welder repair
Hello all,I am new here and would like to get some help with my welder that I am trying to repair.I need to replace the 4 resistors in the center of the picture.They are .75 ohm 20 watt and I have been having a great deal of trouble finding that exact combination.Would it be alright to use 1 ohm 20 watt instead or do I need to use some combination?Thanks 50cal
Last edited by 50cal; 1st February 2009 at 03:13 PM.
How do you know they are bad? In order to read the correct resistance in most cases, one side has to be disconnected from the circuit. And I don't see any fresh solder. It is possible that the heat from the resistor has oxidized the solder joint if it was made with 60/40 solder. The rig dos not look commercial, so that is possible. If the resistors are not open then I think I would try re-soldering them with silver solder. Clean all old solder of the wires first .Originally Posted by 50cal
I guess you don't have the circuit so we don't know what the resistors are doing or how critical they are.
Last edited by Rolf; 31st January 2009 at 03:11 AM.
If you use the 1 ohm 20 watt in parallel with a 5 ohm 5 watt and a 10 ohm 5 watt it will be pretty close to 0.75 (0.769). These are all commonly available.
Also, on ebay, WestFloridaComponents has lots of 5 1.5 ohm 20W resistors (2 in parallel make 0.75 ohms) for $5.89 plus $3.85 shipping - that's about two dollars per resistor.
Here you go, friend -
RH025R7500FE02
.75Ω, handles up to 25 watts, high precision, insulated metal body that screws solid onto that chassis instead of flapping around in the breeze like them ceramic doohickys you got now.
Thanks duffy,I looked all over there sight and vishays and the closest I could get was 1 ohm your the Man50cal
I have another question on this same welding machine.I need several transistors,and I see mouser has them but there are several different brands and price varies part #s 2n6055,and 2n3055 origionals are motorola.Are any of the brands more reliable than others?The motos lasted 23 years I have had this welder since new and this is the first trouble it has given me.I do have the owners manual but my printer scanner software is not compatable.I will work on that.Thanks 50cal
Can't say, but if you're going to shotgun the thing, those transistors are a good bet. Do you know how to check transistors with a multimeter? You use the diode check function. NPN bipolars like that will read as two diodes, cathode-to-cathode for collector-base-emitter. The 2N6055 is a darlington, one junction will read 1.4V, all the rest should read about .7V one way, open the other.
Not sure what shotgun means in this context,but I have several,if its like parts changing,no I have been working with a welder technician online and doing the tests he has suggested.From the symptoms the welder has he has assured me that I am going in the right direction.As I understand it this is the fine tune circuit for welding current.The resistors that I want to replace are fusistors on the schematic.I questioned just using a resistor and he siad they dont use fusistors anymore and to just use resistors.I think there is plenty of room on the heatsink for fuses or circuit breakers of some sort if it would not be overkill?In the test that I did there was deffinatly a open circuit on 2 of the resistor transistor combinations so the tech suggested replacing all.But since parts are all but non excistent,here I am
That's the shotgun repair technique. You get it down to as small a subsection as you can and just replace everything in the hopes that you hit it.In the test that I did there was deffinatly a open circuit on 2 of the resistor transistor combinations so the tech suggested replacing all.
Hey, a schematic! Ugh, another Darlington drive arrangement, and common emitter at that - doubles the heat in the drive transistors just to save a few parts. I think the fuses are a fine idea. Tough to guess what the original designer had in mind, but the 2N3055 is rated for 15A. Do the fusistors have any markings on them?
The transistors look like they are in sockets. This makes them easy to replace, but unfortunately in this case you might want to replace those sockets, too.
Digi-Key - 4601K-ND (Keystone Electronics - 4601)
Take a real good look at the old ones - I've seen those TO3 sockets get corroded and start causing problems. Corrosion increases the contact resistance, and it doesn't have to go up by much when you're dealing with a fraction of an ohm circuit like this. Just a little corrosion can ruin the transistor from a voltage drop or just plain old resistive heating. Don't forget to check and replace those lousy sheet-metal screws, too! They are part of the circuit. Be a shame to go through all this then kill the new parts off because of a few rotten fifteen-cent screws.
Last edited by duffy; 1st February 2009 at 09:32 PM.
duffy,thanks for taking time for this, here are the markings on the fusistors.
405196-1p-501
.71Ω 10% 2148024
thanks for the tip on the sockets,I will be ordering parts tomorrow.Northern sconsin I was at Coloma last spring for classes that related to work nice country.50cal
Afraid I can't tell anything from the numbers. Try 12A fuses, so it's a little lower than the transistor's rated for. Use fast blow.
7' of #30 AWG = ~ 0.75 Ω but the temp. coeff. of resistance is a bit high. In shorter lengths you could use it as a trimmer resistor.
Last edited by Willbe; 1st February 2009 at 11:30 PM.
Uhhh, 12 amps or more through 30AWG - ?
Nah, it melts at 8 to 10A in free air; I thought somehow the current was lower.
High Voltage Fuses
My bad.
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