After doing some research on this and a couple other sites, I made a few modifications to my Harbor Freight 151 Dual MIG this past couple weeks. The information was all out there, but I didn't feel there was any straightforward how-to guide for the mods - so I decided to document my work and put one together myself. When following this or any how-to, be sure to unplug your welder and keep safety in mind. Double check your own work. I'm not responsible if you or your wife, husband, brother, sister, cat, dog, etc get hurt. Without further adieu...
Here is a diagram of the rear section of the welder in stock form. The rectifier takes in the AC current that is output from the transformer on the terminals marked ~ and outputs DC current on the +/- terminals. A lot of the questions I saw were regarding where to attach the capacitor. It connects to the +/- terminals of the rectifier.
The first thing that needs changing is that fan. I searched on eBay and found a 220v "muffin" fan meant for a tanning bed. It came with a pigtail for wiring it. The auction was for about $15, coming to about $22 delivered. Make sure you get a 220v one though, since there are also 110v versions.
This one is pretty easy, just cut the wires a couple inches from the old fan and splice in the wires to the new fan. I trimmed the wires so that they would reach with a little slack to work with, but not so much as to leave a mess of extra wire in the welder. I prefer to solder and heat shrink but I suppose crimp connectors would do in a pinch. After wiring, I unplugged the electrical connection and mounted the fan.
For mouting, I used #8 x 1/2" sheet metal screws. I had them on hand and they were just the right size to bite into the frame of the fan and hold it in place.
I used the fan as a template to drill the mounting holes by holding it up to the back of the welder and drilling through the holes in the fan using the biggest drill bit that would fit through the holes without damaging them (about 1/8"). Then I set the fan aside and drilled the holes out slightly bigger so that the screws would fit through easily. Then I hooked the power wire back up to the fan, placed it in the welder, and ran the screws through the back of the welder and into the frame of the fan. Make sure the fan is facing so that it blows in. The airflow direction should be marked on the side of the fan frame.
Grab yourself a beer (or some iced tea if you're not of drinking age) and pat yourself on the back. Your first mod is done. Double check your work, plug it in, turn it on, and listen to the sweet sound of success
After searching for a bit for a suitable capacitor, I figured I'd just check to see if the Miller part was available that they use in the Millermatic/Hobart Handler 180. It is available as part 228 176, as well as the holding clamp as part 108 105. I ordered them from miller4less.com. There is an extra $10 charge for orders under $50 and they came in just under $50 so you might want to throw in some "filler" to break the $50 mark. If you don't have anything else to order, I'd suggest a couple of 229 895 wire and metal thickness gauges.
Here is the capacitor (the dents are by design -- how odd):
And the clamp:
I also needed a bleed resistor. I found one on eBay for about $4 plus a couple more bucks shipping. 49.9 ohms, 20 watts. From what I read 50-75 ohms is acceptable, but the Miller uses a 50 ohm so I got the closes thing I could find.
The first task was to mount the capacitor. I went to Lowe's and bought the remaining hardware to mount it. 1/4"-28 x 1/2" bolts to thread into the capacitor, 1/4" flat washers to help hold the wires, #8-32 x 3/4" screws and nuts to hold the clamp, #8 lock washers for use with said screws/nuts, and 2 of 1/4" x 1" hex bolts, plus 4 of 1/4" flange nuts. I also bought a couple feet of 10-2 Romex for wiring the capacitor.
There was only one place it would fit, horizontally between the rectifier and the inductor. I attached the clamp to the capacitor (I oriented it so that the screw portion of the clamp pointed straight up with the positive terminal on the left), held it in place, and used a Sharpie to mark the holes for drilling. Then I drilled out the holes. A little tip - remove the wire spool first! One of my holes was behind the spool and when the bit broke through the metal, it ran into the spool, breaking the spindle. The nuts and washers are a bit hard to get on to the screws, so you may want to lay the welder on its side and let gravity help you out. Here is what it looks like mounted:
Now that the capacitor is mounted, we should probably wire it. The + terminal needs to be connected to the + plate of the rectifier, and the - terminal to the - plate. I cut a couple pieces of the black wire from the 10 gauge romex and wired it up using that. I used the 1/4" x 1" bolts on the lower holes of the rectifier plates along with a flange nut on each to attach the wires. The bolts were a bit snug and tricky to fit, but they worked.
Next, we need to add the bleed resistor. This will dissipate the energy from the capacitor when not in use so the torch doesn't stay hot very long after you let go of the trigger. I cut the remaining bit of black 10 gauge wire in half and soldered each half to the resistor:
I then wired it up similar to the capacitor, using the second flange nut on each bolt to attach it to the rectifier. The resistor is non-polar, so you can attach either wire to either post. Since it is only rated for 20w when mounted to a panel, and I couldn't find a convenient way to mount it to any of the panels, I bent the wires so that it was directly in the airflow of the fan:
Now the capacitor part is complete, but if you use it as-is, the energy from the capacitor will continue to run the wire feed motor for several seconds after you let off the trigger, expelling about 4 feet of wire! That's obviously no good. The easiest solution is to add another small rectifier specifically for the trigger and wire feed. I happened to have one in my parts bin that I salvaged from a computer power supply a few years ago.
It is rated for 6A and 600V but we don't need nearly that sort of power so it will be more than adequate. The torch trigger and wire feed power wires are the small ones that are connected to the +/- of the main rectifier, as seen here:
I clipped the cable ties holding the wire bundle to make the wires easier to work with.
I first removed the - ones, and cut them about 4 inches from the terminals. I stripped and twisted together the remaining wire, slid some heat shrink over them, and soldered them to the - terminal of the new, smaller rectifier. I did the same with the wires connected to the + terminal of the main rectifier, soldering them to the + terminal of the new rectifier. I then took two of the pigtails that were cut from those wires and soldered them to the remaining terminals of the rectifier, which are unlabeled.
Take the two wires with ring terminals that you attached to the AC input terminals of the new rectifier and attach them to the AC input terminals of the main rectifier, as shown in the diagram at the top of the thread. Attach them in the same fashion as they were originally attached to the +/- terminals.
(sorry no pic here)
Additionally, I added a 100uf 35V capacitor to the wire feed motor to help smooth the wire feed a bit. The negative terminal of the motor faces the front on my machine. You can verify which is negative by tracing the wire back to the rectifier you just installed.
After you've completed your mods, use a couple cable ties to replace the ones you clipped earlier and to tie up the rectifier.
Enjoy!
Last edited by mobes; 09-07-2009 at 03:04 PM.
Reason: Added Content
And now what is the output of the welder? I'm assuming originally it was 150A? And with the additional capacitor it's now????
Not sure on that one myself. I didn't personally do the mods for more amps since I mostly weld sheet metal, but for the more stable arc and wire speed.
The only thing you you might still do is to put a metal reel spindle in, if you use the large reels of wire the spindle will break if the welder is bumped hard, or bounced a little in a pickup.
outstanding! i've been wanting to do the cap-mod for quite awhile but wanted an explicit guide like this one. is there a big difference in arc stability? i'm mostly welding on level 3 and it often takes half a second of sputtering before getting a stable arc but after that its good. however, when i've tried the lower levels it seems to get much pickier
i recently replaced the junky plastic liner with an after market metal ditto from the LWS. it was pretty easy and it would have been easier yet had i bothered reading the users manual which explicitly explains how to do it without wrecking the blue plastic fittings in the gun anyway, it made wire feed much more stable, in particular when the gun cable is curved
Bluewelders - I saw your posts in some other threads, and they were helpful research for this guide, so thank you. That spindle looks like a good idea. I have a Miller 10 lb spool adapter that won't fit over my HF spindle since it is just barely too big around so that would provide me the benefit of being able to use the adapter as well (and having a spindle that would actually hold a 10 lb spool).
I think if I could do it over again I'd just buy a Hobart - but I already had the money invested in this one so I figured it was best to try to work with what I had. I do like the fact that it's on wheels and has a built in bottle holder.
d94jonca - It's way more stable on level 1 and 2, which is all I typically use so I'm happy with it. The wire feed speed is still a bit fast on level 1 though. I may add a couple diodes as suggested elsewhere. You'd think level '0' would be a bit slower. LWS, is that local welding supply? Is the liner a specific fitment, or was it universal?
Has anyone thought about moving the power cord to the back of the welder?
At the time it seemed I was the only technical type willing to pull a 151 apart and redo it.
I suppose I should have typed up the details, but the demand didn't seem very high.
Since then I've done a 131 and helped do a Hobart 125.
I used a spare liner from my Miller Cricket, cut to length.
I made the new spindle so that a Lincoln M15445 reel adapter would fit it.
Some 2amp diodes will drop the speed with plenty of safety margin
If I was to guess, I would say I gained about 25% more amps on the high range,but I never measured it before the changes .
The capacitor totally changes the way it sounds and welds.
Hello all, I read in another thread that the same trick could be done with the other HF welders? What would need to change in order to apply this to the 90 amp variety?
So I have ordered everything accept the small cap, resistor and bridge rectifier. I can't find that stuff and don't have a clue. Any chance you can show me where to get it?
Yes, LWS is the Local Welding Supply. a little outfit named airgas in my case. the liner i used was a 15' tweco style steel liner made by radnor. not that the style actually mattered as the first thing i did was to dremmel of the connector. i just made sure it was for the right diameter wire, and that its outer diameter was about the same the original liner. in the end i had to make the liner a tad bit thicker at the welder end by putting shrinking tube on it so as to get a good fit into the welder.the replacement liner was actually more than twice as long as the plastic one that came with the welder so i guess i now have a steel spare laying around for the unlikely case the first one ever wears out. except since the remaining half is all uninsulated i'd have to insulate the part that goes into the welder somehow. if someone wants to do this i'd be happy to tell about my mistakes so they dont have to be repeated. the original plastic liners wear out quickly so if you have a few # of wire through you are likely to notice a pretty big difference.
I have to say this thread is amazing reading. When I've read the older threads on this topic i just couldnt get the point with the new rectifier but after comparing your pics with the schematics in the users manual I think i'm getting it: the original design uses one heavy-duty rectifier which primary purpose is to rectify the welding current and secondary purpose is to power the feed motor. but once we've added the capacitor to further stabilize the welding current, this no longer works well as the motor keeps running while the capacitor keeps discharging. so we can no longer have the motor piggyback on the main rectifier, we must introduce a new dedicated rectifier (which can be tiny since all it does is run the motor) and this new rectifier is essentially sitting in parallell with the old heavy duty one. i.e. like the heavy duty rectifier, it uses the transformer output as its input, and the wires to the motor/driver are simply moved from the output of the original rectifier to the output of the new rectifier? corrrect?
i got a few questions: why does the motor run faster after this mod? it used to run off the output of one rectifier and now it runs of another, right? the only reason the motor would run faster that i could imagine is that the voltage somehow is higher from the new rectifyer but why would that be? also, i've read in some threads to introduce a diode in serial with the motor to slow it down. why cant one just use a regular resistor instead?
furthermore, the cap mod is said to make it easier to establish an arc. in my stupidity i would have thought it'd be the other way around; the cap would result in *less* juice during the beginning of a weld since the cap is building up with energy that would otherwise have gone into the arc. is this not noticable at all?
last: what exactly is open circuit voltage on this machine and how does it relate to the voltage of the cap? the manual says 40 volt and checking (on welding level 4) my multimeter reads exactly 40.0 vdc. but the voltage really isnt "normal " dc since it varies between 0 and x volts with a frequency of 120 hz. and so im suspecting the 40 volt value should be considered an average, with a peak voltage probably around 60 volts - correct? doesnt this mean that the cap added really should be rated for 60 volts?
The bridge diode for the wire feed doesn't share quite the same voltage drop while welding, as the pcb had originally.
So the voltage ends up a little higher.
The control board is a Pulse Width control circuit that runs at 120hz, and the motor voltage is regulated to a max of 15v, the 100-200mfd capacitor across the motor shunts some of the spikey voltages away from the board and presents a higher average voltage to the sense circuit..
The higher average in the feedback voltage makes the control board run the wire feed slower.
A PM motor like the one used in the wire feed, will run at a certain speed at a certain voltage, volts is rpm amps is torque.
If you are running a motor at 6V@1A and 3000 rpm, and you load it down,the motor would pull 2A but if you maintain 6V it will stay at about 3000rpm
A resistor to slow the motor will drop a certain voltage at a certain current draw,but if you increase the load the voltage and rpm on the motor will drop.
But a diode will drop about the same voltage whatever the current.
The choke coil in a welder used to be called a reactor and was used to make the arc easier to maintain.
Adding the capacitor makes the choke function more efficiently while the arc is established, and provides the equivalent of thousands of amps to start the arc .
Capacitors have a rated voltage and a surge rating, the higher OC voltage with the capacitors, should be within the surge rating of a 50V capacitor.
If you do slightly exceed the surge rating of a capacitor it will start leaking power, the leakage will make the capacitor start to heat and shorten its life.
"But" the OC voltage is only applied to the Capacitor when there is no arc and the torch button is pressed, so even a underrated capacitor should last for years.
Sorry for so long and boring
blue: definitely not boring. i just learned something new about electric motors - thanks just ordered me a pair of 30000 uF/60v caps of ebay - really looking forward to doing this mod now that i finally understand how to.
backon: good luck on swapping the liner. as indicated before its pretty straight forward. the only challenge is how to remove the old liner from the blue coupler inside the gun. i used brute force and broke the coupler in the process, although still managed to fit the new liner nicely. after the fact i realized the users manual have a section on how to do it right - you are supposed to push the coupler forward while pulling the liner backwards and it should come out w/o resistance.
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Originally Posted by MarkBall2
i've been wanting to do the cap-mod for quite awhile but wanted an explicit guide like this one. is there a big difference in arc stability? i'm mostly welding on level 3 and it often takes half a second of sputtering before getting a stable arc but after that its good. however, when i've tried the lower levels it seems to get much pickier