Harbor Freight Mig 151 Capacitor Upgrade Modification
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  1. #1
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    Harbor Freight Mig 151 Capacitor Upgrade Modification

    I have been welding with this machine for quite some time now. Seems to be a good welder for around the shop and more than enough for the work I come across. I was browsing the forum and read a few threads on adding a capacitor to help stabalize the arc.

    Is there a nice walkthough that someone could point me to that I could follow along to help me complete this modification?

    Thanks.

  2. #2
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    Re: Harbor Freight Mig 151 Capacitor Upgrade Modification

    It's relatively easy to do, usually the hardest part is getting a capacitor or capacitors to fit in the available space.
    If you have a small familiarity with electronics,basically you hook a large capacitor across the output rectifier and add a small bridge rectifier, a capacitor and a couple of diodes.
    Robert

  3. #3
    Join Date
    Aug 2006
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    194

    Re: Harbor Freight Mig 151 Capacitor Upgrade Modification

    There is a thread on here that I started that ha lots of good information in it. Blue pretty much broke the whole procedure down and added some interesting pics as well. I am sure you probably saw it if you did a search. If you need pics of anything else let me know.

  4. #4
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    Re: Harbor Freight Mig 151 Capacitor Upgrade Modification

    Thanks for the help guys. Yes, if you could provide a couple pictures that would be very helpful. What is the minimum specs on the capacitor needed for this modification?

  5. #5
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    Re: Harbor Freight Mig 151 Capacitor Upgrade Modification

    The capacitor should be 40,000 to 100,000uf or a couple of them wired in parallel to add up to that.
    The voltage rating needs to be at least 50v but higher won't hurt.
    You will need a 50-75 ohm 20w or more resistor across the capacitance to keep the torch from staying live.
    The thread Yellow was taking about is:http://weldingweb.com/showthread.php...282#post279282
    Robert

  6. #6
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    Re: Harbor Freight Mig 151 Capacitor Upgrade Modification

    Blue, I was reading your initial postings and thought your brain fell out, adding a second bridge and bleed resistor didn’t make sense.
    Got into the link and now I understand, the 151 doesn’t have a cap to start with. I can’t tell from the photos, the diode looks to be a button type. With the extra heat you may need a dab of thermal greases under the diode to help wick-off the heat. My Century 170 originally had one 100K uF cap.; I doubled that to 200K uF. At 100 amps, without changing the settings, the second cap adds another 50 amps to the welding current, not much change to the arc voltage. Now I’m working on getting the electronic relay to open more and not run in saturation.
    BTW, you don’t want to use a cap with a WVDC that is too much larger then the open circuit voltage with the cap in the circuit. Remember an Electrolytic capacitor has a Forming Voltage and if the applied voltage is very much less then the WVDC you wouldn’t get the full value of the cap.

  7. #7
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    Re: Harbor Freight Mig 151 Capacitor Upgrade Modification

    Any of the 151 type welders I have worked on, I have added a muffin fan and ducting to keep the diodes cooler.
    The fan that comes on them just flails the air around, the air exchange ratio is pathetic.

    The 151 uses an scr driver circuit for the motor.
    The output capacitor keeps the voltage from going to zero, so the scr can loose carriers and shut off the wire feed.
    Powering the motor from a second bridge was an easier workaround than adding a relay.

    The capacitors should have their rated capacitance at a lower voltage, they will just loose thickness in the oxide coating over time, and drift up in capacitance.
    Robert

  8. #8
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    Re: Harbor Freight Mig 151 Capacitor Upgrade Modification

    I have one of the 90 amp HF welders and I was thinking about adding some diodes and creating a full wave bridge rectifier circuit...as well as a cap to smooth out the DC....I also thought maybe I would order the board for the 151 and use the rectifier circuit on my 90... Does anyone have any opinions or ideas on how to do this? or if its possible?

    Nate

  9. #9
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    lancaster,kentucky
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    853

    Re: Harbor Freight Mig 151 Capacitor Upgrade Modification

    what would it take to make the hf 98871 90 amp a non-live torch welder?or is it not possible at all....doug
    Maxus Pro-125 Mig
    Chicago Electric 90 amp DC flux-core
    Lincoln Electric AC 225 tombstone
    O/A torch
    M/O mini-torch
    10 acres of flatland
    15 acres of holler

  10. #10
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    Re: Harbor Freight Mig 151 Capacitor Upgrade Modification

    Quote Originally Posted by elektro21 View Post
    I have one of the 90 amp HF welders and I was thinking about adding some diodes and creating a full wave bridge rectifier circuit...as well as a cap to smooth out the DC....I also thought maybe I would order the board for the 151 and use the rectifier circuit on my 90... Does anyone have any opinions or ideas on how to do this? or if its possible?

    Nate

    Nate, a lot depends on what’s under the covers. Yes, you can add a rectifier to convert to DC. I think that just adding a cap across the rectifier will be inadequate, causing a harsh arc. For proper performance you’ll also need an inductor in series with the welding cables. Some HF units are equipped with an inductor and no cap. The next problem, is there room? Check around the sites, building a Mig welder for more information, Microwave ovens do have a second life.

  11. #11
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    361

    Re: Harbor Freight Mig 151 Capacitor Upgrade Modification

    Quote Originally Posted by transit View Post
    Blue, I was reading your initial postings and thought your brain fell out, adding a second bridge and bleed resistor didn’t make sense.
    Got into the link and now I understand, the 151 doesn’t have a cap to start with. I can’t tell from the photos, the diode looks to be a button type. With the extra heat you may need a dab of thermal greases under the diode to help wick-off the heat. My Century 170 originally had one 100K uF cap.; I doubled that to 200K uF. At 100 amps, without changing the settings, the second cap adds another 50 amps to the welding current, not much change to the arc voltage. Now I’m working on getting the electronic relay to open more and not run in saturation.
    BTW, you don’t want to use a cap with a WVDC that is too much larger then the open circuit voltage with the cap in the circuit. Remember an Electrolytic capacitor has a Forming Voltage and if the applied voltage is very much less then the WVDC you wouldn’t get the full value of the cap.
    How do you figure that you are getting an extra 50amps by doubling the capacitance?

  12. #12
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    Re: Harbor Freight Mig 151 Capacitor Upgrade Modification

    Billy, I didn’t figure it, I measured the current.

    With the two parallel capacitors in the output circuit.
    I installed an induction ammeter on the ground cable, set the controls, heat and wire speed to indicate 200 amps.
    Without changing the heat and wire speed, I removed one cap from the circuit and measured the current again.
    The current measured was 150 amps, 50 amps less than with both caps in the circuit.

    If you’re thinking of adding more capacitance there are limits you’ll run into. Diodes have pages of specifications, depending on the application, what you must pay attention to is the current and thermal limits.

    The forward voltage drop across a typical power diode is 0.75 to 0.80 volts. The higher values go hand in hand with higher current draw and case tempertute.

    Next are the allowable currents;

    There is the maximum “onetime” inrush current, as when you pull the trigger when the cap is first charged from 0 volts.

    There is the maximum “continuous” inrush current. Once you start to weld, the voltage across the cap does not fall to 0, the voltage only has to be pumped up to match the peak voltage from the transformer.

    There is the maximum “continuous” current. This is the maximum current that can be passed by the diode.
    These first three currents may or may not be the same depending on just where on the sign wave current to charge the capacitor is required.

    There is the maximum rate of change current. The diode “pill” in the case dose not turn-on instantaneously everywhere.
    A spot on the pill, a very small spot, starts to conduct and grows, spreads out until the whole pill is conducting. This takes time and like anything that carries current with a voltage drop that spot gets hot. Hence, the limit to keep from over heating the pill in anyone spot. This is a concern in inverter power supplies where the diode sees a square wave that tries to force the diode to conduct instantaneously. Now you say, that doesn’t apply because we’re working with a smooth sign wave. We can’t guarantee or predict where in the smooth sign wave conduction will begin and that will cause a sudden jump in the forward current.

    Not having information about the diodes limits how much they can be pushed to the limits, not knowing what the limits are.

    An FYI, my mig has 24 button diodes arranged as two half wave diodes with a center taped transformer. I’ve measured the max RMS current at 250 amps. Each button carries 250/24 = 10.4 amps. The power dissipated is 10.4 x 0.8 =8.3 watts each or 200 watts total max. That’s why the heat sink is important.

  13. #13
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    Re: Harbor Freight Mig 151 Capacitor Upgrade Modification

    Did you measure the output on the output leads,or from the diodes.
    The inductor/capacitor ringing, may have fooled your amp meter.
    If you are getting 50% more power , your welder should be getting hotter faster.
    Power can't come from "nowhere".

  14. #14
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    Re: Harbor Freight Mig 151 Capacitor Upgrade Modification

    Put simply it is impossible.
    Sure you didnt replace the transformer and forgot to mention that?
    The main effect of caps is to smooth the DC, so with the original cap, adding another will not have the effects that you are assuming.
    Either your meters are faulty or............................
    Add another 10 caps and see if you can get an extra 500amps !!!!!!!!!

  15. #15
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    Re: Harbor Freight Mig 151 Capacitor Upgrade Modification

    Quote Originally Posted by Bluewelders View Post
    Did you measure the output on the output leads,or from the diodes.
    The inductor/capacitor ringing, may have fooled your amp meter.
    If you are getting 50% more power , your welder should be getting hotter faster.
    Power can't come from "nowhere".
    If you read closely, I said, “I installed an induction ammeter on the ground cable, set the controls, heat and wire speed to indicate 200 amps.”
    I don’t believe that ringing would be a problem, as the L-C circuit is most likely over damped and electrolytic capacitors don’t take to reverse polarity well. If something was to go wrong it would have happened by now.

    The power supply does heat considerably more, that is why I also up graded the rectifier heat sink from 1/16 in aluminum to 1/8 in copper with the same foot print. Copper is more thermally conductive than aluminum, about 2-1/2 times so, diamond is the highest. Increasing the thickness adds in the heat dispersion from the diodes point of contact into and outward into the cooler part of the heat sink. I also added a drop of heat sink grease between the diodes and sink to help lower the thermal resistance of the diode to sink junction. The cooling fan blows up a storm, the only thing is to be sure of, is that air is flowing through and not leaking out of the case seams.

    From the 171 specs, the primary current is 20 amps. The circuit diagram shows a solid state contactor, on the case is printed “max current = 40 amps”.

    At 130 amps welding current and 20 amps to the primary, 40 amps would yield 260 amps for the welding current.
    Setting the controls to max power, I measured 250 amps in a short burst. That’s 96% of what I estimated full power to be. I keep the current below 200-225 amps. Now I’ve run into other limitations, transformer core, inductor core, copper losses. Outside of a major overhaul, I’ll stop here.

    If I need more heat, I’ll tap into a super nova.

  16. #16
    Join Date
    Jan 2009
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    361

    Re: Harbor Freight Mig 151 Capacitor Upgrade Modification

    Quote Originally Posted by transit View Post
    Billy, I didn’t figure it, I measured the current.

    With the two parallel capacitors in the output circuit.
    I installed an induction ammeter on the ground cable, set the controls, heat and wire speed to indicate 200 amps.
    Without changing the heat and wire speed, I removed one cap from the circuit and measured the current again.
    The current measured was 150 amps, 50 amps less than with both caps in the circuit.

    If you’re thinking of adding more capacitance there are limits you’ll run into. Diodes have pages of specifications, depending on the application, what you must pay attention to is the current and thermal limits.

    The forward voltage drop across a typical power diode is 0.75 to 0.80 volts. The higher values go hand in hand with higher current draw and case tempertute.

    Next are the allowable currents;

    There is the maximum “onetime” inrush current, as when you pull the trigger when the cap is first charged from 0 volts.

    There is the maximum “continuous” inrush current. Once you start to weld, the voltage across the cap does not fall to 0, the voltage only has to be pumped up to match the peak voltage from the transformer.

    There is the maximum “continuous” current. This is the maximum current that can be passed by the diode.
    These first three currents may or may not be the same depending on just where on the sign wave current to charge the capacitor is required.

    There is the maximum rate of change current. The diode “pill” in the case dose not turn-on instantaneously everywhere.
    A spot on the pill, a very small spot, starts to conduct and grows, spreads out until the whole pill is conducting. This takes time and like anything that carries current with a voltage drop that spot gets hot. Hence, the limit to keep from over heating the pill in anyone spot. This is a concern in inverter power supplies where the diode sees a square wave that tries to force the diode to conduct instantaneously. Now you say, that doesn’t apply because we’re working with a smooth sign wave. We can’t guarantee or predict where in the smooth sign wave conduction will begin and that will cause a sudden jump in the forward current.

    Not having information about the diodes limits how much they can be pushed to the limits, not knowing what the limits are.

    An FYI, my mig has 24 button diodes arranged as two half wave diodes with a center taped transformer. I’ve measured the max RMS current at 250 amps. Each button carries 250/24 = 10.4 amps. The power dissipated is 10.4 x 0.8 =8.3 watts each or 200 watts total max. That’s why the heat sink is important.
    Most of this is garbage !
    The problem is that some people will believe it, that is a worry.

  17. #17
    Join Date
    Jan 2009
    Posts
    361

    Re: Harbor Freight Mig 151 Capacitor Upgrade Modification

    Quote Originally Posted by transit View Post
    If you read closely, I said, “I installed an induction ammeter on the ground cable, set the controls, heat and wire speed to indicate 200 amps.”
    I don’t believe that ringing would be a problem, as the L-C circuit is most likely over damped and electrolytic capacitors don’t take to reverse polarity well. If something was to go wrong it would have happened by now.

    The power supply does heat considerably more, that is why I also up graded the rectifier heat sink from 1/16 in aluminum to 1/8 in copper with the same foot print. Copper is more thermally conductive than aluminum, about 2-1/2 times so, diamond is the highest. Increasing the thickness adds in the heat dispersion from the diodes point of contact into and outward into the cooler part of the heat sink. I also added a drop of heat sink grease between the diodes and sink to help lower the thermal resistance of the diode to sink junction. The cooling fan blows up a storm, the only thing is to be sure of, is that air is flowing through and not leaking out of the case seams.

    From the 171 specs, the primary current is 20 amps. The circuit diagram shows a solid state contactor, on the case is printed “max current = 40 amps”.

    At 130 amps welding current and 20 amps to the primary, 40 amps would yield 260 amps for the welding current.
    Setting the controls to max power, I measured 250 amps in a short burst. That’s 96% of what I estimated full power to be. I keep the current below 200-225 amps. Now I’ve run into other limitations, transformer core, inductor core, copper losses. Outside of a major overhaul, I’ll stop here.

    If I need more heat, I’ll tap into a super nova.


    And this lot as well

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