Harbor Freight Tig Pedal

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I've been hanging out for nearly ten years on a board where we focus mainly on building motorcycles from scratch.  (Not the "as seen on TV" bikes.  The real ones before those silly shows even existed.)  A couple of guys recently bought the HF TIG machine just to play with, and the subject immediately came up of how to incorporate a pedal into the system.  The first attempt was to simply relocate the current pot into a pedal assembly and connect it back to the machine with a shielded cable.   HF does not offer any support and certainly no schematics or technical publications.  We tried it and the machine was reduced to a sputtering and sparking mess.  The pot obviously is part of a finely tuned voltage divider circuit and the extra wire just blows the balance, or the pot is part of a tuned circuit which is very frequency dependant and the extra wire adds Henrys into the resonant circuit and really screws with the machine's frequency circuitry.  I have no way of really knowing because I've never seen the machine(We're doing it long distance over the internet.) and the schematic that's in the manual isn't any good at all. (What a surprise!)                      Soon we came up with a pedal which had a spring return, a very long bicycle shift cable, pully on the pot with the cable wrapped on it and another spring on that end as well.     Well, it works.    Full pedal travel yields about half the pot's travel.  The pully and pot's shaft can be positioned to any desired range of pot travel and the pedal will spin the pully a good measure in either direction.   I haven't even seen this setup as he and I live hundreds of miles apart, but he's happy to report that it is a complete success with only one minor problem I see that needs to be corrected soon.  Bottom line, for the guys with the Harbor Freight TIGs, there is hope out there to really make the little machine capable of doing even better work by having a real pedal that really works.  Here's the final thread from last night when Whizz finished and tested the prototype:            (Not a G rated board.)http://www.network54.com/Forum/16142...e+Tig+lives%21
Reply:Please let me know how you make out with this project. I also have a HF tig and want to add a pedal. I have done some research but havn't found the answer yet. If I do find something I will post it here. I know their are people that have done it but havn't shared yet.Miller DVI2Lincoln Precision Tig 225Thermodynamics Cutmaster 38Everything else needed.
Reply:That is a really clever solution, simple, robust, and it actually works!Way to think outside the box!  That's using the old "KISS" motto, keep it simple stupid.
Reply:...zap!I am not completely insane..Some parts are missing Professional Driver on a closed course....Do not attempt.Just because I'm a  dumbass don't mean that you can be too.So DON'T try any of this **** l do at home.
Reply:Sorry guys I did check the link posted above. Thats not what I was looking for anyway. I am just going to keep saving up for my new tig with all the options.Miller DVI2Lincoln Precision Tig 225Thermodynamics Cutmaster 38Everything else needed.
Reply:Originally Posted by Beezer  A couple of guys recently bought the HF TIG machine just to play with, and the subject immediately came up of how to incorporate a pedal into the system.  The first attempt was to simply relocate the current pot into a pedal assembly and connect it back to the machine with a shielded cable.   HF does not offer any support and certainly no schematics or technical publications.
Reply:Well you went to a whole lot more trouble than I was willing to.  I respect your attention to detail about things I don't really worry about.
Reply:Here is a schematic and some internal pics I found the other day for the Harbor Freight TIG. It is a PDF attachment on this post at the bottom.http://www.practicalmachinist.com/vb...d.php?p=805672Had to register to get the files. It looks like a neat machine for $200. Attached Images Attached Images91811-schematic.pdf (35.1 KB, 5054 views)
Reply:Originally Posted by uhmgawaThe pot is 10K linear taper in the middle of a voltage divider with a 4.7K to ground and a 10K to the 5V reference of the SG3525 PWM controller.  Thus the tap of the pot travels from 19% (low) and 60% (high) or 0.95V and 2.98V respectively.  Given the impedance of the circuit it would be fairly easy to induce noise in the control circuit during welding via the lines connecting the external pot which could certainly cause the erratic operation you observed.  Using a shield to ground around the leads should help avoid inducing noise as would lowering the impedance of the voltage divider. You mentioned using a shielded cable, however was the shield grounded to GND of the actual control circuitry?  You'd need to access the control ground via one of the internal PCBs.
Reply:Originally Posted by Jim_in_OmahaHere is a schematic and some internal pics I found the other day for the Harbor Freight TIG. It is a PDF attachment on this post at the bottom.
Reply:Here is an update to the 91811 schematic.  Thiscontains the OEM version of the machine [pages1,2], my original modification for external heat andtiming control [page 4], and a substantially simplifiedcircuit providing external heat control alone [page 3].This simplified external heat mod involves no surgeryto the control board.  Tie-in to the PWM controlleris achieved by disconnecting the 3-pin connector fromthe existing front panel 10K heat pot and routing it tothe board containing the added circuitry.  This boardwill supply it's own 3-pin cable/header which will thenconnect to the original 10K pot PCB.  The worst ofthe modification is extracting GND and +12V from themain board which can be found on the voltageregulator U5, at pins #2 and #3 respectively.  Withsome soldering skill one should be able to make thetwo connections without needing to unmount themain PCB from the heat sink.Actually the larger share of tedium in the aboveis the issue of how/where to mount the additionaltwo limit potentiometers and the internal/externalselect switch given the scarcity of front panel realestate on this machine. Attached Images91811-schematic-unified.pdf (87.6 KB, 2960 views)
Reply:Hello uhmgawa ...Can you post some more data regarding how the ferite trafo is built and what type of core does it use?Thanks in advancesavu