SCR Bridge Controller for Arc Welder

Discussion in 'The Projects Forum' started by jackw19, Jan 22, 2009.

  1. jackw19

    Thread Starter New Member

    Jan 22, 2009
    Can someone point me to a working circuit for a single phase fully controlled SCR bridge? I want to controll the rectifier stage on the secondary side of the transformer of a 250 amp welder, and I've got these big honking inverse par SCR power modules that look perfect for the job.

    There are lots of theoretical discussions of SCR bridges, esp 3-phase, on the web, and lots of vendors produce firing boards I can't afford--but I haven't found an acutal detailed, buildable circuit. There must be one out there somewhere.

    I did see Siemens app note for TA785. This has a circuit for a bridge with two diodes and two SCR's.

    Questions: i) Is there any advantage in using 4 SCR's in the bridge vs 2 SCR's and 2 diodes, iia) Can I use one of my power blocks as a pair of ordinary diodes? iib) In the 2+2 configuartion, is there a matching problem with the SRC's and diodes, or is sufficient rating all that counts? (Elementary questions, I know. I'm an economist, not an EE.)

    An alternative strategy would be to use triac control on the primary side, a la ST application note AN308 (attached, pls see Fig 11.) This is a very simple, elegant circuit for triac control of inductive loads using an alternistor. The circuit uses a secondary RC network to generate a pulse train to the main triac. The problem is that being on the primary side of the welding transformer (inductive load), the circuit calls for an alternistor. 40 amps is the max rating of alternistors I can find, and that's not a lot of design margin for this project.

    Questions: iii) Can alternistors be run in parallel, or does that entail all the ususal problems? iv) Maybe I could use one of the power modules in place of the alternistor by adding a snubber and verifying that the gate drive in AN308 would have enough oomph for the big dog?

    A final question: v) Is a welding arc an inductive load? Of course the transformer itself must be an inductive load, so control of the primary side must address that problem, but if one's controlling the secondary side ...

    Thanks in advance; I appreaciate any suggestions.
  2. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    Have you checked the Miller Welding site? Find the model number you need or want to copy, then look up the service manual. Miller gives complete schematics for most of its models.