"Ignoramus17256" wrote in message news:_rcbf.2786$ snipped-for-privacy@fe63.usenetserver.com... | On Sun, 06 Nov 2005 00:27:16 GMT, carl mciver wrote: | >
| > The frequency that drives the output transformer is most likely set up | > on the main board. If you could figure out what controls that frequency, | > you could skip the output diodes and that will be your AC output, and | > override the original frequency control. | | For a tig welder that is small, with a small transformer, the | frequency may well be extremely high. | | > I don't know how to keep the HF from feeding back through the diodes | > when in DC, considering the space on the machine, other than adding a switch | > to take them out of the circuit. | | You can see how it is done in my welder | | | HF transformer | -----------------------------|-------|---/\/\/\/\/\/\/\------ tig torch-- | |( | | | |) |recti | | |( Simply having a second set of output | > terminals wouldn't do the job. There could be some bypass caps around the | > diodes, but that would have to take some thought, obviously, once you know | > all the other variables. | > A pair of 555 timers, or a 556 dual timer, IIRC, is enough to provide | > you variable frequency and duty cycle, so just needs the section to drive | > the output. If you could control the frequency from the main board, as | > noted above, but the duty cycle would have to be controlled | > afterwards. | | the problem with 555 timers is, it is hard to control frequency and | duty cycle separately. | | I actually made a timing circuit which fully works, based on XR2206 | chip. Here I can control frequency with a 11 position switch switching | the caps, and duty cycle with a regular pot.
I haven't messed with stuff like that in many, many years, so practical experience is lacking, and I'm sure technology has helped a bit in the following years. You might be right.
| > If you replaced the diodes with SCR's, and some logic circuitry to | > prevent both a short if both sides are on at the same time, then you | > could do both with the output diodes, be they SCR's, IGBT (which I | > think they already are,) or whatever device you prefer. This way | > your could skip the above bypassing of the diodes and just use them | > to provide the frequency (although likely square wave, with whatever | > smoothing the HF transformer adds.) | >
| | I am not sure what you mean here. If you want to make adjustable | frequency, then you have to rectify first and chop later, no? The | power transformer is made for a particular frequency. Am I mistaken?
Well, transformers operate in a range of frequencies, with one particular one, or division of one, better, with diminished efficiency the farther away you get from that. The small transformer likely inside might be a higher frequency than we'd like, but that can be dealt with. Testing will tell what it is, since the manual won't. You're right about the rectification first then the output control; I had two parallel trains of thought going on in my head at the time and crossed the wires a bit.