Converting cheap AC buzz box to DC

[dbach]

Hello, I'm new here, so I hope I post it in the right category.
The project is to convert a AC welder into a DC welder> The schematics of wiring are clear.
Now I have found some Scotty rectifiers. 400A/200V.
The problem is, that these diodes for a bridge configuration have the (-) base joined.
For one side of the bridge thats ok, but for the other not.
Would it be possible to split the base in half and so making 2 single diodes out of it?
Or anybody has an idea for a wiring schematics without cutting these in 2 pieces?
thank you very much.

 

[rtbcomp]

Welcome to the forum.

You should be OK splitting the base.

You'll need to mount them on heatsinks.

 

[pedrobedro]

Can't help you but Scotty rectifiers made me smile.

 

[Wendelspanswick]

Can't help you but Scotty rectifiers made me smile.

She cannee take the puwer captin!!!

 

[Rig Pig]

This looks interesting, let us know how it works.

 

[dbach]

Thank you for your fast advice.
Yes i have thought to separate only 1 of them and screw all on quite big heatsinks. 2 smaller and one large heatsink for the double. The only issue I have, the threads are only 5mm.
Lets see how i can get around of it for the thick cables which have to be connected.

 

[Rig Pig]

Just fit lugs onto the cable ends to attach them to the diode

 

[Pete.]

Hello, I'm new here, so I hope I post it in the right category.
The project is to convert a AC welder into a DC welder> The schematics of wiring are clear.
Now I have found some Scotty rectifiers. 400A/200V.
The problem is, that these diodes for a bridge configuration have the (-) base joined.
For one side of the bridge thats ok, but for the other not.
Would it be possible to split the base in half and so making 2 single diodes out of it?
Or anybody has an idea for a wiring schematics without cutting these in 2 pieces?
thank you very much.

Are they expensive? If not, why not buy four and have each pair act as one diode with doubled capacity?

 

[dbach]

She cannee take the puwer captin!!!

I think they can

Specification:

Product Name Schottky Barrier Diode Module
Model No. MBRP400200CT
VRRM 200V
IFRM 400A
IFSM 2500A
IRRM 2A
Female Thread Size 5mm / 0.2"
Tstg -55 to + 175°C
Total Size 92 x 20 x 15mm/ 3.62" x 0.79" x 0.59" (L* W * H)
Weight 83g
Package 1 x Schottky Barrier Diode Module

 

[dbach]

Are they expensive? If not, why not buy four and have each pair act as one diode with doubled capacity?

I get them from China in 2 or 3 weeks 2 each £ 8.06.
That's what makes me a little sceptic, but i have thought to give it a try

http://www.ebay.co.uk/itm/121148327712?_trksid=p2059210.m2749.l2648&ssPageName=STRK:MEBIDX:IT

The thing with double capacity is an execellent idea

 

[Wendelspanswick]

I think they can

Specification:

Product Name Schottky Barrier Diode Module
Model No. MBRP400200CT
VRRM 200V
IFRM 400A
IFSM 2500A
IRRM 2A
Female Thread Size 5mm / 0.2"
Tstg -55 to + 175°C
Total Size 92 x 20 x 15mm/ 3.62" x 0.79" x 0.59" (L* W * H)
Weight 83g
Package 1 x Schottky Barrier Diode Module

Sorry, I forgot to add a smiley to my Scotty from Star Trek impersonation.
Its a Schottky diode rather than a Scotty diode which is why I made the poor Scotty pun!

 

[dbach]

Sorry, I forgot to add a smiley to my Scotty from Star Trek impersonation.
Its a Schottky diode rather than a Scotty diode which is why I made the poor Scotty pun!

Oh yeah, right reading is sometimes so difficult :-). Shame on me lol so in future i write about SCHOTTKY, thank you

 

[mike 109444]

Might be worth looking out for a diagram of say an early DC arc (rather than inverter/modern) welder to see what additional components you might need such as caps or inductors. Back emf may be an issue....

 

[mike 109444]

Also a temp probe on the heatsink might not be a bad ideal as while you are busy welding away you may not notice the diodes frying.

 

[Rig Pig]

Might be worth looking out for a diagram of say an early DC arc (rather than inverter/modern) welder to see what additional components you might need such as caps or inductors. Back emf may be an issue....

I'm fairly sure that they have a big wire wound choke as well as the bridge.

 

[Robotstar5]

The only issue I have, the threads are only 5mm.
Lets see how i can get around of it for the thick cables which have to be connected.

Look for copper tube lugs, they are available up to 25mm² for M5 holes

 

[dbach]

hi, an update to my dc welder project. I'm still waiting for the second couple of Schottky Diodes from China.
Meanwhile I'm thinking about the output capacitors.
Some in other forum have put some capacitors of 20000 - 40000 uF / 250 V. They are nearly impossible to find, and prices are between £150 and £300 each.
I think capacitors with a rating of 63V is sufficient.
But what is better:
1 large with 100000uF / 63V or a bench of 6 at 15000 to 20000uF, respecting charging and discharging.
Also I have foreseen a 2kOhm resistor across the +/- output of the capacitor bench to speed up the discharge after shut off.

 

[rtbcomp]

In theorey it's the capacitance that matters, not how it's made up so I'd go for the cheapest option in this application. The tolerance spread of electrolytics is quite broad.

Measure the open DC voltage when you've fitted the rectifier and choose capacitors of twice that voltage, this will allow for the higher DC voltage once you've added smoothing plus a safety margin.

The 2K reistance is going to drain about 30mA so it won't affect the welding but it will discharge the capacitors after a while

 

[sardine]

Your'll need a fan if you haven't already got one.

 

[dbach]

One stupid question: The capacitor shown in the picture going across anode and cathode of each bridge have to be 0.5uF, 250V and polarized.
The question is AC or DC. I suppose they have to be AC isn't it|?
Attached a link with a photo of a similar project. It's about the capacitor in the red circle.

https://drive.google.com/file/d/0B5cxcD95ErSyRno3S2ttdTNaalU/view?usp=sharing>