Big Farnell PSU SCR control circuit problem

[See_Mos]

That's great. I have also sent an email to someone I know who might have the information.

[Chris Wilson]

Got the right paper factory manual this morning complete with A0 schematics, only P2 on the SCR board remains a mystery, but the circuit description may be of help. I will either have to photo the page or type it verbatim later or tomorrow. This will be a big help in setting it up! I will try and get it done as a .pdf and share it in due course. Thanks for the offer of tracking one down See_Mos, no need now

[Chris Wilson]

Below is the section from the correct manual for my supply detailing how the SCR stuff works. In another section it describes how to set up all the adjustment pots EXCEPT for P2 on the SCR board. Does this description of the workings of the circuit give enough info to suggest how I might check P2 is correctly adjusted please? All the other stuff seems pretty straightforward. Thanks.

[See_Mos]

Looks like it sets the minimum firing angle.

[Chris Wilson]

How would you measure / check that, or should I just leave it alone? The problem is I have made images to build two new PCB boards with all new components save the SCR transformer, as these are now in a very poor state with repairs and testing (lifted and missing tracks, bridged with fine wires) So the P2 will need setting with the new boards. I have a scope, but the manual makes no reference to setting P2 at all. Maybe I could measure the firing angle on the other PSU?? Cheers See_Mos

[dave cox]

Tricky & dangerous to measure the actual firing point, unless you have a good current probe / current transformer. Maybe someone has experience of a roll your own version ? Maybe they just tuned it by 'ear' listening for the right 'growl' from the transformer

My tek split-core ac/dc probe is made for this kind of thing but their price has gone through the stratosphere

dc

[Chris Wilson]

You don't mean an AC / DC current clamp do you Dave? As I have one I use on automotive injector harnesses to look at injector waveforms. It's a Pico one I think. I suspect you mean something much more high tech though somehow

[dave cox]

That should be just fine, if its fast enough for an injector then its fast enough for 50Hz (=3000 rpm)
dc

[Chris Wilson]

Does it need to be AC though.... Not sure if it's not DC only. Thanks Dave. I also need to be told what I am looking for when setting it

[dave cox]

My guess is that if the controller tries to trigger the SCR too early it won't have enough voltage across it to reliably fire and hold until the next zero crossing and if it fires too late then it will be rather brutal on the transformer and noisy (although the TX inductance will limit the surge current).

Maybe its adjusted until the SCR triggerering starts to 'jitter' then backed off just enough to fire reliably. Just a guess though ;-)

dc

[Chris Wilson]

I have read the bumf a few times and am wondering if it is to set the max voltage out of the rectifier, as in the manual for the older boards it talks of protection to stop it going over circa 93V? Would the mark / space ratio adjustment do that, I am assuming it is all that could? What I don't understand is why 93V would not kill the pass transistors which, originally, are 60V rated 2N3055's , although I have 140V rated ones in it now as I was sick of the original faults killing one or more. Thanks Dave!

[Herald1360]

The voltage across the pass transistors will be a nominal headroom figure- the SCR servo in this sort of psu usually works so as to maintain this figure constant rather than anything to do directly with output voltage. So if everything works, your 3055s will normally see only a few volts- enough to swallow any ripple but as low as possible to keep dissipation down.

So, as output voltage demand is increased, the 3055s are turned on more, headroom voltage falls and the SCR servo turns up the wick to compensate. The clever bits go into keeping the whole multiloop contraption from turning into a power oscillator and protecting against the load suddenly going short with a high voltage output which would otherwise toast the 3055s in double quick time.

[Chris Wilson]

Herald1360: Thanks, yes of course, I see now, the pass transistors, *ASSUMING* the SCR stuff doesn't go berserk, which I believe is what was occurring, only see about 8.5 V drop across them. Thanks!

[Chris Wilson]

Originally Posted by dave cox

My guess is that if the controller tries to trigger the SCR too early it won't have enough voltage across it to reliably fire and hold until the next zero crossing and if it fires too late then it will be rather brutal on the transformer and noisy (although the TX inductance will limit the surge current).

Maybe its adjusted until the SCR triggerering starts to 'jitter' then backed off just enough to fire reliably. Just a guess though ;-)

dc

Thanks, I don't suppose you or anyone can up with some more concrete way of finding out what P2 does and how I could ensure it's set right? Bear in mind I have the second supply where P2 I know has never been touched.... Thanks

[See_Mos]

I haven't heard back from Simon yet so maybe he doesn't have the informataion.

As a last resort I would probably either transfer P2 to the new board or when the board is ready measure the resistance from the wiper to either end and set the new pot to the same value.

[Chris Wilson]

I think I will have to settle for assuming it has never been twiddled and do as you suggest, but if you or anyone else can work out a test procedure I would like to put my mind at rest and KNOW all pre set pots are set correctly. Sincerest thanks to everyone for the amazing amount of help you have all offered, it's much appreciated.