Re: BU406 vs. BU406D

(Replies throughout)

At 02:43 PM 3/6/00 -0600, you wrote:
>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.

Depends on the inductance of the transformer, and the resistance, thus
determining how quickly the voltage builds when the field collapses. There
was also no measured rise in temperate above ambient.

>
>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.

I put mine on the solder side of the board, with the leads as short 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.

Emitter/Base voltage is not the problem. The back-EMF affects the collector
to emitter junction in this particular configuration. The base of the
transistor is connected to the emitter through the secondary of transformer
T1.

 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.

The voltage does not have time to build to several hundreds of volts before
the diode is able to turn on.

  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.

What are your calculations regarding the theoretical voltages that are
obtained using the Wintron transformer in the Amplifone HV board? I'd like
to spot check what you have obtained with my quick calculations to see if I
have made a mistake. In particular, I'm curious as to what the calculated
values of the current through the diode are, and the calculated peak
voltage obtained before the diode starts to conduct.

I'm not saying that the 1N4006 was the best choice, it isn't. I'm still not
recommending that diode to anyone else. I am merely supporting an
alternative to using a BU406D ONLY IF the installer is comfortable with the
alternative. The next time I take the machine apart for other repairs, I'm
going to replace that 1N4006 with something more suitable. At the time, it
was available to me and my initial calculations showed that that diode
would protect the transistor from damage. Temperature measurements and the
fact that the HV power supply has been operating correctly to this point
seem to reinforce my point, although those facts do not completely prove it.

Using this diode was my choice on my game. I am comfortable with that
decision, but recommend it to others with the exclusions I have mentioned.
If you have contrary calculations, I'd like to see them - it expands the
knowledge to the entire group. Some day we may not have access to the
BU406D, and discussions like this will help find an alternate arrangement
to keep the games going.

        - Matt

I never met a Windows box I couldn't crash......

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Received on Mon Mar 6 17:29:01 2000