Electronics 2000

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

To answer your questions:

1. The pot is probably powered by a constant voltage, most likely the same voltage that powers the control circuitry of your welder. In this case a higher value pot will do no harm, but the higher impedance of the resulting control voltage on the wiper might be a problem. A lower value pot might cause excessive current to flow, which might damage (most likely) the pot itself.

2. The pot most likely supplies a control voltage, the current on the wiper will be very low. The value of the pot will not be that important, depending on the voltage across the pot anything from 2k2 up to 10k will do.

3. See 1.

4. See 2.

5. Most standard 15mm carbon track or cermet pots have a power rating of about 100mW.

6. Cermet pots are usually more stable and longer lived than carbon track pots, but also more expensive and harder to find. Wirewounds are rare except in certain niche applications - high power and high precision multi-turns.

Can you open the remote box and check the type of pot it uses and the voltage it has across it?

Hi there, thanks for that!! I cannot see any markings on the pot, unless i dismantle half the welder to get a proper look. I will try to measure the voltage across it too and see what i come up with. I did have this idea, but i do not know if it will work; 1, Measure the voltage across the pot. 2, Turn it all the way up, [to give the largest signal voltage] and see if the signal voltage is the same as that i measured across the pot. I assume that if it is, the impedance of whatever the pot feeds is high enough that the control voltage is not being dragged down... Do not know if this will work or not though, as i know practically nothing about electronics. I do know a fair bit about electrics in general, but it is all wiring up three phase distribution boards and that sort of industrial stuff!!

john

The fact that the original one is a 2K2 pot says that the input impedance to the PCB is relatively high.
Your bigger (resistance) pot might not work with the same % resulting power relative to the position of the pot around mid range but I don't see it being a massive problem. The only issue I can see is if it is in series with other resistors, the potential divider may be thrown out a bit. If it is directly across a power supply rail (which is low impedance) you probably wont see a massive difference.

Actually it is a great idea. I think you should be go forward about your own sequence.
Thanks!!!

For your project work you need to have a strong scene. and keep it up.
Thanks!!!!!

The only issue I can see is if it is in series with other resistors, the potential divider may be thrown out a bit. If it is directly across a power supply rail (which is low impedance) you probably wont see a massive difference.





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