Re: BU406 vs. BU406D

Alrighty then, let's talk about this, shall we?

NEVER use a 1N4000 series diode in a horizontal output (or in this case a high
voltage power supply) stage. Usually the diode will overheat and blow itself up
because it can't turn OFF fast enough to keep from drawing a high reverse
current.

When the transistor first turns off the collector will go several hundred volts
positive (determined by the flyback, resonating capacitors, current through the
flyback at transistor turn-off time, and added to the power supply voltage to
the flyback). At this time the transistor is turned off and the diode SHOULD be
off, but won't necessarily be if it is a slow part such as a 1N4000 series
diode.

The flyback and the resonating capacitors will resonate at a high frequency (a
couple of hundred KHz or so), giving you a sine wave that, by itself would
slowly decay back to nothing, IF there were nothing to prevent it. But instead
of the sine wave driving the collector several hundred volts negative (which
would break down the transistor by forward biasing the collecor-base junction
and reverse biasing the emitter-base junction which will withstand less than 10
volts without damage) the damper diode turns on. In the case of a BU406D the
damper diode is internal. The damper continues to conduct until either the
flyback runs out of current OR the transistor is again turned on. In this case
the transistor turns on first and the whole cycle repeats.

The biggest problem using an external damper is finding a place for it. Ideally
it would be right across the transistor leads as close as possible.

You say you have about 6 to 8 volts forward on that 1N4006. A BU406 crosses to
an ECG/NTE379 which lists the maximum emitter-base voltage at 9 volts. And I'd
like to know how you were able to measure that 6 to 8 volts. If it were with a
scope you would have had to have the gain set high enough that the positive part
(several hundreds of volts) of the waveform would have been saturating the
scopes amplifier. Once that happens it's hard telling how much of your reading
is real and how much is from an overdriven vertical scope amplifier. PLUS, the
peak current across the 1N4006 is in excess of its 1 amp rating.

You are living on borrowed time with a 1N4006 in that circuit. PLEASE replace
it. Use something like an ECG/NTE 506 or 515. The 506 has a plastic case and
would be the easiest to use.

Matthew Sell wrote:

> I used a 1N4006. I do not a recommended using this diode in the future,
> since it has a relatively slow turn on time. I debated with myself for
> quite a while (with my wife watching me with a strange look....) before
> using this diode, since I was concerned about the back-EMF building so
> quickly as to destroy the transistor BEFORE the diode switched on and tried
> to protect the now destroyed transistor.
>
> I dug up some books and amde a few quick rudimentary calculations before I
> was convinced that the diode should be okay, and then made measurements
> with an oscillscope (a decent one - one that was fast enough to view the
> back-EMF) to verify that the diode was handling the task.
>
> The slow response time of the 1N4006 allows the reverse voltage to build to
> about 6-8 volts before clamping. If I had used a faster diode, this voltage
> would have been much lower. The BU406 can handle about 100-150 volts across
> the emitter-collector juction before shorting (I believe - without looking
> up the true specs). If somebody knows the specs of the BU406D, please make
> a comment here.....
>
> Eventually when I get back to working on Star Wars again (and replace my
> faulty NVRAM), I'll replace that 1N4006 with a much more suitable diode.
> Until then, the Force is with the Amplifone.
>
> : )
>
> - Matt
>
>
> At 08:44 AM 3/6/00 -0800, you wrote:
> >
> >--
> >
> >>I replaced a faulty BU406D in my Star Wars with a generic BU406
> >>replacement, and used an external diode to complete the fix. Mine has been
> >>working fine for almost a year now with 3 months "on location" with no
> >>problems. I'm also using the Wintron replacement transformer.
> >
> >
> >That's great information Matthew!! thanks! :-)
> >
> >Would you mind letting us know the part number of the diode you used, for
> use in a pinch?
> >
> >Thanks,
> >Noel
> >
> >
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> >
> I never met a Windows box I couldn't crash......
>
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