Is it a Diode, or Zener or what

[boater sam]

This component is out of an old electric fencing unit, make unknown. It is on a simple pcb with a TIC126  SCR which pulses to charge and discharge an 8uf cap into a transformer to produce the kick.

It is 4mm long, 1.6mm in diameter, the ends are red bands, the centre is blue. There are no markings at all, no symbols either.

My Chinese box says unknown or faulty. It is open circuit as far as my meter is concerned.

Can anyone help please?

[PerdioPal]

A diac?

[boater sam]

Brilliant! exactly correct. Its a diac.

I can't see the yellow band on the lead though.  How can I test it?

[ppppenguin]

First thought. Connect to variable power supply via a resistor to limit the current. Ammeter in series, voltmeter across. Wind up the voltage until it conducts. Note the readings. Then try it with the DIAC reversed. Should give same result. You may need 40V or so to make it conduct.

Unless you have the luxury of a curve tracer. Then you can see the whole characteristic on the screen.

[boater sam]

First thought. Connect to variable power supply via a resistor to limit the current. Ammeter in series, voltmeter across. Wind up the voltage until it conducts. Note the readings. Then try it with the DIAC reversed. Should give same result. You may need 40V or so to make it conduct.

Unless you have the luxury of a curve tracer. Then you can see the whole characteristic on the screen.

Thanks, that's tomorrows job.

[Mike Watterson]

A 20 M Ohm input DVM on volts (or a valve voltmeter) and a current limited capacitor leakage detector (mine has 2 x 1 MOhm in series and gives 400V no load.
The DVM reading is approximately the Avalanche voltage. I can select 1N4148 as known 100V+ regulators. It's non-destructive if current is low. Also good to check Vcb and Vce.

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.

[`FRANK.C]

I am not sure if you just want to test the diac and keep if OK or if you want to repair the fence.

If you are repairing the fence diac's are generally reliable.
One end of the diac will connect the the gate of the SCR. The other end connects to the junction of a capacitor and resistor. The other end of the capacitor connects to ground. The other end of the resistor connects to HT. The capacitor charges via the resistor and when the voltage reaches a certain level the diac triggers and dumps the voltage stored in the capacitor into the gate of the SCR (there is usualy a resistor of around 100R in series with the diac to limit I). Turning the SCR on. The diac then resets allowing the capacitor to charge again.

The diac used in a lot of fences is a BR100. The BR100 has a breakover voltage of 36V.

A neon is used in some fences in place of the daic but circuit values would be different.

The weak link is usually the resistor or series of resistors connected between the capacitor and HT. They are usually many 100's of K and tend to go high or open circuit.
Or the capacitor can go low in value.

Frank

[boater sam]

I am fixing the fencer.
The diac checks out OK. Thanks for the advice on testing guys.
The resistor values are way out, a 270 ohm is 102, the three 270K are now between 670k and over 1M ohm!  They are the 6 band type, I hate them as its not obvious what value they are with my ancient eyes. All will be replaced of course.
The gate trigger cap is sound.  There is a neon that flashes as the trigger voltage come up, I can hear the tick.
The triac is getting gate voltage but not triggering, its duff, ordered some.

[`FRANK.C]

I am sure you know this already but maybe worth pointing out.
Make sure the TIC126 has the same or better voltage rating. Personally I would only fit the TIC126N (N = 800V).
The SCR has to withstand the back EMF from the transformer which can be much higher than HT.

Frank

[PerdioPal]

"The SCR has to withstand the back EMF from the transformer which can be much higher than HT." Does it? The SCR will (almost) discharge the cap. before turning off, not much back EMF.