Unijunction transistor trigger

[MM0TJR]

Trying to fix an AC electric fence energiser, which is not triggering.
the capacitor is charging up nicely to 600V
there are 2 SCR's in series across the HT, connected HT----A-K-----A-K----0V
the gate circuits on each are a bit of a mystery with surface mount parts which I cant identify.
there is a 2N6027 UJT which I presume is supposed to trigger both SCR's using its Kathode

There is about 5V dc on the Gate of the UJT.
the Anode of the UJT appears to be oscillating between 0 & 5V ok via a little SMD cap, with a period of a couple of seconds. The kathode is earthy.

I just cant figure out how the UJT is meant to be connected to the SCR's, whether it's supposed to be firing only one SCR or both or what.

does the UJT gate bias at 5V sound plausible? seems rather low to me.
are the SCR's supposed to be fired both by the UJT? or is only one fired and the other avalannches?

 

[(*steve*)]

I sounds like the UJT is set up as an oscillator, which it's doing from your description.

This signal presumably should be making its way to the SCRs and it sounds lie either there's a fault there or the SCRs have failed in an unusual manner.

 

[duke37]

Having two SCRs in series seems a little strange.
If you wish to do a bit of bodging, you could use a small capacitor charged with a resistor and then via a DIAC to the SCR gate (used by Gallagher) or you could use a similar system but use a neon indicator as the trigger, this will need a sensitive gate SCR.

When you get it working, you can advise on a Copel A4 which I have, I might post here if I get to dead end.

 

[MM0TJR]

the UJT kathode should be connected to the SCR gate, right? (by a voltage divider)

 

[duke37]

I looked up the data for the 2N6027 and it should be limited to under 40V A-K, you seem to have this.
The SCR normally will have a fairly low resistance gate-cathode (earth), perhaps 1k. I would expect the cathode of the UJT to be connected directly to the gate of the SCR.

If the anode of the UJT is bobbing up and down, then the UJT is working. The fault then looks to be the SCR. These can be tested at low voltage with a current limit to see if they will switch on. I have used a 12V battery charger, 12V bulb and a 1k resistor anode to gate to provide trigger.
I do not understand how the two SCRs are fired together unless there is a pulse transformer. Firing one only and overvoltaging the other to get it to trigger seems a bit hit and miss.

 

[(*steve*)]

Perhaps a view of both sides of the PCB may be in order?

It is possible that optocouplers are used to trigger the SCR's too. It may be that one trigger circuit has failed.

We're working in a vacuum at the moment.

 

[MM0TJR]

Working in a vacuum, I know the feeling. hopefully (if my upload has worked) the schematic as I worked it out can now be seen.
the "?" components are unidentified SMDs. some of them I am pretty sure are resistors from checking and I have written in odd values. but some of them must be caps.

the 2 places where I have drawn "X" are SMD locations but no component present (and looks like none were ever placed there.

Ill check the SCR indivisually next.

 

[duke37]

I wouldn't think you could work in a vacuum, there's plenty of wind in the Orkneys.

Have you tried to get information from Gallagher? After some effort, I managed to get some information on the use of their DVM3. The address I used was [email protected] and had a reply from Allan McKie, perhaps he hails from your part of the woods!

 

[MM0TJR]

funnily enough I did call Gallagher in England but they were totally unhelpful technically. apart from telling me the the PCBs are made in NZ
could you try the email address again pse, the BBS removed it.