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Bridge rectifier with capacitor and choke filter

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tfleming

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#1
This probably (maybe) is a wee bit off-topic for machining, however, it does cover basic electronics, and might help others when needing to do similar things. I want to get into TIG welding for giggles (already stick, gas, and MIG weld), but I also don't want to drop a load of cash (just yet). So, I am building a rectifier with filter to plug into my trusty old Lincoln tombstone 225 AC welder and turn it into a scratch-start TIG machine. Here is the schematic for it:

1532343710394.png

It is a basic full bridge rectifier, with 2 capacitors and a choke to smooth out the DC waveform. Will use at least 1, possibly 2 muffin pan cooling fans to keep the old girl comfy. Also, I will not be running this at "full throttle". Will probably never go over 150 amps on the input.

Input and comments are welcome, as I haven't started this yet.
 
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TonyRV2

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#3
Not knowing much about the power requirements for tig welding, I can say that the rectifier circuit that you've come up with should perform to the requirements you've listed. The caps on their own would give you a ripple on the D.C. output of only a few volts p-p. You didn't give a value for your inductor but I would think that anything over a few henries would do the trick.
 

Smithdoor

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#4
Here little data

Dave New-relayHF-170-wiring-diag with capacitor.jpeg


Sent from my SAMSUNG-SM-J320A using Tapatalk
 

tfleming

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#5
Not knowing much about the power requirements for tig welding, I can say that the rectifier circuit that you've come up with should perform to the requirements you've listed. The caps on their own would give you a ripple on the D.C. output of only a few volts p-p. You didn't give a value for your inductor but I would think that anything over a few henries would do the trick.
Tony, the choke came off a 150hp dc drive at 460volt. I am not sure what the henries rating is, but since they were surplus and sized for that application, I think it should work just fine. As far as TIG output requirements, they are pretty straight forward. Most welding I will do will be in the 80-150 amp range, so I oversized things a wee bit just to keep things from "smoking" (the listed input and output values are the specs of what this setup should be able to do). I might even put a switch in to eliminate the filter portion, as pulsed DC current has applications in TIG as well. Again, everything I bought for this is well under $100, and it will give me some flexibility with the AC welder without dropping multiple C-notes...........
 

RJSakowski

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#6
This brings back memories of the good old days when all power supplies were designed like this. Entire books have been written about linear power supply design.

As I recall, choke (inductor) input filters were used for high current applications. A choke input filter has better voltage regulation than can be attained with with a capacitor input although the voltage will be lower. Both my Miller welders use a choke input filter.

A full wave bridge rectifier has a cycle period of 8 msec. When the voltage from the bridge drops, good filter will supply current until the voltage rises again. A capacitor does this by storing charge and discharging into the circuit when the voltage drops. The equations governing the charge/discharge are q = CV where q is the charge in ampere-seconds, C is the capacitance in farads and V is the voltage and i = dq/dt where i is the current and dq/dt is the time rate of change in the charge ( a little calculus here LOL).

The take-away here is the to sustain high currents, a large stored charge is required. The exact calculations are fairly complex but a back of the envelope calculation shows that to sustain a 50 amp current over an 8 msec. time would require capacitances on the on the order of a half a farad. Most of the farad class capacitors that I have seen are rated for lower voltage than would be found in a welder. Capacitors can be wired in series to increase the operating voltage at the expense of capacitance. Two 1 farad 12 volt capacitors in series would have an operating voltage of 24 volts and a combined capacitance of .5 farad. For a 75 volt maximum, you would need six capacitors in series for a combined capacitance of .17 farad.

Chokes, on the other hand, store energy as magnetic flux. Magnetic flux is proportional to current through the windings and it is fairly easy to design a choke which can handle the high currents that welders experience. When the voltage drops, the magnetic field begins to collapse developing a back emf which boosts the dropping voltage.

I have used old transformers by replacing the original windings with a single winding of wire capable of carrying the required current. An old battery charger transformer could be used effectively as a choke by just using the secondary winding.
 

TonyRV2

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#7
Your theory is sound RJ. I taught electronics engineering courses for 32 years until I retired a year ago May. One of my 'specialty' courses was linear applications including power supplies, transistors, etc. One way to remember what the filters do is to remember that capacitors delay change in voltage while inductors delay change in current. As far as the amount of change in voltage on the caps, which is usually referred to as the a.c. 'ripple voltage', it boils down to a simple approximation of the DC current draw divided by (frequency X capacitance) where the frequency is the output pulse frequency of the bridge, in most cases 120Hz (double the line frequency). For the OP's circuit, assuming 150A nominal draw, this becomes 150A divided by (120Hz X 1F). Note the two .5F caps in parallel combine to give 1F total capacitance. So the peak to peak ripple out of the capacitor section becomes about 1.25Vpp. This relatively small change in voltage then produces a relatively small change in current which is then further filtered by the inductor. With this amount of capacitance the inductors job becomes easy.

Of course then all of this assumes a steady current draw of 150A. The welding process itself will cause large fluctuations in current, so oversizing the inductor is prudent in an application like this in order to account for the system variables. That's my story and I'm stickin' to it. ;)
 

tfleming

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#8
RJ and Tony, thanks. One thing to keep in mind here is that I am looking to do some pretty basic TIG work to see if I want to invest the $$$ for a decent, TIG welder. This is definitely a "backyard" setup, but it am hoping it will produce a relatively stable DC current. Plus, it is a fun little project. If there are slight pulses, that is actually not a bad thing, as some of the high end TIQ welders have variable DC frequency capabilities (they are also $3k machines). Also, again, I plan on the 150A being the upper end of how hard I will push this. It will spend a lot of it's active time around 80a-100a. Consider this a "proof of concept" setup. I don't own an oscilloscope, but I would love to see how "clean" the DC output from this when complete. For sure, the capacitors are the expensive part of this, and if I need to add a couple more, I can certainly do that.

I do have a question though, for these caps, is there any suggestion on whether to put them in series or parallel. They are rated for the 75 OCV that the welder produces. Voltage drops off to 25 VDC when the arc is initiated. Lastly, I designed this prototype from looking at how some others have approached this, so the choke was added in addition to the CAPs. Does it make any different to run the choke on the + leg or the - leg? I would presume not, but have to ask.
 

markba633csi

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#9
Hi Tom, the choke can be in either leg. As far as caps, they add in parallel but divide in series. Two 100uF caps in parallel are 200uF, but in series only 50uF
Basically the same circuit is used to convert an ac buzz box to a dc welder, without the caps, since dc stick welding doesn't require quite as constant a voltage source, you can get by with just a rectifier and choke, and even the choke is optional for dc stick if you aren't too picky about your welds
Mark
 
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TonyRV2

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#10
The higher the effective capacitance the better the filtering, so I'd leave the caps wired in parallel. It makes no difference which leg the inductor is on as long as its in series with the load.
 

RJSakowski

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#11
This probably (maybe) is a wee bit off-topic for machining, however, it does cover basic electronics, and might help others when needing to do similar things. I want to get into TIG welding for giggles (already stick, gas, and MIG weld), but I also don't want to drop a load of cash (just yet). So, I am building a rectifier with filter to plug into my trusty old Lincoln tombstone 225 AC welder and turn it into a scratch-start TIG machine. Here is the schematic for it:

View attachment 272547

It is a basic full bridge rectifier, with 2 capacitors and a choke to smooth out the DC waveform. Will use at least 1, possibly 2 muffin pan cooling fans to keep the old girl comfy. Also, I will not be running this at "full throttle". Will probably never go over 150 amps on the input.

Input and comments are welcome, as I haven't started this yet.
Looking at your circuit, you show an OCV on your ac input of 75 volts. This is almost certainly RMS voltage. Peak voltage would be 1.414 times that or 107 volts. This is the voltage that your capacitors will see with no load. The capacitor voltage rating should be somewhat higher so something like 120 volts.
 

tfleming

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#12
Thanks guys, I really appreciate the feedback. RJ, unfortunately, I have already ordered the CAPs. If they "smoke", then I'll jump up to the 120 V flavor.

Most of these components will arrive in the next 3-4 days. Then I'll need a weekend to patch them together appropriately. Then one more weekend to test drive the setup. My promise is to report back with the results (or pictures of the smoked CAPs! LOL)

Additional info:

Here are the specs for the CAP

  • Capacitance: 50000mfd
  • Voltage: 75VDC
  • Tolerance: -10+50%
  • Temp: 85°C
  • Lead Type: 2 High Post Screw Terminals
  • Diameter: 2 1/2"
  • Height: 4 1/8"

Am I reading this correctly that they will take +50% on the voltage?
 
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markba633csi

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#13
Tolerance applies to the uF value, but you can sometimes push the voltage a bit past and get away with it- for a while anyway
m
 

tfleming

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#14
Tolerance applies to the uF value, but you can sometimes push the voltage a bit past and get away with it- for a while anyway
m
Well, interesting enough, when the arc is struck, the voltage drops to around 25VAC on the input, and ignoring the voltage loss going through the rectifier, I would expect that the operating voltage to be the 25VDC. Now, one could be judicious in not turning on the AC power source until ready to strike the arc. I would expect the exposure time to the higher voltage to be around 30 +/- seconds. So, the next logical question is, does the capacitor recover after the voltage drops, or is there cumulative, incremental damage to it? Inquiring minds want to know! LOL.

Worst case, I could install a solenoid to energize the source only when I am ready to strike the arc. Then the capacitor exposure to high voltage would be minimized. I wonder if the OCV could be adjusted internally in the welder...….
 

Bi11Hudson

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#15
My nickle's worth, if I may. The caps you have listed won't work long, MTBF on the order of a few cycles at best. Put two of them in series to cover that problem. An "old school" rule of thumb is for 1000uF per amp as a filter. That's for electronics to run smooth. You will want a little ripple so that rule of thumb is a good fit. I'm not at all familiar with your welder. The conversion I did was an old Lincoln buzzbox into a stable DC welder. Just the bridge, no caps or choke. I stole the diodes out of scrap for a DC crane in a mill. 600 Amps,600 volts. Worked well enough for what I did.
 

RJSakowski

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#16
Electrolytic capacitors can be "reformed" to a higher voltage. The dielectric is a thin coating of aluminum oxide. By applying a higher voltage you can grow the dielectric layer, making it thicker and giving it a higher breakdown voltage. because the layer is thicker, the capacitance will decrease. Capacitance is proportional to surface area and inversely proportional to the dielectric thickness.

There are limits as to how much you can push the breakdown voltage and the voltage should be increased gradually to give the dielectric layer time to form. If you plan on using the 75 volt capacitors in your circuit, you would be wise to ramp the voltage slowly. This can be done by substitutng a Variac for welder in your circuit and monitoring the voltage at the capacitors as you go through the reforming process.

If you Google reform capacitors, you will find a lot of information about the process.
 

TonyRV2

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#17
That's interesting RJ...I'd never heard that before about ramping up cap operating voltages. Looks like I'll be taking some time off from machining this afternoon to blow up a few capacitors. :)
 

tfleming

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#18
Ok, so I came up with a hair brained idea to solve the open voltage issue (or at least a theory). What if I put a 200 AMP NO (normally open)solenoid on the one leg between the bridge and the first CAP. It would operate manually off a small momentary switch mounted to the TIG torch. Press button, solenoid closes and energizes the CAP, choke, and TIG gun..............all when I am ready to strike the arc. In fact, with scratch start, there truly would not be an open circuit state while the caps are energized, as the tip will already be in contact with the work (ground). Now, the inrush voltage might be interesting, however, would that really matter for the charge lag on the capacitors? that initial voltage spike is going to be extremely short lived....................

Thoughts?
 
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TonyRV2

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#19
I never met a capacitor that I didn't enjoy blowing up on the bench. I say go for it and see what happens. Its a win-win situation. :)
 

homebrewed

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#20
I'd recommend adding a resistor across the capacitor, to drain the charge away when the welder is turned off. Otherwise the capacitor will remain charged, which could cause an unpleasant surprise for the unwary. Voltage on the capacitor will decay at a rate determined by the time constant, R*C. For a 50,000uf capacitor and a 1-second time constant you will need a resistance of 1/.05 (50,000uf = .05 farad), or 20 ohms. It will have to have a high enough wattage rating so it doesn't burn up. I went with a 50 ohm resistor when I modded my HF welder for DC -- high power 50 ohm resistors are fairly common. Stores selling stuff to ham radio enthusiasts would be a good place to look.
 

markba633csi

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#21
You could accept a bit longer on the drain down time- say 100 ohms, at about 50 watts. That would drain it in about 3-5 seconds
and wouldn't get as hot (it will still get pretty warm)
mark
 

tfleming

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#22
I'd recommend adding a resistor across the capacitor, to drain the charge away when the welder is turned off. Otherwise the capacitor will remain charged, which could cause an unpleasant surprise for the unwary. Voltage on the capacitor will decay at a rate determined by the time constant, R*C. For a 50,000uf capacitor and a 1-second time constant you will need a resistance of 1/.05 (50,000uf = .05 farad), or 20 ohms. It will have to have a high enough wattage rating so it doesn't burn up. I went with a 50 ohm resistor when I modded my HF welder for DC -- high power 50 ohm resistors are fairly common. Stores selling stuff to ham radio enthusiasts would be a good place to look.
Homebrew, good suggestion, as I have ruined a few screwdrivers over the years shorting CAPs. Let me pose a question on this: Since this is TIG welding, if I use the solenoid and just let off the momentary switch, wouldn't the arc drain the CAPs until it went out? This would eliminate the need to "LIFT" the torch to extinguish the arc...…...
 

markba633csi

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#23
No the arc would extinguish leaving some charge still in the caps Tom
 

homebrewed

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#24
You could accept a bit longer on the drain down time- say 100 ohms, at about 50 watts. That would drain it in about 3-5 seconds
and wouldn't get as hot (it will still get pretty warm)
mark
Yep. The exact time to drain the cap isn't critical, as long as it happens in a few seconds. If you wanted to get fancy you could connect an LED (with the proper series R) to indicate when the cap is discharged.
 

RJSakowski

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#25
The power dissapated by the resistor is V^2/R so the smaller the resistor, the higher the power dissipation. On the other hand, the voltage after time t is equal to Vo * 2.73^(-t/R*C) or .37*Vo after t = R*C and .14*Vo after t =2*R*C. If the voltage is removed from the torch on completion of the weld, there should be no exposed high voltage present. If you are doing repairs or immanence and cracking open the case, you presumably are aware the high voltage may be present (if not, put a warning label) and you can either wait for the capacitors to discharge or discharge them yourself., (Whenever I worked on high voltage power supplies, I took the precaution of discharging any caps before sticking my fingers or other sensitive items inside. After getting bit by a few charged CRT's, even an old dog can learn.)
If the assumption of no unintended exposed high voltage is valid, the discharge can be much longer; say on the order of a minute or more, the design intent being not to leave a charged capacitor for some unsuspecting individual to play with. The plus of using a higher resistance is a much lower power resistor is required. A 1K resistor coulld get by with a 7 watt power rating for an OCV of 75 volts. You are also not dumping welder power into heating a resistor.
 

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#26
Couple things I see right off.
Biggest is a full wave rectifier off a 80 or so volt AC current transformer.
Current transformers attepmt to maintain a specific current output by allowing the voltage to change.
So in short the OCV (open circuit voltage) could get up beyond 80 volts RMS from the transformer.
Second issue, running 80 volts OCV into a rectifier and a capacitor bank is NOT going to have 80 volts DC output.
Transformers and most other AC voltage numbers are in RMS (root mean squared) or average output. THe other number to be aware of is the Peak to Peak voltage. this is going to be the RMS voltage devided by .707 roughly. That works out to about 113 volts. Now that is going to be a actual DC output once the AC off the transformer is fed to the rectifier and the resulting DC is filtered with the capacitors.
Be aware that the OCV will exceed 100 volts if you have 80 volts RMS going into a rectifier. That level of voltage is MORE likely to shock the hell out of you.

Selecting capacitors is critical if you are going to run this in a scratch start configuration.
Reason being is surge current off the capacitors. They will dump tons of current into the electrode as you try to scratch start to weld. Make sure that you put in the correct inductor AFTER both capacitors to help keep this in check.

Now on a side note, as far as power output regulation. You can build this with 4 SCR's in place of the diodes in the rectifier and vary the gate voltage to them causing them to conduct for less than the full 180 degrees of the sine wave input. This is similar to how a light dimmer works.
 

tfleming

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#27
Thanks to all who have posted. The info and suggestions are great. I will either swap out the 50,000 mf 70VDC CAPs for 35,000 mf 120VDC CAPs to handle the OCV voltage OR I will put the 2 50,000 MF CAPs in series. Either way, it is a drop in capacitance, but hopefully won't make that big of a difference in the arc. As I have previously stated, this is more of a fun prototype project than anything else.
 
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warrjon

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#28
I never met a capacitor that I didn't enjoy blowing up on the bench. I say go for it and see what happens. Its a win-win situation. :)
Takes me back to my days at TAFE charge a LARGE cap and toss it to someone.

I've been repairing electronics for nearly 40 years and never heard of reforming caps, head in sand maybe, or just replaced with OEM as military and aviation we were not allowed to buy anything but from the manufacturer.
 
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