Paralleling bridge rectifiers for a power supply

[jsheerin]

I need to make a high current linear power supply - 160VDC at 80A - for powering some servo drives. I already have the transformer and capacitors but am a bit hung up on the diodes. From reading and doing some calculations, I need diodes rated at > ~320 PIV and 160A. I can find really expensive diodes that do this, but I am trying to save money (so I can spend it on other machine parts, inevitably...). I've found 50A 400V bridge rectifiers for ~$6. Can I parallel 4 of these off of my transformer and parallel their outputs into the capacitors? Are there any practical concerns here like putting some series resistors in front of the bridges so they share current better?

I've searched here and on the rest of the net and not really found any information about this, so if any can help me out or point me towards some reading material, I'd appreciate it.

[TarHeelTom]

Here are some part numbers from Newark Electronics.

16M0147

88K1166

63K6151

64K2274

88K1167


Tom

[jhovel]

I had the same issue a few years ago when I wanted to build a rectifier for my welder. Although I was strongly advised against using 6 parallel 35A rectifiers on the basis that if the current flow was uneven, one rectifier bridge might blow and distribute the then overcurrent to the left over ones, blowing them in short succession.
To safeguard the idividual bridge rectifiers, I soldered 30A fusewire in line with each one, so that the fuse wire would blow instead of the rectifiers.
Nothing ever blew - the rectifier is still in use and working fine....
Go for it.
Joe

[vger]

Fuse wire is a special alloy that melts at a lower temp than copper wire. It has a very low resistance, but is higher resistance than copper. The fuse wire that you put placed inline with each rectifier is probably acting as a small resistance and is balancing the current across the rectifiers. This is common practice when paralleling power transistors. The resistor is known as a "Ballast Resistor" and it regulates the current by it's resistance increasing slightly as it's temperature rises.

In place of the fuse wire, you could use some very low value resistors. 0.01 Ohm 5 watt should work fine. About 2 1/2 feet of #16 solid copper wire would give you about 0.01 ohms to do it on the cheap.

Steve

[jhovel]

That's a very interesting thought and I've filed it in my memory.
I'm not sure that the 1" long piece of fuse wire would have a signifcant effect though or would it?
Cheers,
Joe

[jsheerin]

Tom, thanks for the part numbers, but those are all low current devices except the last one according to the data sheets and manufacturer's website, and the last one costs $220. For some reason they all say they have an average forward current of 150A, but most of them are in the 10-30A range according to the data sheets. It looks like I could use one of these guys for $55, but it has a 106 day lead time:
SEMIKRON|SK 100 B 12|SK 100 B 12 | Newark.com

Thanks Joe and Steve for the resistor tips. I might give that a try first as I found 50A bridges from Digikey for $6 each.

[doorknob]

One other approach that you might consider (if you haven't already) - you mention that you want to power multiple servo drives.

How much current do you need for each drive?

If capacitors are inexpensive enough, you could presumably drive multiple rectifier/capacitor combinations off of the same transformer, if the maximum current drawn by any single drive is low enough that you could get by with inexpensive bridge rectifiers.

[jsheerin]

Thanks, that's a good idea. Yes, I need to power 4 servo drives, so I could do that... I already have about 85000 uF of capacitors, so I could use ~21000uF per supply. Looking at the various cap sizes I have, I could split them up so I have between 20-22000uF for each supply. I'll plan on doing that.

[TarHeelTom]

Just curious, but what are you building that needs such big servos?

Tom

[jsheerin]

A big milling machine, but small enough that it will fit in my garage (ie, I can't fit commercially available ones, plus designing and building one gives me something fun to do for years to come). See the link to the design thread in my signature. I'm planning on using the Granite Device drives with Fanuc 10S servos.