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Diode failure

:: EPE Chat Zone ­:: ­Radio Bygones Message Board :: » EPE Forum Archives 2010 - » Archive through 01 June, 2011 » Diode failure « Previous Next »

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alec_t
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Username: alec_t

Post Number: 638
Registered: 03-2009

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Posted on Tuesday, 17 May, 2011 - 10:54 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

The other day a CFL went pop. On dismantling it I found one of the 1N4007 diodes making up the mains supply rectifier was short-circuit and the 10R resistor in series with the input lead had (not surprisingly) done its duty and fused (presumably the source of the sound).
This is the third gubbins I've had (the other two being a PC power supply and the control board for a central-heating unit) where the point of failure has been a diode of the 1N400x series. In 2 out of the 3 cases the diode failed short-circuit. The heating controller diode (a 1N4006 IIRC) was being powered from a ~20V transformer secondary and feeding a tens-of-mA load, i.e. well within its rating, so its demise was unexpected.
Has anyone else experienced high failure rates among 1N400x series diodes?

Alec
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bruce
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Post Number: 628
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Posted on Tuesday, 17 May, 2011 - 11:03 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Also, is it not highly unusual for a diode to burn out and remain s/c?

bruce
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alec_t
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Username: alec_t

Post Number: 640
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Posted on Tuesday, 17 May, 2011 - 11:19 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Apparently not, Bruce, if 2 out of 3 is representative of the average (it may just be a fluke). I would have expected o/c to be the most common failure mode but I suppose, when a diode fails from over-heating, the molten material inside can bridge the end contact regions on cooling.

Alec
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epithumia
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Username: epithumia

Post Number: 857
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Posted on Tuesday, 17 May, 2011 - 12:11 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Not familiar with the application, but is the current possibly at high frequency?

I've found the 1N400x series to be very slow - bad enough that I couldn't even use them in a breadboard at high frequency.

I've not seen failures, but I do wonder if you've got high reverse current through the diodes because they're slow.

Epi
If you need me, Neil and me will be hanging out with the Dream King. - Tori Amos
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alec_t
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Username: alec_t

Post Number: 641
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Posted on Tuesday, 17 May, 2011 - 02:11 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

It's not a speed problem, Epi. The diode is only handling 50Hz mains (as part of a bridge rectifier). A 2.8uF reservoir cap fed by the bridge shields the diode bridge from the higher frequency (~50kHz?) switching spikes in the CFL. In the PC supply (SMPS) there was a similar fat reservoir cap. Can't recall what the heating controller had after the bridge; but I'd bet there was a cap there, too.

Alec

(Message edited by alec_t on 17 May, 2011)
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gordon
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Username: gordon

Post Number: 673
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Posted on Tuesday, 17 May, 2011 - 03:50 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

They are normally pretty rugged, with good surge ratings. Wonder if they are a cheap Chinese/Far East brand?
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alec_t
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Username: alec_t

Post Number: 642
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Posted on Tuesday, 17 May, 2011 - 04:05 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

@gordon
The CFL lamp was branded Phillips, but where they source their diodes from these days goodness only knows. No idea of brands/sources for the other diodes. The CFL, PC and heating controller were 3 different makes *and vintages*, so I expect their respective failed diodes were also of 3 different makes.

Alec
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cjaysharp
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Post Number: 79
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Posted on Tuesday, 17 May, 2011 - 05:23 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

IME, years of repairing switch mode PSUs and sundry electronics, rectifier diodes usually fail SC first and then if the fuse isn't rated correctly, OC with all the attendant smoke/flames you'd expect.

The CFL diode possibly failed because the filter capacitor was under-rated and/or it was a cheap specimen, having said that I've not seen a CFL that had *any* ventilation for the control electronics yet.
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alec_t
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Username: alec_t

Post Number: 643
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Posted on Wednesday, 18 May, 2011 - 11:38 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

The CFL reservoir cap is rated 400V, but *is* bulging slightly. The solder joint of one of its legs is blackened, suggesting a dry joint. As the board is marked RoHS, I guess this particular failure could be down to the dreaded lead-free solder syndrome.
I agree about lack of ventilation. The CFL base has a token few holes, but they're above and facing the FL tube so probably let more heat *in* than out!

Alec
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echase
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Username: echase

Post Number: 461
Registered: 07-2007

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Posted on Wednesday, 18 May, 2011 - 01:00 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I repaired a HP laptop power brick recently. That had an exploded 450V cap and failed bridge rectifier. Not sure if one diode shorted causing destructive ripple into cap or if cap shorted destroying the diode.
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echase
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Username: echase

Post Number: 462
Registered: 07-2007

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Posted on Wednesday, 18 May, 2011 - 01:44 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

According to datasheets for larger power dimmers/power controllers (1-5kW) they only dissipate one watt per one amp current. That is not a lot. E.g. http://docs-europe.electrocomponents.com/webdocs/0b69/0900766b80b69c7c.pdf But not sure if it is the same at all mark/space ratios and any power factor correction or suppressor components will add to dissipation.
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echase
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Post Number: 465
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Posted on Thursday, 19 May, 2011 - 10:03 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Oops. That last post was supposed to go elsewhere.

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