Any Electronic Buffs On Here! Help!

[apprentice87]

[SIZE=medium]Hi All!! Hope someone can help!!!![/SIZE]

[SIZE=medium]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..[/SIZE]

[SIZE=medium]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[/SIZE]

[SIZE=medium]Now here is the problem…..[/SIZE]

[SIZE=medium]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….[/SIZE]

[SIZE=medium]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..[/SIZE]

[SIZE=medium] 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….[/SIZE]

[SIZE=medium]Here are my questions;[/SIZE]

[SIZE=medium]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??[/SIZE]

[SIZE=medium]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??[/SIZE]

[SIZE=medium]3, What is liable if anything to go wrong if I use the larger pot?? Could I damage the PCB in any way??[/SIZE]

[SIZE=medium]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??[/SIZE]

[SIZE=medium]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??[/SIZE]

[SIZE=medium]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??[/SIZE]

[SIZE=medium]I know very little about this sort of thing so I hope someone can help…[/SIZE]

[SIZE=medium]Thanks all[/SIZE]

[SIZE=12pt]john[/SIZE]

 

[Sidewinder]

john,

Firstly think about Ohms law, a higher resistance pot will pass less voltage/current etc.

Therefore it is not going to damage the circuit it is connected to.

You need to find out what the pot does when the wire speed increases, i.e. does the resistance increase or decrease?

Get me the make/model of both devices & I'll try to find something out if I can.

 

[ProDave]

If it IS just being used as a potential divider, and IF the source it's feeding is a high impedance, then it won't make any difference.

I would say it's safe to try it.

You might find the range of adjustment, i.e welding amps for a given potentiometer position, may be a bit different when using the remote control.

 

[apprentice87]

Hi Paul,

It is just a current control on a stick welding set, no idea what make the pot is, but it is a murex set built by stel.

I do not think the murex are too bright, as their wiring diagram in the manual is simply wrong.. It did not "look" right to me, adn when i traced the wires it was not right!!

Hi Prodave,

How do i know, or how can i find out if the load is high impedance?? It is the input signal to a PCB, but how it works i have no idea!!

john..

 

[OnOff]

I that the remote type with 3 wires? 5V, Control and an earth wire?

 

[apprentice87]

Hi Onoff,

I have no idea of the voltage used, but yes, it is a circuit like the one you describe!!

john..

 

[Sidewinder]

john

get some brand names up & we'll look to try to help

 

[apprentice87]

Hi Paul,

The welder is a murex DC203i Apparently they are made by Stel. Very nice to use, which is why i do not want to break it!!! How do i post images?? I could post a wiring diagram then!

Where can i get a crimping tool to crimp tweeny weeny spade terminals?? Not the insulated red yellow and blue sort, [You know what i mean!] but the little small uninsulated ones?? The ones on the set are crimped on and then have a plastic moulded thing pushed on them.. Faston terminals i think they are called?? Would somewhere like Total have the terminals and crimper?? I have seen crimpers on farnell and RS but the price is unprintable never mind payable, and i do not want to buy ten thousand terminals for £400 either!!!!

john..

 

[OnOff]

This link any use:

http://diy-welder.com/docs/rweld-s-manual.pdf

 

[steptoe]

like bootlace ferrules app87 ?

 

[Sidewinder]

This:

http://www.murexwelding.co.uk/manuals/other/Instruction%20Manuals/Transarc%20DC203i.pdf

Does not "seem" at first glance to show an option for a remote?

I "MAY" have missed some bits due to my current status!

 

[steptoe]

This:

http://www.murexwelding.co.uk/manuals/other/Instruction%20Manuals/Transarc%20DC203i.pdf

Does not "seem" at first glance to show an option for a remote?

I "MAY" have missed some bits due to my current status!

:slap

page 8

J1 [bottom right] page 13, Im assuming is the remote connections

 

[Sidewinder]

So the wiring diagram for this is where?

I don't give a hoot about the connection plug, I want to know how this is integrated into the control system for the welder, this is NOT obvious.

Hence my comment.

 

[steptoe]

page 13 of the schematic you linked to

 

[steptoe]

Thats not a circuit diagram - Thats a block diagram - Doesnt tell you anything for the input divider circuit

I know,

but thats what sidey linked to and said there was no option for a remote,,,,,,,,,

 

[Sidewinder]

Steps,

Stop being a prick, there is no remote option I can see on the block diagram, and that in itself is about as much use as a pair of prostitutes knickers when you need to know where to connect to and what the values etc. of the connected devices need to be in this application.

Hence, there is no option for a remote IMHO as I can't see what is needed for the spec for the remote.

 

[steptoe]

sidey, there is,

bottom right page 13

S2 - internal/external

J1 is the connection point,

yes, admittedly, it shows nothing of the connections on the PCB, but surely that shows the potentiometer being bypassed for external controls....?

 

[apprentice87]

Hi Everyone!!!!!

Yes OnOff, that is interesting!! I will have a read of that!!

Hi Steps!! Yes, i have a set of bootlace ferrule crimpers, use them all the time, i was wondering if they were the same....

Hi Paul, Yep, that is the one!! It definitely does have a remote facility for current control, it is pins 1, 2, and 3, at J1 and i have already had that working, albeit with the "wrong" size pot..!!

Now, here is where it gets interesting... For a start off the wiring diagram is wrong... See the connection between "3" on J1 and "3" on the PCB?? Well that is wrong, it actually goes to "6" on the PCB....

Anyway have a look here at this machine http://www.murexwelding.co.uk/manuals/other/Instruction%20Manuals/Transarc%20DC303iE.pdf

As you can see, this machine is very similar except it is TWO power sources in parallel in the same case! Anyway, see the wiring diagram, ALL the connections are exactly the same on my machine, EXCEPT, this machine also has pins 4 and 5 from J1 connected up to give remote arc force control too. So, i believe that the control PCB's are the same, [and so do the people that sold me the remote] although they did not realise that it was not connected up in my model!!! My scheme is bound to work [i hope!!] as my machine has already got the arc force control, same as the other model, and it is connected up to the PCB in exactly the same way too. Soooooo, all i need to do, is to do a little rewiring, and change the "internal/remote" selector switch from a dpdt to a 3pdt one, [to cater for the arc force] and off i go!!!

john...

 

[steptoe]

P14 identifies it as being a 2k2 pot

and the arc as being 10k

 

[apprentice87]

Hi Steps,

Yes, that is right what you are saying, have a look at the wiring diagram for the other machine i have just linked to, and you will see that this is wired exactly the same was as mine, except it has some more wire, and a three pole changover switch [instead of a 2 pole like i got] so you can control arcforce too! The switch just bypasses the internal pots and connects up the external ones like you are saying! Bear in mind as you will spot a difference between the two diagrams, as on the one for my machine, terminal 3 of J1 is shown as going to terminal 3 on the PCB. It does not, it goes to terminal 6 as is shown for the "other" machine!!

john

Hi Steps, that is what i am worried about... It is shown as a 2k2 pot, when i "took the back off" it IS a 2k2 pot, so i am worried about using a 4k7 one, as the set makers did not decide on a 2k2 one for reasons of fun!!

I did have this idea.... If you use a bigger pot, for a given load impedance you will have a bigger volt drop. If i measure the "input to the pot, and then measure the feed to the PCB when it is "turned right up" if the wiper output from the pot is the same as the input across the pot, the can i assume that the load is of very high resistance/impedance and thus all will be fine??

john..