Triacs in Series on Pfaff 1473 Motor PCB

[Mains]

   First post here. I am retired welding engineer who never had the time to become knowledgeable about electronics.  Fixing things has always been a passion. Maintained my own autos, but the circuits there were usually easy. Wired two houses, but nothing fancy there. At 73, I shouldn’t play tennis, ski, climb ladders, or ride dirt bikes anymore, but bench work, why not?  Lots of great electronics content on youtube, and right here on EEVblog. So I am setting up a bench, consuming a ton of new information, and having fun doing so.
   My wife loves her Pfaff 1473 sewing machine, 1988 YOB( Pfaff, not my wife). The 1400 series (1983- 1995) were Pfaff's first computerized machines. This one has two stepper motors to move the needle and fabric in ways impossible with mechanical cams. The 1400 series machines are sought after because they have an integral dual feed and were extremely well built in Germany. Unfortunately, Pfaff has been sold to private equity multiple times and factory support ended decades ago. Nonetheless the 1400 series are worth saving and repairing. The main PCB is often cited as the reason they cannot be fixed. Yet there are documented cases of these boards being repaired on this blog and on other forums around the world. So I want to contribute.
   Recently our 1473 stopped working although it did turn on. Taking these machines apart is a challenge of its own. Pfaff hid screws of various types including M3 pozidriv. But I was not deterred and took out the main pcb to look for trouble (shown in the attached pic). To my beginner mind the main pcb looks modern, somewhat understandable, and fixable. I measured the ESR of all the capacitors and they were okay. I did remove and replace the big yellow one, it measured 0.061 ohms ESR, 2630 uF. (Spec 0.20ohms/2200uF). I had trouble getting solder out of the via so I left the others alone.
   There were no obvious problems on the main PCB so I decided to look at the motor. The motor is also difficult to get to with screws accessed from top, front, and bottom. I had thought that with all the digital complexity on the main PCB that the main motor would be a DC Servo type. But when I final got it out, I recognized it as a series-wound universal motor with no positional feedback wheel. I wanted to test the motor but was confused by the components on the small PCB attached to it. I found two crude schematics on the internet that both indicated the Pfaff motor PCB used two triacs. I could not find any similar circuit designs that used multiple triacs on the internet, and indeed several sources said triacs should not be used in series or parallel.
   I don’t like working on things I don’t understand, so I carefully sketched up the traces and component locations, and drew the attached pdf schematic in KiCad. After replacing the badly worn-out brushes and re-installing the motor the machine seems to be working fine again.
Since this forum is “a place to ask questions”, I have a few:
1.   It seems that after successfully operating for many decades these machines prove that Triacs can be wired in series. What did Pfaff do that worked so well?
2.   The motor control board is very small and simple in design. There is no heat sinking of the Triacs either. Are the Triacs used because of the small space?
3.   I don’t have a scope so I could not see what the output looks like on the gates of the Triacs from the main PCB. Can anyone suggest what that might be? The connecting wire size is very small diameter.
Thanks for reading this.

[Konkedout]

I think you wrote your post very carefully, and your photos are good, so you deserve a response.  Right now is not a good time for me to try to confirm your triac connections based on the board photos.  But connecting two in series seems unlikely.  That would normally mean connecting main terminal 1 of Triac A to main terminal 2 of Triac B.  As I write this I cannot see your post, but I think you said that the machine has 2 motors.  In that case it seems most likely that one triac is controlling each motor, and that the MT1 terminals of the two triacs might be connected together and to one side of the AC power input in that case.

Basically a triac is the device for an electronic logic (or analog) output to control ON/Off in an AC motor.  Triacs can switch in microseconds (much faster than a relay) so they are also used wall lamp dimmer switches.  They basically replace relays but do not provide the isolation between control and AC output which a relay can provide.  They are smaller and cheaper than relays, and perhaps more reliable (but that depends on many factors.)

I think that most often a triac is likely to fail stuck ON (shorted) rather than OFF....but I could be wrong about this. Any chance that there is a blown fuse somewhere?

I have professionally designed with a triac a few times in the last 50 years.  Heck the first time was in the 1970's when we I designed one into a big console computer room printer.

[Mains]

Thanks for your post!  There are two stepper motors in the machine but they connect separately to the main PCB. The third motor, the main ac drive motor is attached to the small square PCB holding the 2 Triacs. I have attached another pic showing the direct connections.
Konke, you have sort of confirmed to me that this is unusual design. I did find a schematic from ABCelectronique in France for a different Pfaff model (Tiptronic) that shows the Triacs in series (see attached). I also found a schematic in Germany that shows the Triacs connected in parallel. The confusion is why I traced it out myself. Pretty sure my schematic showing the Triacs in series connected directly to the AC motor is correct.

[Konkedout]

This is an interesting design indeed.  I can only make an educated guess as to why it is done the way it is.

Dang I cannot read your post as I type this....but I think you indicated that this is a European design.  In North America, 120 VAC will have hot and neutral lines with the neutral generally connected to ground somewhere (such as at the power service entrance to the house).  It is fundamentally an asymmetrical system.

But (I think) that in some (maybe most?) European countries, both wires (with 230 VAC) at a wall socket are hot.  My educated guess here is that

A) It could be that the higher voltage was causing unreliable operation with a single triac.
B) The other more subtle point is that on a transient basis, when switching on and off, a single triac would cause an asymmetrical (a common mode) transient  in what was a more balanced system.  It is possible that this was coupling into the control electronics, or maybe causing some sort of violation of European (such as IEC) requirements for safety or Electromagnetic Compatibility.  I have some thoughts about the first option but this is getting deep for a forum post.  Basically if you have some sensitive electronics, and (something in your sewing machine) suddenly jumps 200V with respect to ground, this can couple into the control electronics or maybe the person doing the sewing.  Unless it is carefully shielded, everything on a circuit board is capacitively coupled to the world around it.  It would only be a quick blip but might be a problem.

Either of the above seems like the result of taking a 230 VAC design which tends to be more rigorous and (maybe just swapping out the motors and the logic power supply) to make it all work with 115  VAC.

Does this make any sense?

DUH!  I had not thought of using another browser tab so that I can still see your post while working on mine.  This does not significantly change my answer above however....

[Mains]

Konked, When I right click on reply, I get an option for split view which shows the post and reply side by side. Very handy.
   Yes the machine is designed for 230V and 110V operation. Our machine is 110V. Those Triacs are rated for 800V. I wonder if two Triacs would run cooler than one and that would be another reason.
   I also have read somewhere that Triacs are often chosen specifically to isolate the AC voltage from low voltage PCB's.
Thanks again.

[tooki]

But (I think) that in some (maybe most?) European countries, both wires (with 230 VAC) at a wall socket are hot.

No, absolutely not. On a standard 3-pin single-phase European plug (regardless of country variant), one pin is line, one is neutral, and one is PE (protective earth, i.e. ground).

In three-phase systems it gets more interesting. Mostly you see 5-pin plugs for that (L1, L2, L3, N, PE, 230V L-N voltage, 400V L-L voltage), but in some places (like rural Norway), they still use 4-pin plugs (L1, L2, L3, PE, 230V L-L voltage).

[Konkedout]

But (I think) that in some (maybe most?) European countries, both wires (with 230 VAC) at a wall socket are hot.

No, absolutely not. On a standard 3-pin single-phase European plug (regardless of country variant), one pin is line, one is neutral, and one is PE (protective earth, i.e. ground).

In three-phase systems it gets more interesting. Mostly you see 5-pin plugs for that (L1, L2, L3, N, PE, 230V L-N voltage, 400V L-L voltage), but in some places (like rural Norway), they still use 4-pin plugs (L1, L2, L3, PE, 230V L-L voltage).

I have been to Germany and I did remember that Germany has 3 phase.  And...(is it Norway or Denmark?) that is particularly sensitive to harmonic current on the neutral wire.  Harmonic current COULD be any harmonic but most often is the blips of current you get when running AC into a bridge rectifier to electrolytic DC filter capacitor, without "power factor correction."

By the way, I was going to suggest that if "Mains" can safely try shorting out MT1 to MT2 of one of the triacs (try one at a time) it might be worthwhile.  One way to experimentally limit current would be to connect the machine in SERIES with (one or more incandescent light bulbs connected in parallel).  If we end up shorting the AC then we would simply light the bulbs with full mains voltage.

[Renate]

I'm no expert but always intrigued by weird (and clever) designs on consumer things.

First, I can't believe that the schematic for Pfaff1473mtrPCB.png is correct.
Could it be that the two triacs are drawn backwards?
I'm thinking that the MT1's (A1's) should be connected together?

This is a brushed motor so there's a lot of weird signals going around.
I'm thinking that they have two triacs in series to prevent false triggering of both.
Both can't be in the most sensitive quadrant at the same time.

[David Hess]

First, I can't believe that the schematic for Pfaff1473mtrPCB.png is correct.
Could it be that the two triacs are drawn backwards?
I'm thinking that the MT1's (A1's) should be connected together?

I agree, but it does not matter for analysis.

This is a brushed motor so there's a lot of weird signals going around.
I'm thinking that they have two triacs in series to prevent false triggering of both.
Both can't be in the most sensitive quadrant at the same time.

That is what I am thinking; they used two in series because there were commutation or false triggering problems despite the snubber.  Ordinarily a 600 or 800 volt TRIAC is completely sufficient for 240 volt AC control.

What was the original TRIAC part number? Is the BTB08 the part they used, as shown in the later schematic?

[Renate]

I agree, but it does not matter for analysis.

Well, it kind of does. But I missed looking at the bottom of the PCB where it does show the MT1's together.