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Topic: Rectifier Diodes

William Phillips
Expert Film Handler

Posts: 117
From: Cardigan, Wales, UK
Registered: Feb 2002

posted 07-31-2002 03:09 AM

Whats the best way of checking the efficancy of rectifier diodes and capacitors.
I have just taken out a Osram 1600watt HSC thats done 1750hrs, the picture was starting to flicker.
The cathode is looking a bit pitted. All the other lamps out of my machine look ok.
The lamphouse is a Cinemeccanica CX21H with an IREM N3-X75DM

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Pete Naples
Phenomenal Film Handler

Posts: 1565
From: Dunfermline, Scotland
Registered: Feb 2001

posted 07-31-2002 03:45 AM

Do you mean how to measure the ripple current?

I use a AC/DC clamp adaptor on my Fluke meter, which makes it very easy. If you buy/beg/borrow/steal one of these make sure it and the meter are true RMS, otherwise the reading isn't especially accurate.

There is a method involving measurement taken across the ammeter shunt, it's detailed on the Osram site somewhere, I'll bet John P can point you in the right direction.

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William Phillips
Expert Film Handler

Posts: 117
From: Cardigan, Wales, UK
Registered: Feb 2002

posted 07-31-2002 04:36 AM

Cheers Pete,
I will try and borrow a clamp meter.
I was just woundering if there was a way of checking the diodes to see if they are going to give out before they do, also the cap.
We have had a few very nasty power surges since this machine was in, one of them we had a lot of damage done to equiptment in the building, computers, emerngcy lights, fridges etc. The projector was running at the time. So I would prefer to find out before anything nasty happens.

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John Anastasio
Master Film Handler

Posts: 325
From: Trenton, NJ, USA
Registered: Sep 2000

posted 07-31-2002 05:58 AM

There's really no way to know when a diode is "ready" to go. When it goes, it's a sudden breakdown usually caused by too much inverse voltage applied to it. A diode is a one-way gate, and if you suddenly push too hard against that gate, the latch is going to come loose! It's those nasty surges that do it. They don't exhibit any characteristics that get gradually worse over time. They just blow. Same thing with capacitors. You can't really measure how well they're "aging". The breakdown is caused by a catestrophic failure of the dielectric, the substance that seperates the two plates. When a high enough surge enables an arc to jump from plate to plate, pushing right through the dielectric, it causes a hole. That hole is now a dead short and the cap is fried. What you need to do if you have surges is to either get rid of the surges or to install diodes and capacitors with a higher rating so that the components can take the extra strain.

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Demetris Thoupis
Phenomenal Film Handler

Posts: 1240
From: Aradippou, Larnaca, Cyprus
Registered: Apr 2001

posted 07-31-2002 07:40 AM

Well,
I had a huge problem with IREM rectifier. Diodes kept blowing too often and I decided to change the whole set with bigger ones. After two years of hard working these diodes have shown no problem. Try that to ensure a safe and reliable projection.
Demetris

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Rick Sanjurjo
Film Handler

Posts: 26
From: Omaha, NE, USA
Registered: Jul 2002

posted 07-31-2002 06:16 PM

Checking the ripple with a clamp on ammeter is a good method. The other way would be to measure AC voltage across the shunt with your meter and convert that reading to amps. Most shunts will be stamped with a number like 50mv 200a that indicates how to convert your voltage reading to current. In the case of a 50mv 200a shunt you would multiply your reading in millivolts by four and that would be your ripple current. The ripple current divided by the operating current will give you the percent ripple in the power supply.When we check ripple in the factory we use a Tektronix's current probe connected to a scope.

A good way to check the condition of the diodes is to put a temperature probe on them and see how hot they are running. If one is running hotter than the others then there is probably a problem. Another method is to check the voltage drop across the diodes, it should be the same on all of them.

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Rick Long
Jedi Master Film Handler

Posts: 759
From: Toronto, Ontario, Canada
Registered: Nov 1999

posted 07-31-2002 10:48 PM

Diodes are like fuses - they're good until they ain't. Generally, when replacing them, go for the highest current and P.I.V. ratings you can get in the same case size.

At Eaton's Centre, Toronto, I used to have a lot of problems with diodes blowing on the horrible little orange Irems, until I put small blowers on the top of them (blowing upwards). They were attached to the grill, just above the diodes.

Where before, we were lucky if a diode lasted a year, after the fan installation, there were no rectifier calls until the theatre closed, six years later.

Pitting, as you know, is a sign of insufficient current regulation. Might be time for a capacitor change too. Electrolytics do dry out over time.

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Gerard S. Cohen
Jedi Master Film Handler

Posts: 975
From: Forest Hills, NY, USA
Registered: Sep 2001

posted 07-31-2002 11:48 PM

Hooo--HA! I didn't know you can make a career of clamping rippling
dildos with a Fluke meter and blowing gently on them (upwards)?
I'm certainly glad they dry out by themselves!

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Paul G. Thompson
The Weenie Man

Posts: 4718
From: Mount Vernon WA USA
Registered: Nov 2000

posted 08-01-2002 12:10 AM

I remember an IREM that blew a diode at the Keno Theatre in Kenosha, Wisconsin around the year of 1981. No diodes in my kit, and no replacements in the booth. I had a king-sized diode that I clamped on with a vise-grip pliers, and affixed a blower on the power supply.

It kept the theatre from going dark until the next service call.

In short, keep a full complement of diodes in the booth in case one gets blown.

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Pete Naples
Phenomenal Film Handler

Posts: 1565
From: Dunfermline, Scotland
Registered: Feb 2001

posted 08-01-2002 08:41 AM

My only gripe with using the drop across the shunt is that you have to attach leads to it, which may or may not be easy according the design of the rectifier/lamphouse. Also it's far too easy to leave the meter connected and strike up the lamp, which will almost invariably result in goodbye meter! I've knackered two in ten years that way. My current Fluke is alleged to be good to 16kV overload, but I don't fancy trying it. That said you shouldn't leave the clamp adaptor on whilst striking either, I've found the splash from the striker upsets it too. Safest is to strike up, then clamp around one of the DC leads.

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Mark Gulbrandsen
Resident Trollmaster

Posts: 16657
From: Music City
Registered: Jun 99

posted 08-01-2002 12:09 PM

Indeed ALWAYS replace all of the diodes in a rectifier even if there is only one bad one. Diodes are under tremendous thermal stress while operating and these repeated stresses, after thousands of thermal cycles are what cause many to fail...the junction becomes mechanically unsound due to the epoxy filling deterioating. This epoxy(glass in high quality diodes)eventually begins to loosen up and the junction then fails. Many times you can actually wiggle the stem of the diode after you remove it. Also, its best not to solder directly to the stem of any stud mount diodes. Most are installed with a small screw, lockwasher, and nut, and this is by far the best way to go, especially in the field.
When I was back in the Chicago area I met a retired semiconductor design engineer that really knew his stuff. Some of the above are his reccommendations and some are from my experience over the years.
Most important thing to look at when selecting a replacement diode though is to get one that is glass filled.
Mark G.

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Paul G. Thompson
The Weenie Man

Posts: 4718
From: Mount Vernon WA USA
Registered: Nov 2000

posted 08-01-2002 01:34 PM

Mark is correct. Change them all out if one goes goofy. If one is ready to go, they all have the same hours on them, and the other ones will follow. Diodes get leaky, and that can cause some problems.

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Mark Gulbrandsen
Resident Trollmaster

Posts: 16657
From: Music City
Registered: Jun 99

posted 08-01-2002 06:53 PM

I forgot to mention that the really correct way to check them is on a semiconductor curve tracer. There are also most likely more sophisticated computerized tools for doing this now but a curve tracer is still pretty much standard for checking actual specs on each diode(or transistor).
Mark G.

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Rick Sanjurjo
Film Handler

Posts: 26
From: Omaha, NE, USA
Registered: Jul 2002

posted 08-01-2002 10:35 PM

I agree using a clamp on ammeter is easier and safer than going across the shunt, but if you don't own an ammeter the shunt is an alternative.
Rick

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Rick Long
Jedi Master Film Handler

Posts: 759
From: Toronto, Ontario, Canada
Registered: Nov 1999

posted 08-02-2002 10:08 PM

OK, now how the hell do I tell if a diode is glass filled? Drop it from a great height and see if it rattles?

What is the advantage of a glass filled diode?

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