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 Diode ratings

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FrankB
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PostSubject: Diode ratings   2012-02-21, 12:10 am

Here is an interesting story on diode ratiings. This happened at a TV repair shop I worked at inthe late 70's.
The organ tech replaced 4 diodes in the PS of an organ. He used the ECG 116's, as they were rated at 1 A 1KV.

They kept blowing. He measured current and it was just below the max ratings and the voltage, which was just fine.

After blowing 3 sets of diodes he contacted the mfg, ECG.

He explained the problem and they came up with the answer in just a few minutes.

He had CUT THE LEADS short, rather than using their full length. This DERATED the diode, as the diode MUST have FULL lead length to dissipate the heat properly, to carry the FULL RATED current.

Now this explains why I saw TV sets with a "Curley cue" loop or 2 in the diode lead, but thought it was some sort of machine insertion revision to make the diode fit the holes.

He put a loop in the diode leads and used their full length, and had no problems or callbacks on the organ.

Just goes to show you, things are not always the way you might think they should be.

FrankB
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Rickey

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PostSubject: Re: Diode ratings   2012-02-28, 7:45 am

Interesting article, thanks!
Rickey

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Cliff Jones
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PostSubject: Re: Diode ratings   2012-02-28, 11:27 am

Good info to share.

When I was doing major overhauls of military oriented computer cards and power supplies we always put a horseshoe bend in all component leads to assist in relieving mechanical and heat stresses. We had a High voltage power cards that used 5 watt carbon resistors, and they continued to burn up 27 resistors in each card. The card was designed with heat shields under each resistor. We finally ended up completely gutting the card and doing a lot of through the hole trace repairs. All the resistors were conformal coated (note: that was engineered design). We replaced all carbons with film resistors and used a tongue depressor to make space between the card and the resistors and added a stress relief in the lead bends, We had thousands of cards to do this to.

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FrankB
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PostSubject: Re: Diode ratings   2014-04-12, 12:27 am

After re-reading this article, with Cliff's comments, I have to wonder if the ratings of resistors are like that of diodes, where the lead length effects the wattage and heat dissipation?
That might explain why many of the Phillips/ Magnavox TV sets kept burning up a resistor inthe screen ckt on the CRT boards. I'd replace them with 1/2 w rated (Not the 1/3W factory installed ones) and always left longer leads on them. I noted on some models the factory also left long leads onthe resistors and mounted them high on the board - for heat dissipation? None of the ones I modded ever came back for repair of that ckt either.
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Cliff Jones
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PostSubject: Re: Diode ratings   2014-04-12, 8:38 pm

Now see, even I just learned something. With lead length aiding heat dissipation.
I just though the body itself met the requirements, But lifting the resistors and diodes would allow for more air circulation underneath and would also keep the board from getting charred.
Those boards I worked on all (everyone) had cold solder joints from the heating expansion and cooling.

Thanks for that tip on lead length.

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FrankB
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PostSubject: Re: Diode ratings   2014-04-12, 11:04 pm

I loosely based that surmise on the fact that Dale and others make a metal bodied resistor that MUST be bolted to the chassis to achieve its full dissipation ratings.
I cant help but think a dab of heat sink thermal compound under it would also help.

If you old timers remember the 50 watt & up resistors came with end clips to mount them above the chassis to dissipate the heat better. I have even seen devices that used a fan across the resistors to aid in heat dissipation.
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