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 arc welder. I've got these big, heavy inverse parallel 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 TCA785. 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 bridge configuartion, is there a matching problem with the SRC's and diodes, or is sufficient rating all that counts? (Elementary questions, I know, but 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 to drive it?
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 should address that problem, but if the design controlls the secondary side ...
Thanks in advance; I appreaciate any suggestions.
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 TCA785. 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 bridge configuartion, is there a matching problem with the SRC's and diodes, or is sufficient rating all that counts? (Elementary questions, I know, but 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 to drive it?
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 should address that problem, but if the design controlls the secondary side ...
Thanks in advance; I appreaciate any suggestions.
