Bridge rectifier with capacitor and choke filter

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.
 
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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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
 
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...….
 
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.
 
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.
 
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. :)
 
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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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. :)
 
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.
 
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