Wire speed control - exponential!

[RogerT]

Hi folks
i have a Weldability-Sif MIG160T, which has been doing sterling work since my last visit here. However I’ve always found the wire speed control very hard to adjust effectively, and at last pulled my finger out and did a little investigating. Here’s the results…

As you can see, the wire speed increases linearly initially, but then goes exponential, which is no use at all. What is the likely cause (or is it just typical of a cheap machine)? What can I do to rectify this? I was wondering about replacing the control knob assembly, or adding another potentiometer (have I got the right word?) I’m a total electrical novice, but willing to learn and try things.

no photo yet of the internal gubbins behind the wire speed control knob, but that can be arranged.

Thanks in advance, collective.

 

[Guineafowl]

You can get either log or linear potentiometers. Have you got a multimeter to test your one, ie resistance vs position? The part number might also tell you.

If you find it’s a log type, then swapping to a linear one would be feasible.

 

[RogerT]

Brilliant, thanks for the reply so speedy! I’ll not be up at the lockup again today, but watch this space again tomorrow or the next day

 

[Chippie]

A=log, B= linear…..

 

[eddie49]

Here is the circuit diagram of the Weldability MIG160T welder:

Section 10 : Circuit Diagram - Weldability MIG160T Instruction Manual [Page 11]

Weldability MIG160T Manual Online: Section 10 : Circuit Diagram. Weldability MIG160T EXTRAORDINARY MAINTENANCE: DIRECT CONTACT WITH MOVING PARTS. Page 10 of 11...

www.manualslib.com

There is no detailed diagram for the wirefeed speed control board, except to show that it uses a 1K potentiometer.
If you could post a photograph of the component side of the board, it would be possible to guess at the type of regulation that it provides, such as simple current control, or more sophisticated Pulse-Width Modulation ( PWM ).

The diagram does show that, in common with many low- to mid-level machines, there is no separate DC power supply for the wirefeed motor. Instead, power is "borrowed" from the main DC welding output. This means that at higher settings of the 6-way voltage range switch, the speed control board will be fed with a higher voltage. At what position of that switch did you make the above speed measurements, and does the graph vary at other settings?

 

[minimutly]

Log law pots are usually used for audio applications, so what you’re seeing is a function of the type of circuit used?

 

[voipio]

A linear potentiometer, wired as a variable resistor in series with the motor, will also give a non-linear motor speed. How much so depends on the resistance of the pot.

 

[RogerT]

Back in the room…
Measured wire speed at different power settings (C1 is 1 on the power switch > C6 is 6…)

So they follow the same curve, and the wire speed is uniformly slightly higher with increasing the power setting.

Looked at the potentiometer

Hungyun RV28 B102

Havent been able to find online if it is log or linear, and struggling to manipulate the probes on my meter while turning the knob. Which contacts should I aim for of the three?

 

[RogerT]

Ah, traced the wires to a three pin plug, that should be easier to contact. Will do it later tonight.

 

[RogerT]

It’s linear. When the knob is at the halfway point it’s reading 500ohm. So, what can I do? Anything, or do I just have to make up a better control knob indicator and scale, and powerful reading glasses to see it?

 

[Chippie]

B102 is a 1K pot with a linear taper…

 

[Guineafowl]

It would be interesting to try an antilog potentiometer (1KC, C102 or ‘reverse audio‘ type ) to see if it cancels out the nonlinear response.

 

[eddie49]

It’s linear. So, what can I do?

If you could provide a photo of the components on the speed control printed circuit board, it may be possible to suggest modifications, upgrades, or replacement with a more sophisticated circuit.
A photo or details of any labels on the wirefeed motor & drive assembly may help to confirm that it is a 24 volt motor.

 

[RogerT]

Photos here on a previous thread…

Weldability mig160T not working.

First post, so hello everyone, and I’m hoping the collective can help me out. I bought this in 2015 I think, using it to do a restoration of my VW T25. Working fine until yesterday, when after completing one bit of welding I turned it on again for some more, but nothing happened. I run it off a...

www.mig-welding.co.uk

 

[RogerT]

12V motor

The board

Thingy Behind the fuse

Then this

and these

Moving left along the bottom

Theseareboth 100uF

 

[RogerT]

Missed these two between the two caps

 

[Misterg]

What problem are you trying to solve?

It's not uncommon for controls to have a constant ratio between settings (so the ratio between (say) "7" and "8" will be the same as between (say) "1" and "2"). This makes the response of the control 'feel' linear.

As an example, this is if each setting is 25% more than the last:

It could be that your speed control pot has worn out and isn't giving a stable setting - cheap enough to replace as a precaution.

If you need more sensitivity on the settings, you could replace the pot with a 10 turn one.

 

[RogerT]

The problem I am trying to solve is that at higher wire speeds, the control is overly sensitive, so a small adjustment makes a big difference in wire speed. I’d just like to have more control over wire speed. I don’t think the pot is worn out, from looking at the graph I produced, the curves are very similar.

 

[RogerT]

Thanks for the suggestion of a ten turn pot, misterg. It looks like it’s either that or fashioning a large diameter control knob\pointer and scale (which may in fact be fairly easy if I raid my 50 yr old Meccano in the loft…). Looking at probable best settings for the range of thickness’s that I’m welding (auto restoration) I’m looking at wire speed settings just where they start to ramp up, so it’s relatively easy to over- or under-shoot when I’m setting the speed. A ten turn pot would overcome that, now that i know what such a thing is.

 

[eddie49]

This motor speed control circuit is based on a thyristor ( silicon controlled rectifier - SCR ), the BT152, being switched on a variable time after the start of every full-wave-rectified half-cycle of the DC welder output voltage, and then ceasing to conduct at the end of that half-cycle. The pulses are therefore similar to a sawtooth waveform, rather than square, and they occur at only 100 Hz, as opposed to PWM which typically runs at 2kHz or more.
The circuit is described here:

PCB clicking.

OK Update..... Power on voltage position (0) and trigger pressed the voltage at the tip and the reading was 19.24v Voltage position at (1) and trigger pressed.....voltage was 25.3v Checked the CN1 & CN2 with the thyristor shorted. Wire feed motor was at full speed. Tested with trigger...

www.mig-welding.co.uk

and here:

A better control board

If someone has an old SIP wire feed control board,working or not,I would like to get it. If shipping is not too high that is. It seems to me that adding and moving a few components, would make the thing work far better. I would like to see if a cheap easy fix could be made. I could pay...

www.mig-welding.co.uk

and a diagram by @R Kraft here:

https://www.mig-welding.co.uk/forum/media/img_4389_5966-jpg.3519/

The 1k potentiometer biases the conduction of the small A42 transistor, which controls the trigger start point of the SCR by shunting the rising gate voltage to ground.
Although it is not traditional PWM, this circuit usually works well enough. The extreme non-linearity that you are seeing begins at very high speed settings ( "7" to "9" ), which are not often used. I am surprised that the motor is rated at only 12 volts, because it is powered by the welding output, which typically varies from 15 to 30 v.

If the great suggestion by @Misterg to use a 10-turn 1k pot - e.g.

Multi turn potentiometer and counter knob Spectrol 1K 5% model 534 DC02M7598 | eBay

Find many great new & used options and get the best deals for Multi turn potentiometer and counter knob Spectrol 1K 5% model 534 DC02M7598 at the best online prices at eBay! Free delivery for many products.

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does not help, then you could use a separate power supply and a real PWM controller:

12V-40V 10A PWM DC Motor Speed Controller Dimmer Voltage Regulator with Knob*YO | eBay

Find many great new & used options and get the best deals for 12V-40V 10A PWM DC Motor Speed Controller Dimmer Voltage Regulator with Knob*YO at the best online prices at eBay! Free delivery for many products.

www.ebay.co.uk