Tradesmig 245 mystery component

[Barlidge]

Was inside my tradesmig investigating wire feed issues which I believe is the pot and have a new one coming tomorrow but while there spotted what I think is some kind of 'add on' resistor which has at some point broken one of its legs as it was just left hanging.


.

Any ideas as to its purpose and a suitable replacement? (google wasn't very helpful for the original).

Thanks

 

[Seadog]

27Ω 16W

https://www.ebay.co.uk/itm/0-01-5k-...e-Wirewound-25-50-100W-Resistor/183603453219?

 

[dan treg]

is it not 27k or am i not reading it correctly

 

[Seadog]

Here's another similar, lower wattage

http://www.ibselectronics.com/ibsst...27rk-7-watts-27-ohms-10-radial-lead-rohs.html

 

[Barlidge]

Thanks, any ideas on what its purpose is? And should it be mounted somewhere it can dissipate heat?

Cheers

 

[Seadog]

any ideas on what its purpose is? And should it be mounted somewhere it can dissipate heat?

Nope. If someone has a circuit we might be able to arrive at an answer. I assume the detached end is at the resistor body.

 

[Barlidge]

I'l dig around, I believe there is one online. Off out now so will be tomorrow.

 

[eddie49]

It is a 27 Ohm bleeder resistor, used to drain the voltage from the DC capacitors.
There may be two thin wires attached to the capacitor bank ( as opposed to the heavy bus bars there ) which go to a pair of normally-closed ( NC ) contacts on the main contactor ( heavy-duty relay ). The resistor should be connected on the other side of that pair of contacts. When you press the torch trigger, the contactor pulls in, this pair of contacts opens, taking the bleeder resistor out of the circuit, and the capacitors charge up. When the trigger is released, the contactor lets go, so the NC contacts close, putting the 27 Ohm resistor back across the capacitors and draining them down. This ensures that there is no residual voltage, which would otherwise cause a "live torch" effect, and give a small spark at the end of the MIG wire before you press the trigger.
The vibration of the contactor clacking in and out often causes the solid copper resistor leads to fracture. If you look carefully under the screws on the contactor, you should be able to find the broken resistor leg still clamped in there.
EDIT: I think the resistor lead is there on the right, with some orange sleeving on it.

 

[eddie49]

Some wirewound resistors use brass end-caps pressed onto a ceramic tube. You may be able to scrape the paint off and solder the broken wire back on. Using flex instead of solid-core may prevent it fracturing again in the future.

 

[Barlidge]

It is a 27 Ohm bleeder resistor, used to drain the voltage from the DC capacitors.
There may be two thin wires attached to the capacitor bank ( as opposed to the heavy bus bars there ) which go to a pair of normally-closed ( NC ) contacts on the main contactor ( heavy-duty relay ). The resistor should be connected on the other side of that pair of contacts. When you press the torch trigger, the contactor pulls in, this pair of contacts opens, taking the bleeder resistor out of the circuit, and the capacitors charge up. When the trigger is released, the contactor lets go, so the NC contacts close, putting the 27 Ohm resistor back across the capacitors and draining them down. This ensures that there is no residual voltage, which would otherwise cause a "live torch" effect, and give a small spark at the end of the MIG wire before you press the trigger.
The vibration of the contactor clacking in and out often causes the solid copper resistor leads to fracture. If you look carefully under the screws on the contactor, you should be able to find the broken resistor leg still clamped in there.
EDIT: I think the resistor lead is there on the right, with some orange sleeving on it.

Some wirewound resistors use brass end-caps pressed onto a ceramic tube. You may be able to scrape the paint off and solder the broken wire back on. Using flex instead of solid-core may prevent it fracturing again in the future.

Thank you kind sir, yes the other leg was still under the clamp and yes those two wires are present and go to the contactor in the pic, there was only a tiny stump left on the resistor but I did manage to solder it back on and test it and indeed with it connected 10 seconds after releasing the trigger there is no live torch

I would like to replace it and mount it so it doesn't break again, is one those listed above oK?




With regards to the wire speed potentiometer, I was looking at it as I've always struggled with consistency and dialling in the settings. I mapped out the wire speed against the settings and was surprised how coarse the resolution was, there was an old post on here regarding a tradesmig 130 which led me to believe it should have a linear pot fitted and mine actually had a logarithmic one fitted, on close inspection it didn't look original as the plastic shaft showed sign of being cut down. New pot came today (love Prime) and fitted and remapped against the settings and the resolution looks to be a lot more workable now. I am not sure I will ever need the upper end feed rates so wonder if its worth turning down the max feed pot on the board?

If any one else has a similar thing on one of these the pot I used was part number 'rv24yn20s b103'

 

[eddie49]

I am pleased that everything has gone well!
Yes, the first resistor that Seadog linked above ( the gold-coloured 27 Ohm 25W ) would be ideal as a replacement. It has the additional benefit of an aluminium case with mounting holes, so it can be rigidly mounted, and will therefore be better than new.

 

[Barlidge]

Replacement ordered, many thanks Graham and Eddie