Half-wave rectification vs. full-wave

[Kat M]

Hi,

I've yet to start on the thin stuff on my Viva (floor section mostly.) But my limited experience on the now-scrapped BMW was it was all too easy to blow holes even on the lowest setting (25A on my SIP.) I've now watched the 'pulse' technique video and, maybe with more experience and the wire-feed issues sorted out, I'll find it easier.

But I'm still pondering something. It's a single-phase welder with a centre-tapped transformer secondary using a pair of diodes for full-wave rectification. So, by replacing one of the diodes with an SCR I could 'switch a diode off' hence switch between full- and half-wave rectification.

I'm kinda "thinking out loud" here... So, instead of 100Hz pulses of DC, I'd have 50Hz pulses, missing every other pulse. Could this make it easier to avoid putting too much heat into thin car-body steel or am I barking up the wrong tree?

I can try it out (when it stops raining...) by disconnecting one of the transformer secondary connections but I thought I'd run this past those with more knowledge/experience first

Cheers, Kat

 

[bill_gj]

Gone quiet hasn't it.

If you disconnect one half of the transformer, you'll get a lower rectified voltage won't you? I need to find my old electrical book.
Single diode, two diode and full bridge give different voltage outputs don't they?

 

[shenion]

Disconnecting one side of the transformer will cut the average power in half.

My miller goes down to probably 15 amps. I found such low a setting just makes things worse as you are going so slow, there is more time to blow holes and the arc to wander. I finally increased the power setting and moved faster and bot better results.

You can try it, but I doubt it will work that well. You are using 0.6mm wire?

 

[piman]

Hello Kat,

surely there is a smoothing capacitor(s) which evens out the D.C. waves from the rectifiers?

Alec

 

[shenion]

No smoothing caps on most MIGs as they are expensive. At 150A, it takes about 200,000uf/50V. Would add about 50% to the cost of the unit. They will wear out due to the high currents.

Most have about 0.5uf just to reduce noise feeding back to the power line.

 

[piman]

Hello Shenion,

thank you for that, pretty crude devices then really?

Alec

 

[shenion]

Yes, they are real simple: transformer, rectifier and a reactor.

If you don want massive spikes on your power line, add some small caps on the rectifiers and a MOV on the power line.

The real cheap MIG have no caps, no MOV and some don't have the reactor (Wolf MIGs are this way.)

The Clarke's have the caps and MOV. Still my 130EN would reset my satellite box at times.

I think the SIPs leave out the MOV.

 

[SAE140]

No smoothing caps on most MIGs as they are expensive. At 150A, it takes about 200,000uf/50V. Would add about 50% to the cost of the unit. They will wear out due to the high currents.

I've got 6 x 10,000 uF caps rated at 100V which I had intended using on a home-brew, as on the cans it reads: "Ieff (100Hz) 28A (50°C)".
I was hoping that these would smooth up to 168A. (28A x 6)
Any thoughts ?

Colin

 

[shenion]

I added two 30,000uf 68V caps to my CP-250. The arc is now a soft hiss.

They increase the output at low settings. What was 18V is now about 28V. I can go down to about 12V with the unit so it is not bad (it has infinte voltage settings between 11-37V).

I did a rough calculation athat they would raise the output up to about 80A. Then they just add smoothing. On 50HZ, it will be less.

60,000uf will have about 5V of ripple at 30A with an 18V RMS setting. The result will be about 21.5V.

As long as you have enough range, then use the caps.

If you want to calculate the gory details of the amount of ripple you can check this document.