Adding a pedal to lincoln AC-225 tombstone
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  1. #1
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    Adding a pedal to lincoln AC-225 tombstone

    I posted this over on the hobart board since thats where the build threads originated but haven't got any responses...maybe someone here can help with my predicament.

    Hi guys I saw this thread:

    http://www.hobartwelders.com/weldtal...ead.php?t=4089

    And Mike W's Schematic:
    http://smg.photobucket.com/albums/v1...ontrol_gif.gif

    Between those threads and all their info, and the lincoln buzzbox and hobart chiller/torch I've got sitting in the back of the shop I've decided to make this project happen.

    I bought a cheap pedal with a 7-pin plug intended for one of those chinese machines from ebay, as well as an IRKT71 series SCR rated for 70A 1200V and a package of about 18 more diacs than I think I'll need for this project, but hey, at least I'll have them.

    My question is about the necessary values for the variable resistor in Mike W's circuit. I haven't yet got the pedal in my hands to throw a DMM on it, but from two other sellers descriptions of a similar pedal with the same 7 pin connection its likely either a 4.7k or a 10k linear pot inside the thing.

    I don't have a problem with pulling it apart and changing out the pot with something different, but if I can change values elsewhere on the circuit to avoid purchasing another pot I'd rather that instead. I guess thats my big question, what values would I need or am I better off sticking to Mike's values and changing the pot?

    Please refrain from turning this thread into the tenets of buying a syncrowave or an old linde machine instead of building this circuit. I sold my linde already, it just took up way too much floor space and weighed almost as much as my bridgeport and as much as I'd love a syncrowave I'm not gonna max out a credit card to get one. Thanks

  2. #2
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    Re: Adding a pedal to lincoln AC-225 tombstone

    How are you going to control the output?
    A traditional foot pedal isn't designed to carry the load on the output side of the rectifier.
    It is designed to offer a variance in control voltage that sends a message into a control board/shunt/Hall device that regulates the output.
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  3. #3
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    Re: Adding a pedal to lincoln AC-225 tombstone

    Quote Originally Posted by Maedar View Post
    How are you going to control the output?
    A traditional foot pedal isn't designed to carry the load on the output side of the rectifier.
    It is designed to offer a variance in control voltage that sends a message into a control board/shunt/Hall device that regulates the output.
    If you look closer at the circuit you realize the pedal is controlling input voltage to the buzzbox thru the SCR. The output side is left alone (although in this case it will get a high frequency stabilizer later on)

  4. #4
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    Re: Adding a pedal to lincoln AC-225 tombstone

    Regarding your question about whether it is necessary to change the value of the resistor in the foot pedal - here is an opinion by an electronics hobbyist, not an electrical engineer.

    While a workaround involving added active circuitry to simulate the effect of the 250K pot specified in the source schematics while retaining your 4.7K or 10K pot in your foot pedal is always possible, I would recommend against trying to simply diddle circuit values in order to avoid changing the value of the pot in your pedal. Assuming the values shown not only result in a good, workable unit but were also selected on the basis of good circuit engineering (and it would be stretching my capabilities to actually evaluate the engineering), simply changing component values to make your pot work could get you into expensive trouble.

    The control pot operates in conjunction with the two 0.1 uF capacitors to determine at what point in the AC waveform the diacs (and, thus, the SCRs) fire. The 0.1 uF capacitors and 1K to 250K range of the control resistance presumably provides the desired range of control of the firing point. To maintain the same range of time constant in the firing circuit (and, thus, the control range) you would have to increase the capacitance values by the inverse of the resistance change, or a factor of about 25 to 50 (for 10K or 4.7K). Now you have 25 to 50 times more charge on the capacitor discharging into the SCR gate via the diac which might exceed the current or power ratings of the SCR gate or diac. Whether this is actually a problem or just my imagination is a reasonable question, but should be checked against device specs unless you want to perform a smoke test. The pot may also not be up to the required power dissipation. I'm not sophisticated enough to know how the 100K resistors across the diacs would affect the firing range, but I suspect they are there primarily to drain off gate leakage current to prevent unintended stray firing of the SCRs.

    I'd change the pot.

    awright

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    Re: Adding a pedal to lincoln AC-225 tombstone

    Quote Originally Posted by awright View Post
    The control pot operates in conjunction with the two 0.1 uF capacitors to determine at what point in the AC waveform the diacs (and, thus, the SCRs) fire. The 0.1 uF capacitors and 1K to 250K range of the control resistance presumably provides the desired range of control of the firing point. To maintain the same range of time constant in the firing circuit (and, thus, the control range) you would have to increase the capacitance values by the inverse of the resistance change, or a factor of about 25 to 50 (for 10K or 4.7K). Now you have 25 to 50 times more charge on the capacitor discharging into the SCR gate via the diac which might exceed the current or power ratings of the SCR gate or diac. Whether this is actually a problem or just my imagination is a reasonable question, but should be checked against device specs unless you want to perform a smoke test. The pot may also not be up to the required power dissipation. I'm not sophisticated enough to know how the 100K resistors across the diacs would affect the firing range, but I suspect they are there primarily to drain off gate leakage current to prevent unintended stray firing of the SCRs.

    I'd change the pot.

    awright
    Thanks for the explanation. I think I probably could've clarified my original post by saying that I have two similar schematics to accomplish the same thing here, one I purchased off of dans workshop www.dansworkshop.com which I paid for and don't want to repost here for fear of getting sued. You can see the diagram I'm speaking of, or at least intending to use part of here http://www.dansworkshop.com/electric...arc-welder.htm about 3/4 of the way down the page on the RH side. I don't plan on using the MOTs, overheat switch or the easy start circuit

    Obviously you've seen mike W's schematic the concern I have is that a 250k 2W pot is not something the local radio shack is going to have in stock and I don't really want my potiometer to be pushing enough current that it needs to be fused and is likely going to get hot as well. Fear of failure and the relatively high price/rarity of a linear taper pot of that value that will dissapate that much heat has me a bit scared of the mike w schematic

    The circuit from dans workshop uses a 1M linear taper pot, doesn't specify a value for wattage but the pot and a .1uf cap run in series between the gates of the SCR module, the output of the pot runs thru a diac and into a triac which triggers the gates. From the looks of dans circuit it does the exact same thing. All the components on the dans workshop version are easily sourced from the local radio shack except the SCR module, which I already have 4 of thanks to an ebay sellers mistake and a triac, which I could steal out of a light dimmer I can get for ten bucks or less at the local hardware store.
    Last edited by greenbuggy; 06-01-2010 at 06:02 PM.

  6. #6
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    Re: Adding a pedal to lincoln AC-225 tombstone

    Mouser has stock on several styles of 250K, 2Watt, 15% tolerance linear pots in the $6 to $12 range.

    awright

  7. #7
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    Re: Adding a pedal to lincoln AC-225 tombstone

    if this was the AC/DC version ... might be worth the effort
    52, 59 & 66 Lincoln SA200, Tombstone Idealarc 250, Thermal Dynamics Cutmaster 50, Marquette 250 amp mig, Miller 330 A/BP

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    Re: Adding a pedal to lincoln AC-225 tombstone

    Quote Originally Posted by _CY_ View Post
    if this was the AC/DC version ... might be worth the effort
    While it is just the cheapo AC version, I can always add a rectifier later if need be. For now my concern is tig welding aluminum so the fact that its AC only isn't a big deal to me. I intend to build all the control electronics into a different box than the welder anyways.

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    Re: Adding a pedal to lincoln AC-225 tombstone

    Will the SCR / diac circuit work on the AC/DC version of the Lincoln 225 tombstone?

    My current TIG setup is a small DC inverter welder running through a Miller HF-251D-1 high frequency box, but I would like to add the capability to do aluminum. Obviously I'd need to change out the welder to something with AC and was thinking I might as well get something that has AC and DC. Just wasn't sure about the circuitry differences in the AC only version vs. the AC/DC version of the 225.

    Also, I have a Miller RFCS-14 foot pedal that I got along with the HF-251D-1 when I bought that used. Don't know what value the potentiometer is in that, but hopefully it's a standard physical size in case I need to change it. Need to dig that out and see...

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    Re: Adding a pedal to lincoln AC-225 tombstone

    I dug out the pedal. It's a Miller RFCS-14, style KJ-25. Inside is a Clarostat C0416136 1k-Ohm pot that gets moved about 3/4 of a turn by the pedal over its range. Couldn't find much about that particular pot online, except something on a page in Chinese. The pot has a really long shaft - the Chinese page said it's 63 mm long (almost 2.5 inches). I need to measure the range of resistance at the pins on the connector.

    So, anyone ever been inside a Lincoln AC/DC-225 tombstone?

  11. #11
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    Re: Adding a pedal to lincoln AC-225 tombstone

    Quote Originally Posted by dbotos View Post
    Will the SCR / diac circuit work on the AC/DC version of the Lincoln 225 tombstone?
    Should make no difference what the output side of the transformer is doing, just as long as the SCR/Diac circuit is properly sized to the load on the input side.

    My current TIG setup is a small DC inverter welder running through a Miller HF-251D-1 high frequency box, but I would like to add the capability to do aluminum. Obviously I'd need to change out the welder to something with AC and was thinking I might as well get something that has AC and DC. Just wasn't sure about the circuitry differences in the AC only version vs. the AC/DC version of the 225.

    Also, I have a Miller RFCS-14 foot pedal that I got along with the HF-251D-1 when I bought that used. Don't know what value the potentiometer is in that, but hopefully it's a standard physical size in case I need to change it. Need to dig that out and see...
    In the chinese pedal I bought off ebay the pot was a standard size and easy to replace with something at radio shack. YMMV.

    Quote Originally Posted by dbotos View Post
    I dug out the pedal. It's a Miller RFCS-14, style KJ-25. Inside is a Clarostat C0416136 1k-Ohm pot that gets moved about 3/4 of a turn by the pedal over its range. Couldn't find much about that particular pot online, except something on a page in Chinese. The pot has a really long shaft - the Chinese page said it's 63 mm long (almost 2.5 inches). I need to measure the range of resistance at the pins on the connector.

    So, anyone ever been inside a Lincoln AC/DC-225 tombstone?

  12. #12
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    Re: Adding a pedal to lincoln AC-225 tombstone

    Sweet. Looks like this will be my road to aluminum TIG'ing then. So the DC+/DC-/AC selector on the AC/DC-225 just controls whether it gets rectified (and the polarity if it gets rectified)?

    The pot looked pretty standard except for the long shaft. Guess I could always make an extension if I got one that had a more normal shaft length. Can the SCR / diac circuit be massaged to work with a 1k-Ohm pot or should I just switch out the pot in the pedal?

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    Re: Adding a pedal to lincoln AC-225 tombstone

    Quote Originally Posted by dbotos View Post
    Sweet. Looks like this will be my road to aluminum TIG'ing then. So the DC+/DC-/AC selector on the AC/DC-225 just controls whether it gets rectified (and the polarity if it gets rectified)?

    The pot looked pretty standard except for the long shaft. Guess I could always make an extension if I got one that had a more normal shaft length. Can the SCR / diac circuit be massaged to work with a 1k-Ohm pot or should I just switch out the pot in the pedal?
    You would be better off switching it out, I'm sure values could be changed to work with a 1k pot but I wouldn't know enough to tell you what values to change it to. Otherwise it would be a lot of trial and error and surplus electronic components.

    Like I said above the polarity/AC selector switch is all on the output side of the transformer, so it will have nothing to do with the SCR circuit. For all practical intents and purposes you could actually wire the incoming power and SCR circuit in a separate project box with a plug for the welder power and avoid modifying the actual welder altogether.

  14. #14
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    Re: Adding a pedal to lincoln AC-225 tombstone

    Cool. I'll just switch out the pot then.

    I was thinking about adding one of the 14-pin remote sockets to the front panel of welder as well as a second pot (also on the front panel). Connect those up with a DPDT switch to the SCR / diac circuit and you'd have the ability to select panel or pedal amerage control...

  15. #15
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    Re: Adding a pedal to lincoln AC-225 tombstone

    Quote Originally Posted by greenbuggy View Post
    While it is just the cheapo AC version, I can always add a rectifier later if need be. For now my concern is tig welding aluminum so the fact that its AC only isn't a big deal to me. I intend to build all the control electronics into a different box than the welder anyways.
    this sounds like a fun project. considering that its been a year, did you get around to do this? did you build your own high frequency module as well?
    if anyone was unwilling to work, neither should that one eat - 2 Thessalonians 3:10

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    Re: Adding a pedal to lincoln AC-225 tombstone

    Quote Originally Posted by greenbuggy View Post
    While it is just the cheapo AC version, I can always add a rectifier later if need be. For now my concern is tig welding aluminum so the fact that its AC only isn't a big deal to me. I intend to build all the control electronics into a different box than the welder anyways.

    Quote Originally Posted by d94jonca View Post
    this sounds like a fun project. considering that its been a year, did you get around to do this? did you build your own high frequency module as well?
    I did build my own high frequency module based on the casano schematic posted over on hackaday.com, which was the next mod after I got the pedal situation worked out. I did put quite a few hours on the whole rig and put it on its own dedicated cart, but then had the opportunity to purchase an awesome L-tec (used to be linde, now owned by Esab) Heliarc 307i tig for $199, and ended up selling the homebrew tig to someone in the next town over for a couple hundred bucks. As far as I know its still welding. It did work great, but unfortunately I don't have the experience to modify the lincoln to do pulse, have ac balance and frequency control to give me ultimate control over the arc, and my welds have improved greatly thru the additional tig experience and additional control the new welder gave me. It was a great bang-for-my-bucks project and I'd do it again if need be, but once you've used something like a Dynasty, Heliarc or Caddy Tig runner, its awful hard to turn back.

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    Re: Adding a pedal to lincoln AC-225 tombstone

    So how exactly does the SCR / diac circuit work? I get that the SCRs act like a switches, turned on at a certain gate-to-cathode voltage and shut off when current flow from anode to cathode drops below the holding current value. And that the diacs are diodes for AC. But how does it all work together to control what's going to the primary side of the transformer? Is it controlling current or voltage going to the transformer?

  18. #18
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    Re: Adding a pedal to lincoln AC-225 tombstone

    Quote Originally Posted by dbotos View Post
    So how exactly does the SCR / diac circuit work? I get that the SCRs act like a switches, turned on at a certain gate-to-cathode voltage and shut off when current flow from anode to cathode drops below the holding current value. And that the diacs are diodes for AC. But how does it all work together to control what's going to the primary side of the transformer? Is it controlling current or voltage going to the transformer?
    Its controlling phase-angle of the AC power going to the primary. At the zero crossing phase the SCR turns off, and depending upon the position of the pedal, recharges the capacitors till they trip the diacs and fire the SCR's, next zero crossing the process repeats itself.

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    Re: Adding a pedal to lincoln AC-225 tombstone

    Quote Originally Posted by greenbuggy View Post
    Its controlling phase-angle of the AC power going to the primary. At the zero crossing phase the SCR turns off, and depending upon the position of the pedal, recharges the capacitors till they trip the diacs and fire the SCR's, next zero crossing the process repeats itself.
    Cool. Going to pick up the AC/DC 225 today. Will probably play with it this weekend and open it up to plan how I'm going to hook things up.

  20. #20
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    Re: Adding a pedal to lincoln AC-225 tombstone

    Finally got around to testing out the AC/DC 225/125 this past weekend as a stick welder and then today I opened it up to figure out the details of how to wire in the SCR/diac circuit.

    When looking at the front of the unit, the DPST on/off switch is in the lower left. Coming into the top of the switch are two black 12 AWG wires from the power cord. Connected to the lower terminals of the switch are two small gray wires (for the fan) and two flat aluminum leads (primary side of the transformer).

    My plan is to disconnect the flat aluminum lead from the left side of the switch (while leaving the fan lead still connected). Then bring two 10 AWG wires (cause I have some spare sitting around) through grommeted holes in the back panel. One will connect to that flat aluminum lead (that was formerly connected to the switch) and the other will connect to the switch, effectively breaking the circuit between the switch and transformer on one side. The other ends of those two 10 AWG wires will connect to the SCR/diac circuit in separate enclosure that will probably get piggy-back mounted to the back of the welder (in a way that it doesn't block any cooling).
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    Last edited by dbotos; 09-07-2011 at 11:12 PM.

  21. #21
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    Re: Adding a pedal to lincoln AC-225 tombstone

    Okay, I need a little help with my shopping list. The resistors, potentiometer, capacitors, diodes, and fuses all seem straightforward, but I'm a little lost on what to spec for the SCRs and diacs. The Lincoln website says the AC/DC-225/125 draws up to 50 A in AC mode or 36 A in DC mode, so that means I'll need SCRs that can handle at least 50 A, and probably be rated at a few times the working voltage. The other values I'm clueless on, but I'm assuming they need to be chosen correctly in order for the circuit to work correctly...

    R1 = 100k, 1/2 W
    R2 = 100k, 1/2 W
    R3 = 1k, 1/2 W
    R4 = 250k, 2W, linear

    C1,C2 = 0.1 uF, 600 V
    D1,D2 = 1N4007 (1000 V, 1 A)
    F1, F2 = 0.5 A, 250 V

    SCR1,SCR2 = >50 A on-state current, _?_V off-state voltage, _?_mA gate trigger current
    DI1, DI2 = _?_V breakover voltage

    Thanks,
    Dave
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    Last edited by dbotos; 09-12-2011 at 12:55 PM.

  22. #22
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    Re: Adding a pedal to lincoln AC-225 tombstone

    For Diacs I used DB-3, easiest to find them on ebay.

    For SCR's I used the IRKT71's as specified in the OP. Beyond that I don't think you need anything too fancy, and I was able to pick them up cheap on ebay.

  23. #23
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    Re: Adding a pedal to lincoln AC-225 tombstone

    Not sure if any of these parts are of use to you as you are over my head in what you are doing but i thought the main amp control rheostat?could be used to make a remote tig pedal or something. anyway check my thread here and see pics http://weldingweb.com/showthread.php?t=60017

    for DC, why not get a little everlast or Longevity stick/tig for about $200 seems you would be saving money over what you are trying to do now.. plus you will get the torch and all you need to start..
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  24. #24
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    Re: Adding a pedal to lincoln AC-225 tombstone

    I just ordered some stuff yesterday from Mouser. Diacs were 32 V trigger DB3TG from ST Microelectronics and the SCRs were SK055R from Littelfuse (55 A, 1000 V, 1.5V/40mA gate trigger). Hopefully those SCRs will work okay. I guess I'll soon find out...

  25. #25
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    Re: Adding a pedal to lincoln AC-225 tombstone

    Still waiting on backordered caps from Mouser. Started roughing out a layout on a scrap piece of perfboard. What's the best way to connect a ring terminal to the anode of these TO-220 package SCRs? Maybe some of these 30 Amp PCB screw terminals in the board?

    http://www.keyelco.com/products/prod19.asp

    I know they're only rated for 30 A, but compared to the leads on those TO-220s, they're pretty beefy. What about heatsinking for the SCRs? I've got a bunch of random aluminum blocks and sheets if something simple would work.

    Thanks,
    D
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