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Rufus 01-10-2017 08:55 AM

harbor Freight 90 amp flux core conversion
Hi all:
I see these people on youtube that will buy the Harbor Freight 90 amp flux core wire welder and convert it to a DC welder. DC because these cheap welders weld with an AC output and the welds are nasty and so as to lay a nice bead they convert these welders to put out DC using a bridge rectifier, resistor and capacitor. Now, if my current line of thinking is correct, aren't they losing current in the weld output when a bridge rectifier is installed to take the weld current from AC to DC? And if that is the case, isn't the weld current going from 90 amps AC down to maybe 75 amps DC? I think this might be the case because the Miller Thunderbolt AC 225/DC 160 current drops down to 160 when welding with DC. Now with my Lincoln Tombstone the current is at 125 on DC. So wouldn't this be the case when someone adds a bridge rectifier to one of these cheap HF 90 amp flux core machines?


alchemist 01-10-2017 09:56 AM

You shouldn't lose any current with the rectifier. Are you sure thqt machine is A/C? I thought wire feeds were D/C.

Ironman 01-10-2017 09:58 AM

First off, AC measurement is peak to peak.
In layman's terms, when you cut off the peaks and put them the hollows where zero was on AC, (which is what rectifying sort of does) the DC current is less, The voltage is higher, and I will wager that it is hotter and much easier to light off, and a stable smooth arc and nicer looking bead.

These people mucking about with a HF welder are wasting money, they should be playing with a microwave oven transformer, and would get the same result.

MAC702 01-10-2017 10:04 AM

Rectifying doesn't cut off peaks, it just puts them all on the same polarity, assuming full wave bridge rectifying. If only using a single diode (instead of four), then yes, you'd lose half your power, but no one would do that.

But yeah, I'd believe even these cheap HF wire welders are already DC.

Now, maybe they are using inductance or something to smooth it out.

Any reason you couldn't link the video you are asking about?

terry lingle 01-10-2017 10:05 AM

No they do not lose current.
The rating of the selected diodes or bridge plus the effect of rectification on the transfomer heating sets sets the DC rating.

A practical consideration is that rod sizes must be matched to welder capacity.
A 160 amp welder would be fine with 1/8 or smaller rod but under rated for 3/16 and larger rod.
Cost of manufacturing a transformer dictates final cost of the welder.
Since wire size controls the current rating and the required amount of steel in the transformer and wire gauges are standardized so current output generally comes in discreet steps. Custom wire gauges are possible but at higher cost which makes them impractical for budget welders

Rufus 01-10-2017 10:23 AM

Here is one fellow doing a conversion, taking it from AC to DC using a diode, inductor and capacitor.


Rufus 01-10-2017 10:32 AM


Originally Posted by alchemist (Post 678593)
You shouldn't lose any current with the rectifier. Are you sure thqt machine is A/C? I thought wire feeds were D/C.

Yeah, these HF flux core machines are AC and that is why they put down a messy weld bead and hence the reason a lot of people do the AC to DC conversion.


JohnBoy 01-10-2017 11:40 AM

if you can get the parts for cheap then it's kinda fun to modify some of these cheaper mig welders.

I've done two, both better starting points than the 90A machines, both were already DC, but both had lots of room for improvement.

On the 130A machine I used have I added a seperate wire feed transformer and replaced the motor controller circuit with a generic PWM, Also added a cap and upgraded the cooling fans.

On a cold day it was a mighty machine, the extra cooling fans really pushed the duty cycle and the capacitor added some serious snap to the arc too.

I got a 170A replacement which was a marginally better starting point. Similar treatment of secondary PSU, PWM for the motor, brace on the feed mechanism and two capacitors. plus the obligatory cooling upgrade. That one only made sense on the basis that I bought the welder so cheap to start with as I used bought not scrounged parts. I dont think the maths add up to buy a welder new and add these bits to them.

MAC702 01-10-2017 04:58 PM


Originally Posted by Rufus (Post 678598)
Here is one fellow doing a conversion, taking it from AC to DC using a diode, inductor and capacitor...

Using only one diode?

rustythe4x4 01-10-2017 08:31 PM

Even the cheap wire feeders will be DC.

Don't bother trying to stick weld with it. You will be less than happy with the results.
Go buy yourself an old AC machine off craigslist and rectify that with diodes. It will be a hundred times better, and cheaper.

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