welder trouble

[tonggy]

Okay I took the heat plat with diodes off the body, here are the ID on the diodes. On the left side of the plat shown on the picture one its T25-u-k8 top and bottom and the right side is T25-u-L8. the diodes are riveted on to the plat. looks like the white ceramic O ring is crack in half, does this mean anything? the diodes look like the two of the T25-u-L8 are weld on the O ring together and that goes the with the T25-u-K8. the reading on the T25-L8 .667 ohms and the K8 is .648. This is not right,is the meter suppose to read any voltage at all because I haven't even touch the two ends of my meter and its reading AC 55.6V? I'm not even sure if the reading is going to be right, if this was not suppose to happen with my meter.

 

[user_88]

When you test the diodes, you cannot use the resistance range of your multimeter.
The diodes, when working correctly, turn on with a forward bias of 0.5 to 0.7 volts, and when reverse biased, will give a higher voltage reading. Does you meter have diode checker on it someplace?

When you check the AC voltage at the diode bridge, you have to have the multimeter set to the AC range. Did you read 55.6 V AC volts at the AC transformer connection points of the diode bridge?

 

[tonggy]

hey, well my meter just read 55.6v, now this is before testing the ac connection. And also the meter does have a separate diodes testing area but that doesn't still explain my meter voltage issue. it's not one thing its another funny huh, Okay I've tested the diodes when the power is off its reading was .556 and with power on, it read nothing is this bad or am I doing it wrong. I thing I'm doing something wrong here and also the ac reading when power on its 3.3v

 

[user_88]

... Sounds like you have correctly tested the diodes in one direction .... with power off ..... with the diode range on meter .... getting a reading of .556. Now, the next step of the diode check ... with the power off, using the diode range of meter, check the meter reading after you have switched the + and - test leads on the diode. This test, the reverse diode polarity check, will tell you if the diode is good in the reverse direction.

In the reverse polarity diode check, you should get an off-scale reading.

If the reverse polarity check gives you a low reading .... .1 or .2 or whatever, then that diode is bad, even though it tested good ... .556 .... in the forward direction.

 

[tonggy]

okay here is what I did, on the left plat I did(- to +)and its read nothing. then try (+ to -) it show .569. the other side (- to +) it show .659 and (+ to -) it shows nothing. what your telling me is that both side should show at least something or close to it. if it doesn't come close to what I said then its bad diodes!!....

 

[user_88]

If I understand correctly, you have checked the diodes correctly.
The .569 and .659 sound about right for forward bias.
The off scale ... or no reading .... in the reverse bias direction is also correct .... meaning that the diode is good .... not conducting in the reverse bias direction.
Were you able to check each diode individually .... for each of the four diodes?

 

[tonggy]

yeah they all came out close to the same or a little off. if this mean that the diodes are good then I'm a summing that the transformer is the cause of the problem. replacing the transformer might cost me. Its not that I don't have an extra welder, I just want to see if its low cost to repair. But it looks like I'm out of luck huh? If we cant find anything else that might cause this I just want to thanks for being patient with me and also for your help...:

 

[user_88]

If you have the diode bridge lead wires disconnected, you might want to set your meter to the AC voltage range, and put the test leads on the AC terminals of the diode bridge.

There should be two wires, going from the diode bridge to someplace on the transformer, or at least in that direction. These AC wires would be the test lead connection points.

For safety, unplug the welder, connect the test leads, and then plug the machine in and turn it on. Check the AC voltage on both heat ranges.

The problem might be a thermal protection switch, on the primary side of the transformer ......maybe it is defective, or not completely conductive.

It looks like the diodes are good. So it must be something else.

 

[awright]

tonggy, did you test all four diodes or just two? If you got those results on each diode individually, they are good. But you only reported two values, so I'm asking.

What you are trying to do is determine that each diode conducts current when forward biased and blocks current when reverse biased. Forward bias is with a more positive voltage on the anode (the rear of the arrow symbol) and a more negative voltage on the cathode (the bar across the tip of the arrow.) Reverse bias is just the opposite.

Since conventional ohmmeters gives idiosyncratic results when measuring other than pure resistances (due to differences in voltage applied by different ohmmeters and due to differences in forward voltage drop of semiconductor junctions), the "Diode Test" function has been built into DVMs. This function works by applying a voltage above any ordinary semiconductor junction forward voltage (2.6 volts and 3.7 volts from my two Fluke DVMs) with a series resistor to limit current, then displaying the voltage developed across the load. If the DVM detects no load current, it defaults to "OL" on my Flukes or some equivalent message indicating blocked current flow on other meters. So what you are actually measuring is the forward voltage drop of each of the four diodes in your bridge, individually and assuring that there is no reverse current flow with the leads reversed, as indicated by the displayed "OL."

You can do this with the diodes still connected in the full bridge because the other three diodes will block stray paths of your test current. However, the transformer secondary must be disconnected because it would effectively put a second diode in parallel with the one you want to test, making it impossible to distinguish between the two diodes.

Normally the MOV could be left in the circuit and ignored, but you indicate that it is a very low resistance, so it could also disrupt the diode evaluations. A good MOV would look like an open circuit to a diode tester or ohmmeter because they only start to conduct above their rated voltage - probably a hundred volts or so for your machine. It is only there to protect the diodes against transient voltage spikes.

The reason for this long explanation is to be sure you understand what information we are seeking and to be sure that you have properly evaluated all four diodes individually. (You do not have to disconnect the diodes to do this.)

I am also wondering about your evaluation of the MOV. If, in fact, it has resistance of only 1.2 ohms, not only is it defective, but it would have burned out dramatically and very visibly only a few seconds after turning on the welder. 55.6 VAC across 1.2105 ohms would dissipate about 2.5 KW. The MOV would smoke in seconds. However, it looks quite normal in your photo. Therefore, the measurement of the resistance of the MOV, the measurement of transformer secondary voltage, the transformer winding(s), or the diodes must be defective. That's what we are trying to zero in on.

So please measure the resistance of the MOV again and let us know exactly what the DVM displays. Any "K" or "M" in the display? Measure the transformer secondary voltage UNDER LOAD. Use a power resistor, a hair dryer, a space heater, a water heater or stove element a water load, or whatever load you can scrounge. Connect the load directly across the isolated transformer secondary. At 56 volts the load will be dissipating only about 1/4 of its rated dissipation. Put the load in a bucket of water if it gets too hot to allow you to make the measurement. To avoid damaging the fan motor, don't leave a fan running at low voltage for a long time.

While you have the transformer driving the load, measure the voltage across the primary LEADS (not the switch). If you have normal input voltage on the primary LEADS and very low voltage on the loaded secondary LEADS, you are screwed. It would cost you more for a replacement transformer or a rewind than a whole new machine.

awright

 

[tonggy]

well yes I did tested all four diodes and the 55.6v reading was on my meter with out any testing at all. All I did was turn on my meter and put it on AC I never touch the two ends of my meter to the welder parts, it just started reading 55.6v and it stayed on that reading, still never done any testing. Okay I retested the MOV I started putting the positive on the left end of the MOV as the way the picture is showing its reading at this point 7.47m, then try the opposite putting the negative on the left it reads nothing.

 

[tcmtech]

Been following this for abit. I do alot of welder repair. Its part of my businness.
I have not seen or may by I missed that answer to that all important question.
When you turn it on and pull the trigger does the power transformer hum or growl or inductive pulse rattle the body! The reason I ask is to find out if its actualy powering up.
If it is, take a piece of welding wire and tie it to the positive and then negative side of the rectifiers, dead short. turn it on again and pull the trigger if it blows the wire apart they are good.
If not try it again on the transformer leads going to the rectifiers.
if you have growl but no output on a dead shorted secondary the winding broke open and your transformer is junk.
Also those little wire feeds are constant voltage. the open circuit is usualy in the 15 - 25 volt area.
I have worked on those small welders and I can say with good confidence that the rectifier assy is usualy junk! piss poor diodes. Under sized both in voltage and amperage rating.
They usualy pop two of the four and go open circuit.
Most poeple run those little machines at the limit and the diodes overheat and burnout inside their own case but dont actualy burn though enough to become visable burn outs.
that mov actualy is there to protect the diodes from the inductive kick back from the big filter choke while your welding. Its usualy to small in cappacity or to high of voltage rated to actualy do any good protecting the under sized diodes.
Another thing to look for is bad connections between the choke coil and the lines in and out of it. very few have copper windings they are almost always aluminum.

Some manufactures actualy go to the trouble of copper platting the aluminum to pass it off as copper or so they can solder the alumiunum with cheap tin based solder. The copper aluminum bond breaks down and you get a bad connection that looks good and even ohms good, until you try to pass any amps through it!
Been there seen that, too many times.

They learned that from miller and lincoln! they were doing that in the 70's and 80's.
They even put "copper used thoughout" right on some of the machine faces! I have two commmercial 600 amp and one 800 amp units I bought to scrap for the copper. I even scraped the transformer leads to check and make sure they had copper windings before I bought them.
Went to scrap them and those three coils that should have wheighed about 80 to 100 pounds each weighed about 15. The end of each lead had a thick copper plating about 18 inches back into each coil.
A good buddy of mine runs a scrap yard and has about twenty tons of lincolns, millers and a few others that are built that way too! He cant get anything for them. The smelters wont take them because of the aluminum and no one wants to bust them up to separate everything. He paid to much and now they are not cost effective to tear apart.
If you burn them the aluminum melts out but is contaminated with all the winding insulation residue. Zero value! And the 300 pound iron cores are only worth a few dollars apiece.

 

[user_88]

... It sounds like some useful information/experience from tcmtech.
Maybe tonggy, the o.p., can reassemble his diode bridge and use the shorted wire test to check it out ... and probably also try the transformer shorted wire test.

 

[awright]

A MOV is non-polar and should read the same in both directions. It is basically a pellet of sintered granules with leads on both sides so has no mechanism to develop a polarity. Is it possible that you were holding the probes on the MOV leads and the meter was reading your body resistance? Anyway, it looks like your MOV is good. Seems that your diodes are good.

I am perplexed by your description of your AC voltage reading. Are you saying that you never touched the meter probes to the transformer wires? If that is what you are saying then your meter simply picked up an impulsive kick from the transformer and the reading doesn't mean anything. Apologies if I am misunderstanding your description.

You want to put a load on the transformer secondary leads using strong clips or lugs that can carry the current. You can get copper lugs that tighten onto the transformer leads with a set screw or you can get really large "wire nuts" that bind two wires together. Look around at your hardware store.

The purpose of the load is to draw a few amps to be sure you are not just measuring minute current leaking across a tiny break in the winding or voltage capacitively coupled across adjacent layers of the winding. Your DVM voltmeter draws such low current that leakage paths like these can give you a reading that looks good but the transformer can't actually supply any significant current. By loading down the secondary you avoid false readings due to leakage.

You should attach your meter leads to the two secondary windings (with the load attached) using clip leads. These can be very light duty clip leads like you can buy at Radio Shack. Set your meter to "AC volts" and, if necessary, set the range switch. Turn the welder on and take your reading. Shut the welder down, pull the plug, and remove your meter clip leads. You can't make this measurement without having the meter leads attached to the transformer secondary wires.

tcmtech, if the schematic in the manual linked to by tonggy is accurate, the gun trigger has nothing to do with supplying power to the transformer. Like many of these small machines, the gun is always hot (which makes for some rather spectacular visual effects on occasion). I think tonggy should forget about the gun trigger and pay attention to the on-off switch and where his gun tip is.

awright

 

[tonggy]

Wow that's a lot of information, first I'll try the shorted wire test then I'll try to focus on the on and off switch too. And Awright about the voltage testing, the way you had describe it is correct so no need to apologies. I'll keep in touch and let you guys know how doing on my issue, thanks again.

 

[tonggy]

okay here's what I found out, the good thing is that I did some testing on the bottom of the transformer and it read 27.8 volt and also revers testing it also read 27.8 volt. did some cross wire on those two lead my welder started to vibrate like crazy, before this it was humming like normal. now I did testing on the top transformer the voltage on that is 3.4v not very much I don't know if that's right voltage that was suppose to come out. I think its not good.

 

[tcmtech]

your right about the gun trigger in the schematic. It was way past my bed time when I looked at it. Plus I have worked on so many different machines I just made the asumption that it turned on and off durring the weld cycle. (like a real CV welder)
I used to tease a few of my cheaper cutomers about buying those low low end machines. I told them that if they need real power pull the transformer out of a 250 amp peak battery charger! Stick it in the little welder!
But really, some of those little runts do a fair weld with gas and .023 wire!

 

[tcmtech]

That top transformer is not a tranformer. in your schematic I belive you will see its the Z choke on the output side of the rectifiers. Correct welder parts term for it is a reactor. Thats to give the DC a little inductance to "wet Down" the arc puddle its purely an arc smoothing device, without it you get a very sputtery "dry looking" and crappy weld!
the 3.4 volts your getting is purely inductance leakage from being attached to the top of the power transformer.

 

[tcmtech]

asuming your testing from top to bottom on the trasformer secondary for the 27.8 volts that would give you 13.9 at the center tap. DC output voltage would be around 19.65 volts open circuit. if C1 is good. About dead center of the 15 -25 volt I estimated.
I would say your transformer is probibly Okay.
I am still leaning heavy on the bad diodes though! A great replacenent is made by international rectifier they are 400 volt 125 amp rated stud type. I dont recall the the exact part number but they were aound $4 each. I used to buy them 50 at a time when I worked at the welding supply store.
Permanent cure for the bad diode blues on 120 volt and small 240 volt powered machines!
but any good diode rated at 200 volt and 100 amp or more will solve the rectifier problem in your small one.
hope this helps.

 

[tonggy]

So I guess I should just go ahead and replace the diodes, at least if its not the diodes I have it for spare just in case huh! it doesn't cost me that much to replace the diodes lets just do that first. I hope this resolve the headache wish me luck..

 

[awright]

tcmtech, I also noticed the obvious reactor on top of the main transformer in the photo, but don't see it in the schematic. Are you going to believe your lying eyes or the schematic???? If we can't believe the schematic, what can we believe in? The schematic actually has the same model number as the number tonggy gave us at the beginning of this thread. My faith in CH is being shaken.

Anyway, I agree with you on the function of that reactor.

tonggy, I gather that you did measure the transformer secondary voltage with a load? What kind of load?

I always hate to replace parts on the chance that the replacement might work without having actually found the problem. Your diode test seemed to prove that the diodes are OK but that was with no load. While I agree with tcmtech that the diodes are probably marginally suited for the task, at best, nevertheless, if they are still OK it is a waste of time and money to replace them experimentally.

Assuming that you did measure the transformer output with a load, how about reconnecting the diode bridge, (checking all other connections for tightness along the way) and looking at the DC output from the bridge UNDER LOAD - even under the light load you were supposed to use on the bare transformer leads?

I see no transformer secondary center tap on the schematic - just a full wave rectifier bridge. tcmtech, did you download the manual (with schematic) that tonggy posted?



Not much there, and no transformer center taps or reactor shown (although the primary winding is tapped for Min/Max output selection).

awright