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Need advice on pot choice!! HELP!!!!!

Bigblueflash

Sep 21, 2013
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Sep 21, 2013
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Hi All!! This is my first post here and I hope someone can help!!!!

Now then, I have a welding set, a modern inverter type with electronic control by means of a PCB. Output current for welding is controlled by means of a pot used as a voltage divider to create a signal in the following manner. A voltage from one terminal of the PCB, feeds one end of the track of the pot which is then, in turn, led back to another terminal of the PCB. Another wire connected to the wiper of the pot also leads back to the PCB to provide the control signal..

There is a changeover switch incorporated, which allows you to switch this pot out of circuit and switch in another one on the end of a cable, for use as a remote control

Now here is the problem…..

I bought a remote control for the welder, problem is, the pot in the remote is not the same value as the one in the welder itself….

The pot in the welder is a 2k2 one, and the one in the remote is a 4k7 one…. The supplier of the remote assures me this is not a problem and it will still work ok, but I am not so sure..
I know that when used as a voltage divider the value of the pot does not matter up to a point, but, pot value WILL affect the output signal voltage from the wiper depending on load current. Sooooo….

Here are my questions;

1, As both ends of the pot track are connected to the PCB, could use of a different value damage the PCB by forcing it to provide a higher voltage or whatever??

2, What sort of current value will the control signal from the wiper to the PCB need to be?? Will this be low enough that pot value does not matter??

3, What is liable if anything to go wrong if I use the larger pot?? Could I damage the PCB in any way??

4, I assume that the makers used a 2k2 pot for good reason and did not just pluck a figure out of the air, would I be mad to fit a different pot??

5, I cannot find any markings on the pot in the set, at least not without taking half the welder apart. Is there much of a correlation between pot size and wattage?? The "body" of the pot in the welder is about 15mm square and about 8mm thick. Can I guess pot wattage from this??

6, If I need to get a new pot, what is the best sort for long life and stability and general being right for the job, a carbon track one, or a wire wound, or a plastic film, or cermet one or what??

I know very little about this sort of thing so I hope someone can help…

My area of expertise is welding [properly qualified] and engineering machining, so if anyone has any questions regarding these, fire them off to me!!!!

Thanks all

john
 

john monks

Mar 9, 2012
685
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Mar 9, 2012
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What is the make and model of the equipment?
Your questions seem vague in a way. But if you are supposed to used a 2K pot and if the pot is used and a potentiometer, that is a voltage divider, not a variable resistor, then you can used a 4K pot with a 4K resistor in parallel with the two legs, connections that are the furtherest apart from each other.
 

Bigblueflash

Sep 21, 2013
2
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Sep 21, 2013
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Hi John,

My questions ARE vague as i have no idea what i am doing really!! It is a Murex DC203i welder.

I like your idea!!!! Yes, the pot IS used as a voltage divider and NOT a variable resistance. If i add the 4K resistor, will that still give me the same output voltage and current as if i used a 2k2 pot in the first place?? Sorry to sound thick, but i am where electronics are concerned!!!!

john....
 

john monks

Mar 9, 2012
685
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Mar 9, 2012
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685
Yes because there is no current passing through the wiper, that is the center pin.
 

Rob_K

Sep 20, 2013
59
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Sep 20, 2013
Messages
59
You can check what rating a pot is, with a multimeter set to ohms, make sure the welder is unplugged (checking resistance with a multimeter is means the meter is passing a current and reading the results, not just reading a current) turn the pot completely one way, then just probe the wiper and either of the fixed ends one end should read zero, the other end will be the value you are after. Turn the pot the other way, and the readings reverse. There should be no continuity between the two outside poles. You might have to probe a little, get some crocodile clips for your multimeter or something so you can turn the pot without needing 3 hands, eventually you will see the current change as you turn the pot, turn all the way up and that's your pot rating. Make sure you are reading you meter right as well, I find they can be not very intuitive, mine has it settings as 200ohms, 2K, 20K, 200K, etc. these are the maximum it can check for, so if you want to check for a 2K or 4K pot, set it on 20K if the meter reads 1, go up a setting. Why they can't just go up in powers of 10 to the power 10 I don't know.

This is all assuming that you can get a direct line to the pot, either directly or from a wire. Any components in between will obviously give you something different. Hope this helps.
 
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