Is My Thyristor Gate Driver Circuit Malfunctioning Due to Component Issues?

[FusionJim]

Hey folks. please help me out, I am having trouble finding the problem with a thyristor driver circuit. So this circuit has been working for many years without problems. It works from 2 of the 3 phases, so 400VAC (I am in Europe).

The thyristor is in series with a large coil, aka electromagnet located inside a box attached to a sand separator. The box works like a vibrator. It can be powered directly from 2 phases but in order to lower the vibration amplitude the thyristor circuit is installed.

Recently the thyristor circuit started working intermittently. It works for about 5 mins then stops for about 5 mins or so.

I took the whole circuit box home, checked every part, measured the capacitors (with capacitor tester), measured the transistors, checked the circuit board, connections everything, and it was all fine.

The problem still continues. Then today I was measuring the circuit as it was working LIVE. The thyristor gate driver is supplied from a rectified but unfiltered DC supply of 25 volts, this sags to 22.5 roughly as it begins working.

When the gate drive circuit stops working and the thyristor shuts off, I noticed there is a voltage of 25 VDC across the pulse transformer primary (measuring across the 1N4007 flyback diode) The transformer primary has a resistance of some 2 ohms or less. It is a small gate drive transformer. Clearly a voltage drop of 25 volts across the transformer isn't resulting from current flow through it but most likely I'm measuring from - terminal to which one side of the primary is attached through the unijunction transistor across the 220 ohms resistor connected to + side.

Now here is the weird part. When the gate drive circuit has stopped working, as i touch the cathode side pin of the primary flyback diode (1N4007) with a screwdriver or multimeter probe, it immediately starts working.
Clearly as I touch it even though with an isolated probe, there is a small change in capacitance introduced by my body and this is enough to get the drive going again. Then it stops after a little while again, touch it once more it works again.

It seems to me this could be some kind of a problem with a trimpot maybe changing it's originally set resistance (the two yellow ones are on board 3 pin potentiometers ) or maybe one of the capacitors is bad, although I checked all of them. I even changed the 2N2646 unijunction transistor simply because I had one and I wasn't sure whether the original measured correctly.

Any suggestions? Pleasesee the attachedschematic

 

[FusionJim]

I think i will bypass the BC107 bipolar transistor altogether as I am not sure why it is there in the first place since the UJT 2N2646 can work as a oscillator without it and the frequency is still controlled by the 60k potentiometer and the 0.68 uF capacitor right? Can anyone figure why the bc107 transistor is in this schematic in the first place?

 

[DaveE]

1000μF, really?
It looks like a capacitor multiplier configuration to me. But honestly, I haven't thought too hard about this.

 

[FusionJim]

1000μF, really?
It looks like a capacitor multiplier configuration to me. But honestly, I haven't thought too hard about this.

Yes, the cap in the bc107 base is a 1000uF electrolyte

 

[Tom.G]

I would say that the 1000uF serves to delay the trigger pulse to the thyristor until there is enough line voltage for it to latch on for a half-cycle.

I also consider it to be the prime suspect for causing the observed symptoms.
Replace it.

Cheers,
Tom

 

[FusionJim]

I did some tests today, turns out as the pulse transformer heats up the primary becomes an open circuit...
I opened it up in the hopes of being able to rewind it but its tiny and epoxied in place. This one had silicon steel laminated core. @Baluncore @Tom.G @DaveE what do you think, can i change it with any general purpose pulse transformer, including ones with ferrite core? The original was 1:1 and measured roughly 4mH @1kHz, winding DC resistance was 2.5 ohms roughly

 

[Baluncore]

Any small 1:1 pulse transformer will do that job.
The only requirement is that it have a high isolation voltage.

I have trouble understanding your tangled schematic.
What does the emitter of the BC107, 1000uF, and the 60k pot connect to?
Is it also connected to rectifier bridge negative?

 

[FusionJim]

I have trouble understanding your tangled schematic.
What does the emitter of the BC107, 1000uF, and the 60k pot connect to?
Is it also connected to rectifier bridge negative?

Not sure why you say its tangled. Although i had limited time while i was copying it from what i saw on the pcb. The bc107 emitter in series with the 60k pot connect to the 0.68uF capacitor which then connects to rectifier negative. The 0.68 cap charges up and when the UJT threshold is reached it discharges through it to make a spike like pulse through the transformer as i understand it. There is just 1 thing i don't quite get, the rectifier outputs 100Hz positive half periods, so the 0.68uF cap is charged up basically every half period the moment voltage reaches the point it is high enough to cause the 2N2646 to fire.
The pot can only change the resistance which changes the charging time therefore lowers the pulse frequency on the SCR, how does this result in the SCR regulating total power? Because IIRC SCR can only regulate at which point during each sine half period it turns on as it turns off only after current has decreased below threshold so at or near zero crossing. So does this circuit regulate for example the SCR turn on starting from the falling side of the half period back towards the rising side depending on how fast within each period the first pulse manages to be formed by the 0.68uF cap in tandem with the UJT? I hope i was able to explain myself

 

[Baluncore]

This is 1960s technology.
The KP25A thyristor is an SCR, not a TRIAC, so it always blocks one polarity of the 400 VAC input. It will conduct on the other polarity half cycle, only after it is triggered.
Why is the SCR being triggered every half cycle by the UJT, when it can only catch and conduct on half of the cycles. It could have used a single diode, not a bridge.

Think phase angle, not frequency.
This controller sets the phase angle for the start of conduction, the SCR will then turn off, when the current falls to zero. With the inductive load, that may be well into the next half cycle.

The 2N2646 UJT is only rated to 30 volts. Are you certain the AC is 170 VAC before it enters the bridge rectifier? I would expect 17 VAC.

 

[Baluncore]

OK, so the secondary supply is 17 VAC which is rectified to 25 Vpk.

There is no ground on the floating circuit between the two transformers.
I would expect the transformer insulation to break down.

 

[FusionJim]

The transformer is an old Siemens model. There is written on it 170v 5VA, there is no filtering on the diode bridge, measuring with multimeter set to DC shows roughly 25 volts so if one rectified that it would be 35. This SCR was changed recently, origonally there stood T122 (25A 600V) which also seems like an SCR

 

[Baluncore]

There should not be a capacitor filter after the bridge.
The rectified voltage is being used to measure phase angle.

The BC107, with the 14k5 resistor, and the 1000uF cap makes a 1.33V voltage-dropper, and/or an 8.6mA current regulator, charging the 0u68 F capacitor. That would generate a voltage ramp; C = Q / V ; Q = i * t ; dv/dt = i / C;
8m6 / 0u68 = 12.65 kV/sec.
50 Hz rectified is 100 Hz. T = 10 msec, dv = 12650 * 0.01 = 125 V ramp per cycle.
That seems a bit big for the 25 volt rectified AC. I would verify component values, looking for a factor of ten.

A stud thyristor, T122-25-6 is an SCR, but the circuit as drawn, is triggered to fire twice per cycle. I would expect that to be used with 2x SCR, or 1x AC TRIAC. Either the second SCR has gone missing, or it should have used a TRIAC.

 

[FusionJim]

I changed the pulse transformer with one that has similar inductance and its a success, the device now works as it worked before. Thanks to all that participated