:

If you look at the symbol in between the I and R, you can see a triangle pointing up. That symbol is the standard symbol for an SCR, and the triangle pointing up tells us that this unit has the Anode (+ terminal) on the bottom stud. SCRs with the Cathode (- terminal) on the bottom stud are MUCH more common in my experience. I probably bought this one surplus without noticing the polarity, then found it was the wrong polarity, so couldn't use it, which is why I have it still around...

There should be a similar symbol on the side of your SCRs, and I strongly suspect they will both be pointing the opposite direction to the one in this photo. If you should have one pointing up and one pointing down, then they should have different part numbers, and you must note which one is pointing which direction (I always take at least one photograph before I disassemble anything), so you'll know which way the replacements go.

I'm virtually certain you can do this testing without having to remove the SCRs from the unit. If one or both test good in circuit, then the problem is elsewhere, and there is no need to remove the good one(s). If they test bad in circuit, then I'd remove them and test again out of the circuit. If you should connect the main terminals backwards, it won't hurt anything, but it won't work, either, so if it does not work the first time you try it, reverse the two main SCR leads, and try again.

In case you can't find part numbers or other information on your SCRs, they should be rated at 30 Amps or more, and 400 Volts or more..

Thanks will try it.

I think this is beyond my ability and knowledge of the controller. I thank you for your information and trouble.

Dave

Read this entire post before proceeding!

Yep, these are very similar to the ones I have successfully replaced, although yours appear to have the wire connector soldered to the top connection (the larger part sticking out of the ceramic towards the camera in this view.), while the ones I replaced most recently had a flexible cable coming out of the ceramic. You will need a fairly powerful soldering gun to heat it enough to melt the solder, and don't even try to unsolder it until it has been removed from the heatsink.

But before you replace the SCRs, you should probably test them. Otherwise you might be replacing SCRs, when the problem is somewhere else. Here is a good testing procedure: http://www.circuitstoday.com/how-to-test-an-scr. The author includes a switch between the triggering resistor and the trigger connection. That would be nice, but I'd just touch the resistor or a wire to the trigger connection.

They are indicated by the yellow circles:

There will be a nut, and probably a lock washer, on the opposite side of each heatsink, holding the SCR in place. From the bottom view, it appears that you may have to unmount the heatsinks in order to gain access to those nuts.

Before anything else, a bit of trouble-shooting is in order. In the first photo, I see a fuse. There may well be one or more additional fuses outside the photo area. Have you tested it/them? A multimeter on the Ohms scale should read very close to zero when connected across each fuse. If you don't have access to a multimeter, then you probably shouldn't attempt to repair this unit!

I see a power-on indicator (or possibly a switch) on the front panel. Does it illuminate? If not, of course it could be burned out, so don't use it as a final indication of lack of power. I see the transformer is rated @ 22Amps. Most 120VAC outlets are powered by breakers rated at 20 Amps or less. The short pulses of 22A used would probably not blow a 20A breaker, unless it is used pretty heavily, but you have checked for 120VAC at the outlet, haven't you? Then of course the next thing to check is for presence of 120VAC where the power cable connects to the fuses, and then on the other side of the fuses.

Good Luck, and please let us know what you find...

Although I'm a repair-it kind of guy, and hate to discard repairable equipment, I do tend to agree with Welderman. Get the replacement unit ordered as soon as possible. If your operation depends on this unit, then buy two!

I'm not at the plant, so I can't check for part numbers 'till late on Monday. Without the part number, I didn't quickly find a replacement SCR of this type. I have purchased at least one, within the last year or so, but I don't remember where. I think it was between $40 and $50, so a pair of them will cost almost as much as an entire used controller. This style of SCR is no longer used in new equipment, so they will be getting harder to find. If you can't find and read a part number on your SCRs, then you may be out of luck.

Thanks

I tested the fuse, it read .2 ohms..

That's an appropriate value. Most of that is likely to be the lead resistance. Next, find that 12V battery, lamp, and resistor, and test the SCRs. The size of the lamp and the value of the resistance are not at all critical, as long as the battery can provide enough current to light the lamp. Without going back and looking, I think I remember he specified a few hundred Ohms. Any value up to several thousand should work

For anyone reading this thread, I have since done considerable experimenting. Using a 12VDC source, newer and smaller SCRs will indeed trigger with resistances of a few thousand Ohms, but none of the 1/2" stud-type SCRs I tried would trigger with resistors much above 500Ω, and older ones required as low as 200 Ohms connected momentarily between Anode and Gate to trigger.

I tried IR 37RA120's, an IR 68-4038, and a 70C120BIL, all 1/2" stud SCRs.