Is it a Diode, or Zener or what

[`FRANK.C]

In the photos below the scope is connected across the SCR of a fence.
In order to protect my scope I connected it using a EHT probe.
This is very far from ideal and not at all accurate but I think good enough to show what is happening.
Each division is approximately 400V.  The SCR in this fence has a peak off-state/peak reverse voltage of 1200V
The first photo is with no load on the secondary the second is with 47R load.

The fence transformers primary have a low resistance in this case it is 0.3R. This transformer I would consider as medium power. Lower power or poorer quality transformers can have higher resistance primary's.

Frank

[Murphyv310]

Hi.
Concur with Frank. One reason that thyristor line stages didn't last was due to back EMF off the LOPT and thyristor failure, the theory and the practice often don't mix. Yes there is an efficiency diode and tuning caps in a TV line stage but often the thyristor was pushed to it's limits which would often result in failure a year or so after replacement.
There was a few CTVs with thyristors EG, Grinding & Saba, the use of thyristors didn't last and they were ditched by the manufacturers due to poor reliability.

[PerdioPal]

Theory and leakage inductance, the worst enemies! I should have simulated it in LTSpice (a much better waste of time than TV).
GTOs (gate turn off SCRs or whatever they were called) used in Sony sets for a bit, how well did they cope?
And every day should be a school day, thank you.

[Mark Hennessy]

GCS - gate controlled switch.

I know these have a bit of a reputation - to put it mildly! - but I've only experienced them in the KV1820, which only used 1 in the line output stage, IIRC. I've fixed plenty of faults with my 1820 over the years, but never had a GCS fail. I have a bunch of spare boards, so hope it keep it alive for a while yet (my vintage TV collection amounts to just this one set now).

[`FRANK.C]

Not many TV's with thyristor line stages around this neck of the woods. But I do remember some colour portables that used them.
I think they were Ferguson branded if not probably Grundig. There was a article in Television magazine about the chassis.
All I can remember doing with them is changing the thyristors. Which were always changed in pairs.

Frank

[Mike Watterson]

My Ferguson TX9 chassis I think was Thyristor based PSU. Never understood why it worked. Bought in maybe 1978 (?) and went to recycling maybe 2006 due to mechanical pushbutton station selection failing. Analogue ended in 2012 anyway. Screen went very magenta or green about 1996 (I forget which) and I boosted gain of one of the Video o/p stages. The isolated chassis TX10 (I think?) were easy to modify for Apple II (custom crystal and disable PAL line switch), BBC & Research Machine 380z.

You get a big volts when you switch off the current in a transformer primary compared to discharging a capacitor into it, so CDI for car really needs a different coil to simple points & ignition coil.

There used to be a Fencer beside me that clicked merrily over entire LW, MW and HF. `FRANK.C and the PPPenguin have seen the setup here.

[BusyBee]

You get a big volts when you switch off the current in a transformer primary compared to discharging a capacitor into it, so CDI for car really needs a different coil to simple points & ignition coil.

I seem to remember reading that a sports coil was suggested for CDI systems although in the few systems I have seen the original was mostly used. The fact that many had invertors which stepped the discharge voltage up to around 400V would help.

Tracy

[Mike Watterson]

Yes, a 30x bigger HT will certainly make the original ignition coil work. I guess the Fencers must be like an electronic flash gun circuit. Unlike CDI they don't need to charge up the cap multiple times per RPM. They usually have a 6V or 9V battery pack, though newer ones have solar panels.

[Kalee20]

I suspect a diac can be replicated with two diodes or zeners depending on voltage. Low voltage is Zener action (reverse Vbe or lower voltage zeners). High voltage is Avalanche mode.

Well, no - Zener diodes (whether true Zener, or avalanche, action) pass zero current until the breakdown is reached - at which point they start to pass current maintaining the same terminal voltage.

Diacs, on the other hand, pass zero current until the breakdown is reached, following which they start to pass current and a regenerative action happens and the terminal voltage snaps back to a lower voltage - like, for instance, a neon bulb, only at lower voltages.

[Mike Watterson]

But a high voltage trigger circuit could use an avalanche diode. I suppose a diac can be used to make a relaxation oscillator, like a neon can, but as you point out the Zener and Avalanche modes are like two terminal shunt regulators and don't usually have a negative resistance region.