Film-Tech Forum ARCHIVE: Diodes on Kneisley Rectifiers

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Posted by Elvis Navas (Member # 1676) on 04-23-2003, 12:57 PM:

What´s the optimal value for the diodes in rectifiers of 135amp, 100amp, 75amp, 65amp?
what´s the output voltage?
thanks for your help. [thumbsup]

Posted by Paul G. Thompson (Member # 655) on 04-23-2003, 02:00 PM:

Get the highest rating you can get in current and PIV that will fit without modifications.

Posted by Dave Macaulay (Member # 813) on 04-23-2003, 08:59 PM:

I agree with Paul.
The rectifier's open circuit voltage is probably under 200VDC, I would get 400V PIV rated diodes or higher. Higher PIV doesn't radically increase the cost.
I suggest at least 10% over the rectifier's maximum output amperage for the diodes' rated current. You may find the highest current diode available in the package used in your rectifier won't allow that safety factor, so use the highest you can get; as long as the diode current is at least equal to your lamp current you should be OK. A slightly higher rated diode should last longer as the silicon chip is a bit larger and the thermal performance a bit better.

Posted by Gordon McLeod (Member # 33) on 04-23-2003, 10:43 PM:

remeber that the diode is only conducting 1/2 the output wave form and in a 3 phase bridge even less
I usually use the IR 85amp 600v diode in most recifiers

Posted by German Marin (Member # 918) on 04-24-2003, 12:45 AM:

We've been having problems with one 135A rectifier (it's the only one we keep working) and apparently is fine but the 4KW lamp life is no long than 400 hrs. We think the problems are the diodes but they give us differents values. What should be the exact (or most aproximate) number we get on tester? When the diodes begin to loss its properties?
Gracias.

Posted by Paul G. Thompson (Member # 655) on 04-24-2003, 01:21 AM:

Why are the bulbs failing at 400 hours? I would look at those areas first, primarly ventilation of the lamphouse.

People disagree with me on this, but if one diode goes, replace them all. As far as testing them is concerned, the most accurate way that I know of is testing them under load with volt meters ammeters, and temperature bulbs. Current meters to monitor the current flow through it, volt meters to check the voltage drop across the doide, and a temperature bulb to see how hot the damn thing is getting.

High quality meters will give a very good idea but when these diodes are actually in the circuit working under load, things change. It is cheaper in the long run to just replace the whole complement of diodes.

In addition, old diodes have been known to fail because of thermal stress over years of operation, and will fail without warning. It always amazes me when some people use a Zillion-Watt Solder Gun to solder a 12 gauge wire to a little stud rectifier without even thinking of using a heat sink to route the heat away from the diode's internal junction. Then they cannot figure out why that diode failed in about a month or so.

I tell them they are stupid. The reason why it failed is because it was severely heat damaged when it was installed. And then, they didn't even use new compound when the diode was bolted in and over-torqued, no less....

I once saw a huge shipboard 2-story Fairbanks Morris Diesel Engine in the old Olympic. Each main bearing had a temperature gauge to measure the temperature of the actual bearing. As I looked around, each main bearing had a different temperature, sometimes drastic enough to raise the eyebrow of the Chief Engineer.

Get the point?

ARG!!!!! Hammer Technicians..... [fu]

Posted by Elvis Navas (Member # 1676) on 04-24-2003, 08:26 AM:

Thanks very much [thumbsup]

for your help, specially Paul.

Posted by John T. Mellor (Member # 468) on 04-24-2003, 08:57 AM:

I agree with Paul , the stress on the remaining diodes after one starts to fail should be reason enough to change them all.Its a lot cheaper than giving out passes on a Saturday night. The Knisley rectifier will take a 150 amp diode with a 3/8 stud there is a company in New England named Edal that makes a very good diode . As a matter of fact they are the ones in a Knisley rectifier.

Posted by Gordon McLeod (Member # 33) on 04-24-2003, 09:02 AM:

I rarely ever change all the diodes out as usually it isn't necassary. When a diode fails (usually open circuit the rectifiers overall output drops proportionally.
I use a 100watt Iron to solder the #8 wire onto the stud. #12 is two small as it is rated for 20amps in free air.
If a rectifier is repeatedly loosing diodes the culprit is usually a fan cooling issue or High Voltage spikes either from the lamphouse or sometimes due to the failure of the main transformer

As for only getting 400 hrs on a 4K lamp I would ask what is the failure mode. If the electrode tips are pitted look for excessive ripple or inrush current. If it is overall blackening then it is probably heat. If they are getting hard to strike then i would be suspisious of the high voltage boast supply that provides the noload voltage.
Certain brands of lamps seem to need more noload voltage than others when they get old

Posted by Elvis Navas (Member # 1676) on 04-24-2003, 10:20 AM:

Hi mr. Gordon, the electrode tips are pitted. The ripple is produced by some diode that is not working properly. I test the continuity of diodes (unplugged) in beep position on the digital tester and it shows me a number measured in ohms, what should be this value? [Confused]

Posted by Paul G. Thompson (Member # 655) on 04-24-2003, 01:48 PM:

Elvis, I have seen those values "all over the place". But remember, that is a test that uses practically nothing for current and voltage. About the only good that test is is to determine if the diode is open or shorted. in most cases, that's all you need to know. I have not had too much problems if they read somewhere about .375 to .650. Take a bunch of new ones and try them, and plant those figures in your mind. I'll almost bet you will find that each one (even the new ones) will have a different reading.

I respectfully disagree with of Gordon's first paragraph when he said it was usually not necessary to change them all. I agree with the rest of his comments, and fully agree with him on his second paragraph.

As far as diode lead wire size is concerned, it depends on the power supply. I will agree that a 12 gauge wire is not heavy enough to be using in some larger high current power supplies, but it will probably work just fine in the lower ones such as a 1,000 or 1,600 watt unit. Rarely did I ever see anything smaller than 10 gauge wire anything above 1600 watts. Also, the type of wire used makes all the difference in the world. I would opt for bonding strap style I have seen in some good quality power supplies instead of AWG 12, 10, or what-have-you.

Diodes with pigtails already attached can be purchased, and I will go out of my way to get those styles. The price difference between the two is not that big of a deal.

I never liked the idea of using soldering equipment on diodes. No matter how good of a heat sink you use, damaging heat can still reach the PN junction inside the doide.

At one time, I did just change one diode instead of the whole complement. That was cool, because I knew I would be back in about 2 to 3 months to change another.

Good job security.... but non-professional in my books. It is an injustice to that person trying to make an honest "buck" so he or she can feed themselves and their family.

I discontinued that "change one diode" practice when I left a theatre supply company I used to work for years ago.

Posted by Gordon McLeod (Member # 33) on 04-24-2003, 03:28 PM:

If it was a bad diode that was causing the ripple there would be a terrible amount of flicker
you will need a clamp on ampmeter and measure the AC current value in the rectifiers output.
If it is much over 5% then I would suspect a bad filter cap. If it is a 3phase kniesley they don't have an output filter choke and that tends to increase the inrush current

Posted by Paul G. Thompson (Member # 655) on 04-24-2003, 03:35 PM:

Gordon is correct. I might add that the flicker Gordon speaks of would be a strobing flicker caused by the shutter. With the projector stopped, shutter open, CO open, HD open and FS open, you probably will not notice the flicker on the screen. However, the eyebrow raising question would be low bulb current - much lower than normal.

I also understand a storage scope is a valuable tool for checking in-rush current, but I never had that luxury to have one.

Posted by German Marin (Member # 918) on 04-25-2003, 01:47 AM:

The lamphouse is a Kneisley

The flicker is present but we cannot measure 'cause the "ripplemeter" we have is being repaired.

Posted by Paul G. Thompson (Member # 655) on 04-25-2003, 02:06 AM:

If it is a strobing flicker, you either have one or more diodes headed south, or maybe a bad filter capacitor as Gordon suggested.

However, to be sure, please indicate if it is or is not a strobing flicker. Some people's idea of a flicker is caused by an unstable drifting arc.

Posted by Dave Macaulay (Member # 813) on 04-25-2003, 08:49 AM:

Diode readings:
Digital ohmmeters are not great for measuring diodes. If that's all you have, you should get a very high (reverse) or infinity reading in one direction, and a fairly low reading the other way (forward). The forward reading is actually not a resistance though - it's a voltage drop - and some variation is normal for all rectifier diodes but it should be about 0.5V. Digital ohmmeters interpret this slight variation poorly, so most have a "diode" setting that reads the actual voltage drop.
Also, digital meters can measure a resistance from dirt on the outside of the diode (I know, the inside of the rectifier is always absolutely clean so this isn't a problem... as if) so the reverse reading might be somewhat low. This resistance doesn't affect the rectifier output but can confuse the technician doing the reading.
An old needle type ohmmeter gives more consistent readings, they should be infinity one way and a low reading the other way.
Whatever, if you get fairly high readings one way and fairly low readings the other way the diode is almost certainly OK.
If a diode is "open", not conducting anything, you'll get a lot of ripple and with a single phase rectifier the lamp will flicker so bad you would likely be alarmed. With 3 phase it isn't quite so bad. A diode going "short", conducting both ways, usually blows fuses or causes loud hum & smoke problems from the rectifier... not a ripple or lamp life problem alone.

I don't recall what Kneissley uses, but be aware that many rectifiers use both "normal" and "reverse" diodes - these look almost identical but must not be mixed up. There's usually a "R" in the numbers marked on the reverse type, this may be the only visible difference. Make sure the correct ones get mounted in the correct holes! It's fairly impossible to guess which is which with a meter - do you know for sure which lead is positive voltage when checking ohms on your meter?

Posted by John Pytlak (Member # 331) on 04-25-2003, 09:31 AM:

Don't forget the Osram technique of letting some of the lamphouse light shine directly onto a loaded (about 500 ohms) silicon photocell, and then look at the voltage output on a scope. Gross ripple caused by an open diode or missing leg should be obvious.

http://www.sylvania.com/business/photo/pdfs/ENGR_BLTN11.pdf

(See technique on page 18)

[ 04-25-2003, 11:52 AM: Message edited by: John Pytlak ]

Posted by Elvis Navas (Member # 1676) on 04-25-2003, 10:30 AM:

Paul, yes it is a strobing flicker.
Dave, all diodes are "normal" positive to the base. i.e. christie's rectifiers use both normal and reverse.
John, thanks for the link.
[thumbsup]

Posted by Paul G. Thompson (Member # 655) on 04-25-2003, 11:36 AM:

What is the model number of that power supply? I presume it is a 3 phase power supply?

Several things come to mind.
* 1 or more leaking or open diodes. It may not show up with a diode tester.

* Filter capacitor has lost its poop.

* Fried or burned up connections in the power supply.

* Loss or partical loss of one AC power phase.

* Major ammeter mis-calibration. (Power supply operating outside of parameters)

Can you safely check the ac voltages on the plates of the rectifiers? They should all be the same. Can you safely check the voltage drop across the diodes? They should also be the same.

There is not a whole lot of things that can go wrong in those power supplies. More than likely, you will either find some weak diodes or a bad filter capacitor. Voltage checks will help determine if you have an internal transformer malfunction. Current draw readings on the transformer primary can also provide very valuable information.

If memory serves me correcly, some Kneisley power supplies used selenium stacks. Some were converted to use high-powered diodes. If this is true with your power supply, a possibility does exist a wiring problem could exist - from day 1 of the conversion. This is just a guess...I could be wrong since I don't know exactly what you have for a power supply.

Posted by Gordon McLeod (Member # 33) on 04-25-2003, 11:40 AM:

Is that a prevost favorite 35/70 machine on the front?

Posted by German Marin (Member # 918) on 04-25-2003, 04:50 PM:

Paul, It's a three phase power suply Kneisley model M135RTX2X. Elvis has tested the diodes and the measurement is little different between they. We had to change the breaker 'cause it failed some weeks ago. The input voltage is 208V for each pair we've tested (it's normal for my country). The capacitor is new, it was change at the same time we change the breaker. We'll change all the diodes, I'm sure they are the problem.

Gordon, It's a P60 and it's one of three we keep working. We're rebuilding three more.

It's the old lamphouse its used years ago.
Thanks for all the support!
Regards.

Posted by Gordon McLeod (Member # 33) on 04-25-2003, 06:02 PM:

It looks a lot like the 35/70 machines they sold

Posted by Marc Jones (Member # 29) on 04-25-2003, 06:33 PM:

If all diodes test good one other thing to look at is the incoming power. Verify if you have a fused disconnect near the supply that all the fuses are good and also verify that once the contactor is pulled in that you have all 3 phases.
Good luck

Posted by Paul G. Thompson (Member # 655) on 04-25-2003, 08:29 PM:

In addition to Mark's comments, I would recommend you check the 3-phase rectifier feed from the power panel. If there is a bad breaker or a corroded bus bar where the fuse or breaker seats, it probably will not show up like you think it should.

Even though the load is still connected, one cannot go by a volt meter when the legs are read. The other phases will "backfeed" through the power transformer and give a bogus reading equal to or the same as what you might expect if the circuit is intact. A current circuit tester is highly recommended for tests like that.

A simple voltmeter might not do the job either simply because if the breaker, fuse, or bus bar is defective, there could still be an indication that things are OK. But as soon as you put something on that phase that draws a little current, that phase may have a "bad day at the office." I have also seen some fuses and circuit breakers that check good by the use on an ohmmeter. However, in actuality, they were not worth the powder it takes to blow them to Hell. You may not have a diode problem after all!

Get one of these devices. They are inexpensive, reliable and accurate. This can be one of the most indespensable tools you could have for troubleshooting 3 phase and single phase problems. You see mine is practically worn out. I have had this thing for 20 years. Follow the instructions on how to use it in 3-phase circuits, and you might have the answer for your problem in a matter of seconds.

Here is another tool you should not leave home without. It is also fairly inexpensive.

These are old basic tools that are still available, and will do what you want them to do. There is no need to spend a King's Ransom on tools like this, such as digital readouts, memorys, etc. If you spent more than a hundred bucks total for like items or equivilant, it would really surprise me. Both of these will save you many hours in troubleshooting.

Posted by Marc Jones (Member # 29) on 04-26-2003, 11:46 PM:

One other thing:
I am not familiar with that exact kniesley rectifier, but I know previous ones that I have had a selector bar for different taps from the secondary, just curious if moving the bar to the next position up or down helps the problem.

Posted by Elvis Navas (Member # 1676) on 04-27-2003, 01:56 PM:

Marc, I've checked the contactor and it is ok, the cause of the problem is the diodes.
Conclusion: the current diodes are 25 years old and I think is time to change it [Razz]

, thanks for your help and I know you'll help us with another trouble. [Big Grin]

Posted by Paul G. Thompson (Member # 655) on 04-27-2003, 03:47 PM:

Elvis, let us know of the results when you replace the whole set. [Smile]

Posted by Elvis Navas (Member # 1676) on 05-13-2003, 03:16 PM:

hello my friends, what do you think to place a filter irem with the rectifiers kneisley. [Confused]

[ 05-16-2003, 03:33 PM: Message edited by: Elvis Navas ]