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  1. #1
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    Question Electrolytic Capacitor Series Connection

    Hi guys !

    I added a bridge rectifier to an AC welder + inductors + capacitors to be able to weld DC...

    The capacitors I have are 2x 2200µF @400V the output voltage is only 90VDC but the capacitance is not enough for welding DCEP with 7018 electrodes which keep on sticking instead of initiating the arc.

    I do have a 10000µF @50V capacitor on hand, I am thinking about wiring it in series with the other two hoping it will be ok @ 90VDC across the chain +2200µF-+10000µF-+2200µF-...

    I need advice since I am not sure whether the unbalance will have side effects... I don't want a ~1KG cap to explode on me...a tiny one is scary enough.

    So, is it safe to proceed or will it go BANG!!! ???

    Thanks very much !

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    Default Re: Electrolytic Capacitor Series Connection

    Series will cut the value not add to it.
    You need to add caps in parallel to add.
    Al.

    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
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    Default Re: Electrolytic Capacitor Series Connection

    Hi,

    So, is it safe to proceed or will it go BANG!!! ???
    Don't do it. it will probably go bang.

    You need some inductance as well as capacitance. If you have capacitance alone on the output electrodes will stick and
    you get short circuit current from the capacitors....you need a little inductance as well as the capacitors.

    Your two 2200 uf capacitors are a joke. A Miller Deltaweld 650 has EIGHT capacitors of 22,000 uf each, that is 80 times what you've got.
    Even really crappy Chinese welders have more capacitance than you have.

    Referring to the diagram attached the capacitors should be as big as you can get, say 22,000 uf, 27,000 uf, 33,000 uf and a 100V
    rating. They ABSOLUTELY MUST BE LOW ESR TYPES or they will get hot and fail in short order. Miller and Lincoln always used to use
    good quality low ESR caps and they were expensive, on the other hand they lasted for twenty years or more.

    The you NEED that inductance, 5mH or so. Why don't you find out which electrical companies in your area repair industrial welding equipment
    and ask then for a second hand choke....what's the bet they scrap them and you could get one for a box of beer.

    Craig

    Attached Thumbnails Attached Thumbnails Electrolytic Capacitor Series Connection-dcwelder-png  


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    Default Re: Electrolytic Capacitor Series Connection

    Hi,
    these are more like what you want:

    https://nz.element14.com/vishay/mal2...rew/dp/8820945

    The price is in New Zealand dollars, including local taxes the price is $76NZD which is about $50USD......and that's about typical if buying these type
    of capacitors.

    You should not mix capacitors up, they should all be the same type and spec otherwise one will try to do all the work and then blow.
    If you are going to try an electrical repairer for a choke then you might have some luck with capacitors.

    Capacitors are pretty sought after, I worked repairing welders for seven years, and any good capacitors got snapped up real quick, but you never know
    your luck. Mostly we had to buy new ones. If you buy genuine Miller parts expect to pay $80-$120USD each!

    Craig



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    Default Re: Electrolytic Capacitor Series Connection

    Quote Originally Posted by Al_The_Man View Post
    Series will cut the value not add to it.
    You need to add caps in parallel to add.
    Al.
    Oh yeah, what was I thinking ?

    Thanks very much !



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    Thumbs up Re: Electrolytic Capacitor Series Connection

    Quote Originally Posted by joeavaerage View Post
    Hi,



    Don't do it. it will probably go bang.

    You need some inductance as well as capacitance. If you have capacitance alone on the output electrodes will stick and
    you get short circuit current from the capacitors....you need a little inductance as well as the capacitors.

    Your two 2200 uf capacitors are a joke. A Miller Deltaweld 650 has EIGHT capacitors of 22,000 uf each, that is 80 times what you've got.
    Even really crappy Chinese welders have more capacitance than you have.

    Referring to the diagram attached the capacitors should be as big as you can get, say 22,000 uf, 27,000 uf, 33,000 uf and a 100V
    rating. They ABSOLUTELY MUST BE LOW ESR TYPES or they will get hot and fail in short order. Miller and Lincoln always used to use
    good quality low ESR caps and they were expensive, on the other hand they lasted for twenty years or more.

    The you NEED that inductance, 5mH or so. Why don't you find out which electrical companies in your area repair industrial welding equipment
    and ask then for a second hand choke....what's the bet they scrap them and you could get one for a box of beer.

    Craig
    Oh thanks for the real world numbers... I won't be bothering with those tiny caps any more... my tests with 6013 looked like DC welds but were misleading.

    I have approx the same circuit, with two big air cored chokes (unknown inductance) of 35mm² copper, I got for cheap but, I also have a low voltage (about 45VAC & 40VDC at the bridge) from the welder which is not enough for 7018 (60V min required) and the caps kicked that up to 70VDC... I guess that I'll have to mod the transformer to up the voltage.

    I've been to a local repair shop once, the choke they had in their bin was small air cored and thin section plastic insulated wire so, I left it for someone else who might need that model... their business was more on the electronic/inverter welders.

    Thanks very much !



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    Default Re: Electrolytic Capacitor Series Connection

    Quote Originally Posted by joeavaerage View Post
    Hi,
    these are more like what you want:

    https://nz.element14.com/vishay/mal2...rew/dp/8820945

    The price is in New Zealand dollars, including local taxes the price is $76NZD which is about $50USD......and that's about typical if buying these type
    of capacitors.

    You should not mix capacitors up, they should all be the same type and spec otherwise one will try to do all the work and then blow.
    If you are going to try an electrical repairer for a choke then you might have some luck with capacitors.

    Capacitors are pretty sought after, I worked repairing welders for seven years, and any good capacitors got snapped up real quick, but you never know
    your luck. Mostly we had to buy new ones. If you buy genuine Miller parts expect to pay $80-$120USD each!

    Craig
    Thanks for the link ! "one will try to do all the work and then blow" I had that idea in mind but wasn't sure from where I got it, if it was true or a myth/assumption...

    At this price, if I fail at sourcing cheap second hand caps, I'll probably consider buying a second hand DC welder which might be a safer/cost effective choice.

    Thanks very much !



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    Default Re: Electrolytic Capacitor Series Connection

    Hi,
    open circuit voltage is a prime determinant of ability in an MMA welder.

    40VDC is not enough, 60VDC is adequate and about the norm in cheap inverter welders, and 80-90VDC is good. Some of the old and
    much loved Miller inverter welders had 90VDC open circuit, they were great with low hydrogen and cellulosic rods.

    If your transformer output is 45VAC then the peak voltage is:

    VDCPeak= VAC rms x square root (2)
    =45 x 20.5
    =45 x 1.414
    =63.6VDC

    I'm sorry I think your transformer is marginal for the job. You will need a lot of capacitors to maintain the highest possible
    open circuit voltage.

    Craig

    Last edited by joeavaerage; 01-18-2020 at 02:58 PM.


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    Default Re: Electrolytic Capacitor Series Connection

    Newer Miller inverters have very low OC voltage, in vicinity of 12-13V.

    Make no mistake between my personality and my attitude.
    My personality is who I am. My attitude depends on who you are.


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    Default Re: Electrolytic Capacitor Series Connection

    Quote Originally Posted by joeavaerage View Post
    Hi,
    open circuit voltage is a prime determinant of ability in an MMA welder.

    40VDC is not enough, 60VDC is adequate and about the norm in cheap inverter welders, and 80-90VDC is good. Some of the old and
    much loved Miller inverter welders had 90VDC open circuit, they were great with low hydrogen and cellulosic rods.

    If your transformer output is 45VAC then the peak voltage is:

    VDCPeak= VAC rms x square root (2)
    =45 x 20.5
    =45 x 1.414
    =63.6VDC

    I'm sorry I think your transformer is marginal for the job. You will need a lot of capacitors to maintain the highest possible
    open circuit voltage.

    Craig
    Hi,

    Accounting for voltage drop at the bridge, I measure 40VDC, so it's even lower.

    First time I measured I remember it was reading 90VDC and had to check that it wasn't off limits for welding but, took another measurement today with a different DMM and it was about 60VDC, probably a low battery in that DMM... will have to make sure.

    Might be possible to get higher voltage by wiring two secondary windings in series, if it's not a single multi tapped secondary...will have to check.

    Yeah, caps alone are not worth it.

    Thanks very much !



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    Default Re: Electrolytic Capacitor Series Connection

    Quote Originally Posted by ZASto View Post
    Newer Miller inverters have very low OC voltage, in vicinity of 12-13V.
    Probably an "intelligent" safety feature ?

    Thanks very much !



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    Default Re: Electrolytic Capacitor Series Connection

    Hi,

    Newer Miller inverters have very low OC voltage, in vicinity of 12-13V.
    You'd never be able to start a weld. Its called VRD (Voltage Reduction Device), when you are not welding , ie no current output the voltage will reduce,
    another bull**** safety feature that all welders hate. The first thing they ask you to do is disable VRD.

    Recent Miller inverter welders typically have an open circuit voltage of 72V, quite respectable.

    Craig.



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    Default Re: Electrolytic Capacitor Series Connection

    Hi,

    Accounting for voltage drop at the bridge, I measure 40VDC, so it's even lower.
    That's incorrect, that is not voltage drop in the rectifier, if it dropped that much voltage it would fry up in seconds.

    When you measure the AC voltage you are measuring the RMS average. The peak voltage of the AC waveform is higher than that,
    in fact it is 141% of the RMS voltage. If you had a decent capacitor array hooked to the output of the rectifier you they would charge up to the
    peak voltage, ie 63V

    Because you have so little capacitance what you measured was the DC average voltage which is 90% of the RMS average. If you rely on inductance alone
    too smooth the DC this is the result.

    VDC (capacitive filter)=VAC x 1.4
    VDC (inductive filter)=VAC x 0.9

    Craig



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    Cool Re: Electrolytic Capacitor Series Connection

    Welllllll... Miller Dynasaty 350:

    Attached Thumbnails Attached Thumbnails Electrolytic Capacitor Series Connection-screenshot-2020-01-19-11-59-25-a  
    Make no mistake between my personality and my attitude.
    My personality is who I am. My attitude depends on who you are.


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    Default Re: Electrolytic Capacitor Series Connection

    Quote Originally Posted by joeavaerage View Post
    Hi,



    That's incorrect, that is not voltage drop in the rectifier, if it dropped that much voltage it would fry up in seconds.

    When you measure the AC voltage you are measuring the RMS average. The peak voltage of the AC waveform is higher than that,
    in fact it is 141% of the RMS voltage. If you had a decent capacitor array hooked to the output of the rectifier you they would charge up to the
    peak voltage, ie 63V

    Because you have so little capacitance what you measured was the DC average voltage which is 90% of the RMS average. If you rely on inductance alone
    too smooth the DC this is the result.

    VDC (capacitive filter)=VAC x 1.4
    VDC (inductive filter)=VAC x 0.9

    Craig
    Last time I measured 40VDC was without the caps only the inductors so, VAC x 0.9 it is.

    Are you sure it'd fry the bridge ? BTW, I have made a bridge out of SKN130 diodes if that makes sense, for such voltage drop.

    Thanks very much !



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    Default Re: Electrolytic Capacitor Series Connection

    Well, I have measured OCV without caps with two different DMMs with the welder on its highest setting (n°7),
    and I got 49.5VAC before & 44.6VDC after the bridge rectifier... yes the heat sink was cold after the test.

    BTW, I have fake magnetic copper connectors on the bridge cables... if that makes any sense for the voltage drop ?

    Thanks very much !



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    Default Re: Electrolytic Capacitor Series Connection

    Hi ZASto,
    that is VRD in action. The open circuit voltage is held low until there is 400mA of current and then the VRD is disabled and full open circuit
    voltage (72VDC) is applied.

    In some countries, Australia particularly, VRD is legally mandated.

    VRD is a real PITA with low hydrogen rods.

    Craig



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    Default Re: Electrolytic Capacitor Series Connection

    Hi bmn,
    no you will not fry the rectifiers.

    The voltage drop of a diode in forward conduction mode is about 1V. If that diode is conducting 100A the instantaneous power loss
    is 100W. The diode must be mounted on a substantial heatsink or it WILL fry.

    A single phase bridge rectifier has four diodes with two conducting at any one time thus you could expect a thermal load on the heatsink
    of 200W with 100A output. Note that the voltage drop will be 2V. Trying to measure voltage drops in bridge rectifier circuits with a mulitmeter
    is confusing and ultimately inaccurate. Mulitimeters are designed to display RMS voltages assuming a sinusoidal waveform whereas in a rectifier
    circuit the waveform is now any thing BUT a sinusoid.

    If you really want to see whats going on in a rectifier you need at least a two channel scope with at least one, but preferably two galvanaiclly
    isolated inputs.

    If you want to see full, that is approx. 60VDC, open circuit voltage from your transformer rectifier combination you want at least 100,000 uf but better
    200,000 uf capacitance bank. At $50USD per 22000 uf that is about $450 worth of capacitors for 200,000 uf smoothing.

    If we used genuine parts in some of the big welders like Deltawelds or some of the Lincolns like a DC600 a replacement capacitor set could cost over $1000,
    so $450 is not out of the way for a welder. As I posted earlier good capacitors get snapped up pretty quick because replacing them with new ones is a costly
    business.

    Craig



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    Default Re: Electrolytic Capacitor Series Connection

    Thanks for the help guys, I'll be looking for cheap caps... heck the guy who sold me the chokes had a pack of big caps with no (V/µF) markings on them except for a part number.

    Thanks very much !



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