Paralleling of Bridge Rectifiers

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Besoeker

Besoeker

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UK
mike_kilroy said:
For the OP... hopefully you googled something like "series and parallel of diodes" and learned why you must try to not do it.

The issue as many have said is that the forward voltage drop will not be the same. You can buy 10 bridge rectifiers and check drop on each of the internal 4 diodes and try to pick a better matched set, but why not just the correct size to begin with? My 7kw generator in my 1973 GMC motorhome has a 25amp continuous bridge rectifier - think I paid like $ 1.50 for it.

The issue continues to a potential snowball effect: as one diode passes more current than its parallel friend, it will heat up. Heating semiconductors causes their forward voltage drop to decrease. So it will then pass evven MORE of the uneven load. Heat up more, draw more of the load, heat up. This snowball effect can overload this diode, blowing it. Now you are back to the single diode that is not rated to pass the full so it will eventually blow.

If this is required to be done, adding L or R in series with the diodes helps match them. Otherwise, as one reply stated where they put bazillions of diodes in parallel for hi power power supplies, they no doubt put then on THE SAME HEASINK - so one did not heat more than it neighbors and the snowball effect did not happen (often probably).

Next issue is WHAT SIZE DO YOU REALLY NEED?? Do you really believe your 480v 22 ohm solenoid will draw 18 amps???? Think about that for a bit. Then go measure some other INDUCTORS and COILS like other small solenoids, any transformers... Your 22 ohms is only a SMALL PORTION OF YOUR SOLENOID's IMPEDANCE! The L in that coil will limit the current!

So before you try to pick the proper diode bridge, why not stick an ammeter in series (or a clamp on) with that solenoid and MEASURE the steady state current! I bet you find it is less than 1 amp! The 22 ohms tells me that it likely will have about a 20x inrush current - or 18 amps.
Click to expand...
Why parallel diodes when you can get a single module that will comfortably handle the current.
And, if it's a DC solenoid, there would be no inrush.
 
T

templdl

Senior Member
Location
Wisconsin
ATSman said:
Has anyone ever connected 2 bridge rectifiers in parallel to increase the current capacity?
This will be used to momentarily energize a large solenoid in an ATS (480V @ 80ms.)
I have heard pros and cons, even if they are matched perfectly to share the load, PIV differences, etc.
Just thought I'd throw this out there for to hear any real life experiences on the subject.
Click to expand...
Since rectifiers have become less expensive over the years why not just replace the existing rectifier with one of adaquate ampacity? I designed one to supply DC power to energy a DC brake it jog a ladle while desculling for a continuous caster in a large steel mill. Going to a much larger bridge rectifier than required for added reliability was insignificant.
But it is very difficult to provide a diffinitive answer when we I very little information about the application has been provided by the OP.
 
ATSman

ATSman

ATSman
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When I started this post I didn't expect it to generate so much interest. Thanks to all for your
comments and practical knowledge. One of the reasons I asked was that I have a bunch of these bridge
rectifiers and wanted to put them to use. I am attaching a drawing of a typical ATS. Page 1 shows the coils and rectifier modules. Will post more later
 

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ATSman

ATSman

ATSman
Location
San Francisco Bay Area
Occupation
Electrical Engineer/ Electrical Testing & Controls
mike_kilroy said:
For the OP... hopefully you googled something like "series and parallel of diodes" and learned why you must try to not do it.

The issue as many have said is that the forward voltage drop will not be the same. You can buy 10 bridge rectifiers and check drop on each of the internal 4 diodes and try to pick a better matched set, but why not just the correct size to begin with? My 7kw generator in my 1973 GMC motorhome has a 25amp continuous bridge rectifier - think I paid like $ 1.50 for it.

The issue continues to a potential snowball effect: as one diode passes more current than its parallel friend, it will heat up. Heating semiconductors causes their forward voltage drop to decrease. So it will then pass evven MORE of the uneven load. Heat up more, draw more of the load, heat up. This snowball effect can overload this diode, blowing it. Now you are back to the single diode that is not rated to pass the full so it will eventually blow.

If this is required to be done, adding L or R in series with the diodes helps match them. Otherwise, as one reply stated where they put bazillions of diodes in parallel for hi power power supplies, they no doubt put then on THE SAME HEASINK - so one did not heat more than it neighbors and the snowball effect did not happen (often probably).

Next issue is WHAT SIZE DO YOU REALLY NEED?? Do you really believe your 480v 22 ohm solenoid will draw 18 amps???? Think about that for a bit. Then go measure some other INDUCTORS and COILS like other small solenoids, any transformers... Your 22 ohms is only a SMALL PORTION OF YOUR SOLENOID's IMPEDANCE! The L in that coil will limit the current!

So before you try to pick the proper diode bridge, why not stick an ammeter in series (or a clamp on) with that solenoid and MEASURE the steady state current! I bet you find it is less than 1 amp! The 22 ohms tells me that it likely will have about a 20x inrush current - or 18 amps.
Click to expand...

Mike,
Thanks for the info. In regard to your last comment see the OP. There is no steady state current. This is momentary solenoid operation, then mechanically latched contacts.
 
Besoeker

Besoeker

Senior Member
Location
UK
Ingenieur said:
AC 480
DC 430 full wave rectified ( ac rms x 2 x sqrt2 / Pi)
at 24 Ohm 18 A
Assume a mechanically latching mechanism (since momentary and an ats)

that is a lot of current and power 7.75 kW or so
Click to expand...
But comfortably within the capabilities os a single module.
 
Tony S

Tony S

Senior Member
Location
Resting under the Major Oak UK
These have been happily working in parallel for the last 50 Years

DSC_0264_zpsc9616478.jpg
 
Tony S

Tony S

Senior Member
Location
Resting under the Major Oak UK
Besoeker said:
But something you would neither need to do nor want to do now.
Click to expand...

Why not? The set is rated at 1250A, with a single diode a failure would take the whole set out with dire repercussions. As it is a pair of diodes failing doesn’t affect the plants safety. They flag up an alarm so they can be changed when the opportunity arises.

The upsetting thing is these sets replaced the mercury arc and rotary converter sets.
 
M

mike_kilroy

Senior Member
Location
United States
Tony S said:
Why not? The set is rated at 1250A, with a single diode a failure would take the whole set out with dire repercussions. As it is a pair of diodes failing doesn’t affect the plants safety. They flag up an alarm so they can be changed when the opportunity arises.

The upsetting thing is these sets replaced the mercury arc and rotary converter sets.
Click to expand...

Just curious, why is that upsetting? The mercury was better? With the age of it, I am surprised they are not selenium diode sets.

But back to this thread: Not apples to apples comparison....

THIS reasoning is totally different than OP need. By your logic, EVERYONE should ALWAYS use smaller undersized diodes in parallel for ALL applications? May be wonderful reason for that particular application at that particular plant but not as a general rule for others to follow.
 
Tony S

Tony S

Senior Member
Location
Resting under the Major Oak UK
The mysteries of electricery, I was brought up with MAR’s and RC’s

For high power rectification then I would go for multiple diode packs. The OP’s situation is different, single units are readily available off the shelf so use them.

My only beef with the way things as they have gone in this thread is the insistence that rectifiers can’t be paralleled.
 
steve66

steve66

Senior Member
Location
Illinois
Occupation
Engineer
Ingenieur said:
AC 480
DC 430 full wave rectified ( ac rms x 2 x sqrt2 / Pi)
at 24 Ohm 18 A
Assume a mechanically latching mechanism (since momentary and an ats)

that is a lot of current and power 7.75 kW or so
Click to expand...

Yes, but the diode bridge only drops 1.4 volts, so it only dissipates 1.4V * 18 amps = 25 watts, and only for 80 ms.


Besoeker said:
Why parallel diodes when you can get a single module that will comfortably handle the current.
And, if it's a DC solenoid, there would be no inrush.
Click to expand...

I agree, there can't be a huge inrush because of the inductance. I also tend to agree one bridge would be enough. To be sure, we would need the maximum current rating of the diode bridge, and also the thermal properties of the bridge, so we can know if it can handle 25 watts for 80 ms.

Of course, if one bridge can handle the current and power, adding a second in parallel would only reduce the current and power one bridge needs to handle, although we can't say by exactly how much.

So it really couldn't hurt to put a second one in parallel.

Also, I think the matching problems with diode voltage drops and resistances will matter a lot more if you have a capacitor input filter after the bridge. It won't matter nearly as much with an inductive input.
 
Besoeker

Besoeker

Senior Member
Location
UK
Tony S said:
The mysteries of electricery, I was brought up with MAR’s and RC’s

For high power rectification then I would go for multiple diode packs. The OP’s situation is different, single units are readily available off the shelf so use them.

My only beef with the way things as they have gone in this thread is the insistence that rectifiers can’t be paralleled.
Click to expand...
Of course they can be paralled. The question is why would you when you can get the capacity out of one package or a stack? 10kA from a single bridge is easily achieved.
 
Besoeker

Besoeker

Senior Member
Location
UK
Tony S said:
Why not? The set is rated at 1250A, with a single diode a failure would take the whole set out with dire repercussions. As it is a pair of diodes failing doesn’t affect the plants safety. They flag up an alarm so they can be changed when the opportunity arises.
Click to expand...
Not so sure about that. In my experience, power semiconductors tend to fail in short circuit mode.
 
ATSman

ATSman

ATSman
Location
San Francisco Bay Area
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Electrical Engineer/ Electrical Testing & Controls
Besoeker said:
Of course they can be paralled. The question is why would you when you can get the capacity out of one package or a stack? 10kA from a single bridge is easily achieved.
Click to expand...

You're missing the point.
From post #23 "One of the reasons I asked was that I have a bunch of these bridge
rectifiers and wanted to put them to use."
 
ATSman

ATSman

ATSman
Location
San Francisco Bay Area
Occupation
Electrical Engineer/ Electrical Testing & Controls

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