Electronic circuit help

[johnrev]

Thank you for that charliecambs. I admit I think I've well overstressed about this speed controler. I know most on here who've done the various Sip wirefeed mods have had a 2 to 4amp supply and have had no problems so either it's perfectly fine or they have been lucky- and I doubt everyone has just been lucky. That includes me as I did a Topmig a couple of years ago and used a 3.5amp HP laptop switchmode block supplying the standard board which worked fine for the few months I had it.

My worry was that on this occasion I'm using a PWM controler instead of simply supplying the power to the original Sip board which uses a transistor with a 100volt and 25amp rating. Way overspec'd for the job but therefore probably pretty safe driving an inductive motor load sometimes under non-ideal conditions. I have no idea how rugged the output components of the 'Chinese' PWM board are so thought it best to make sure they were well up to the job and well protected. I'm an old worryguts.

I'm ashamed to admit it but I have a City and Guilds Industrial Electronics 224 qualification and can't even remember resistor colour codes any more.
Mind you it was about 45 years ago and I haven't had much to do with electronics for the last 40yrs. Then it was building circuits with discreet components and the odd 555 or 741 and digital stuff was only just getting mainstream. Times have moved on but sadly I haven't.

John

 

[eddie49]

I'm ashamed to admit it but I have a City and Guilds Industrial Electronics 224 qualification and can't even remember resistor colour codes any more.
Mind you it was about 45 years ago and I haven't had much to do with electronics for the last 40yrs. Then it was building circuits with discreet components and the odd 555 or 741 and digital stuff was only just getting mainstream. Times have moved on but sadly I haven't.
John

Similar story for me, except that it was 50 years ago ( 1970 ) that I failed my electronics degree course. Got a job as a computer service engineer ( 'cos I wanted a free shiny J-reg company car ) and made it last 44 years, on the back of Texas Instruments 74-series TTL with some 2N3055s and a bit of UNIX added in.

 

[johnrev]

Back then I wasn't at all interested in computers- couldn't see the point of them for ordinary folk. Friend had an Amstrad something or other which seemed to get so hot you couldn't touch it and he never managed to get it to 'do' anything worthwhile. 'It'll never catch on' so I went back to engineering as a trade with better prospects.
Pretty inciteful of me eh!

Lovely sunny morning so I imagine there are some things wifey has in mind for me in the garden- oh joy

Stay safe Eddie

John

 

[johnrev]

Don't know if this is of any interest to anyone but thought I'd just jot down what I've done to get the PWM speed controler up and running.

I was (probably unnecessarily) concerned that my 5amp PWM board might not be up to the job on my new (very old) Autoplus 150. With advice as above I changed the output transistor, a Tip 122, for a better Tip 142 to give higher current capability. Looking at the above circuit, there is a link which needs to be removed if operating at higher than 12volts. This isolates the output from the main part of the circuit which needs the 12volts to run. To power this part I put a 7812 voltage regulator in place of the link to supply the 12volts from the 24volts I am supplying from the switchmode supply to the motor. I doubt many will have the same board as me so the foregoing is probably of interest to no-one. Yawn yawn- sorry.

Any road up this works fine and the motor seems to deliver the speed range I expected right down to so slow the drive roller can only just be seen to be moving up to quite a fair lick. I haven't run any weld yet as I've only just finished the conversion and haven't tidied up the wiring yet.

A few more boring details for anyone thinking of doing the mod.

I chose the system where the output of the original motor speed board is simply used directly to operate a relay. This in turn is used to switch the output of the PWM board to the motor.
So in effect the Laptop switchmode power supply is connected directly to the on/off switch so is on as soon as the welder is on. This is connected to the PWM board supply which is therefore also switched on with the welder.
The output of the PWM speed controler goes to the motor (which has been disconnected from the old control board) via the normally open contact of the 24volt relay.
This relay is directly connected to the old control board in place of the motor. Pressing the trigger on the gun gives the usual output from the old board but instead of starting the motor it now closes the relay contact. With the power set to minimum and trigger pressed the old board speed pot is turned up until the relay pulls in (it only took a low speed setting on the pot to pull in). I have yet to try this while welding and may find the voltage drop will then pull the voltage too low for the relay so I may have to increase the speed pot a tad although the hystersis of the relay should stand a drop once activated) I'll see. On any setting above minimum I imagine the voltage will be fine and works perfectly at the highest setting during testing. If the relay burns out I'll know it wasn't 'fine'.

There was a little wire run-on after releasing the trigger and although not a lot I decided to use the normally closed relay contact to short the motor for a power stop. I am not happy with a vicious stop from a dead short so put a 2ohm resistor in series with the shorting wire to make it slightly more gentle. Me being paranoid again. The result is a pretty instant stop so no problem with run-on.

I used two pieces of 12mm ply screwed to the inside walls of the welder to mount the extra bits on. The new speed controler pot replaced the old board item and the old board was simply moved to a new position out of the way but accessible for adjustments if necessary. My OCD kicked in again when I mounted the new PWM speed board and I mounted a small PC CPU fan just above the board to blow a bit of cooling air over it. This is run from a small 12volt 'brick' power plug from some long deceased bit of electronics and is also connected to the mains switch.

Next step is to do a bit of welding and see that all works as it should. If OK I will then need to sort out the euro torch which is sloppy in it's socket and has a badly rotted rubber sheath.

 

[Seadog]

Any photos?

 

[johnrev]

Hi Seadog. Taken a few pics but not all that wonderful. My camera tells me it's in 'green' mode and won't let me exit. No idea what green mode is.

One or two pics show the total mess I'm in at the mo but have now cleared a path along one side and started the roofing today so think I'm getting there.

 

[johnrev]

Still got to tidy and insulate some wiring once I've tested the results properly. Weather today had me outside doing my roof and will carry on with that while I can. At some stage I want to empty half the shed and concrete the dirt floor nice and level. I also hope to take half the carport for an extension to the shed for a working area. Not sure what to do about the Morris Traveller as it really needs a fair bit of tinwork replacing but it's really difficult working outside on the gravel.

 

[johnrev]

Could any one tell me what I need to use the SCR in pic as a standard diode? Can't find much info about them on the net but believe they're about 200amps. Four of these on heatsinks were donated to me and want to keep for use as spares for one of my welders or for future welders.

They are Westcode N105PH06 and have two thin wires which must be the gate and anode. Can I just put a resistor between these wires to permanently switch the gate and if so, could someone suggest a suitable value to try?

Thanks
John

 

[Seadog]

Try a pot but I'd guess around 33k. That's based on around 10mA gate current. The maximum for this device it 150mA so the resistor may actually be a lot lower than that since I can't see a minimum figure for triggering.

Get a 47k pot, start at maximum resistance and work down until it conducts.

 

[johnrev]

Thank you seadog I was hoping you'd be about.
I spent a least an hour searching for that info last night and couldn't find anything on the SCR. Even though I'm sure I found some data on it a while ago I found nothing this time. I couldn't even find anything on using an SCR as a diode replacement. The vid explains it perfectly.

Thank you for the help yet again
John

 

[Seadog]

Glad to be of help, John.

This link was the second hit. Good luck

https://www.datasheets360.com/part/detail/n105ph06/7942432532498109517/

 

[eddie49]

The N105PH06 is a 600 volt 110 amp SCR. The two thin wires are the gate ( white ) and the cathode ( red ). The average gate power needed to switch the device on is 2 watts. Normally the trigger voltage ( typically 3v to 6v ) would be applied between the gate ( positive ) and the cathode ( negative ). To drive the gate "on" all the time, you can connect it to the anode, via a dropper resistor - the value will depend on what voltage is being applied to the anode. In other words, are you planning to use 4 x SCRs as a diode bridge for AC mains, or some lower voltage? The video above states that as well as a resistor, a diode has to be included in the circuit linking the gate to the anode, to prevent reverse bias being applied to the gate.

 

[johnrev]

Oh! thanks Eddie. I thought 'It' was current max but obviously its 'It(av)'. Not as much use as I thought then. I had thought they would be useful to make up a bridge to replace a dead or dying one if I needed one in a future welder. They look a lot more 'chunky' than I'd expect for 110amp- old tech I suppose. Also thought the red wire was anode and reverse bias didn't matter and just shut the scr down on opposite AC cycles- got everything wrong it seems!

Guess I could use them in a SCR power control on the input of a welder for smooth power regulation. There are a few circuits in the forums for that sort of control that I've seen. Would such a circuit be as effective as the switched power settings system if transformer tappings are disconnected and circuit put across the max power winding? Or shunt type adjuster set at max and circuit placed in stick welder?

I've saved the Datasheet360 site for future reference. Don't know why I didn't get that one appear in my searches.


Thanks Eddie and seadog

 

[eddie49]

I think SCRs are too useful to "waste" them as just rectifiers! With a 600 volt rating, they would be good enough for use on mains ( just, 800v is recommended ), and would make a good input power controller for a MIG, or even an Arc welder. This would give you stepless, infinitely-adjustable range, especially down to very low power for thin sheet, providing the adjustability of an inverter without the complexity and potential unreliability.
This thread describes using a Triac "light dimmer" to do that:
https://www.mig-welding.co.uk/forum/threads/tm-150-mig-mods.97298/

 

[johnrev]

Just to round off this thread I eventually got round to testing the Autoplus for welding performance. It really welds very nicely indeed. I discovered that the ultra low speed setting was really too low and meant the speed needed to be set at near 4 to be useable with the .6 wire at it's lowest power setting.
Used a resistor to reduce the speed control range a bit and now at about 2 it suits the lowest power setting for the wire. Could happily reduce the spread some more but it's OK as is.
Turning the power to full I need wire speed way up at maximum (again with the .6 wire). It then sounds like an air leak shhooshing sound and gives good penetration in a bit of bed angle. Not a very practical setting to use but showed me I had a good range to play with and should be nice with .8 wire.

The wire was on the machine when I got it and had been stored for a long while so had a patch of slight rusting over about a quarter of all it's layers on the reel. I peeled off a good bit of wire but it seems to be affected over most of it's length. It fed through without problem and was only just noticeable as a slightly rough crackle every now and then while welding.

Ran out of time before I tested any further or tried .8 wire but I'm comfortable the machine is a pretty good performer.

Now tucked away for the time being while I carry on with the shed renovations. When I have the time and the space I will give the side panels a spray with some silver/grey paint and the whole thing a good clean up inside and out.

The shed roof is now done but I'm waiting for a hospital appointment and possible op so will probably wait till that's sorted before I get down to the concreting and extension.

Thanks again to all who offered advice.

John