Re: Measurement vs. Simulation

On Thu, 8 Jun 2006 17:06:18 +0200, "Ban" <bansuri@xxxxxxxxxxxx> wrote:

A couple of days ago Mr. Fields made some measurements on a bridge rectifier
(A.B.S.E.) discovering huge spikes. Some things puzzeld me and I tried to
find out what might have been the reason. IMHO those spikes can simply not
be so strong to force the diodes into forward conduction.

They can be, but usually aren't. 50/60 Hz isn't fast enough to
optimally snap a diode.

Data collected from the pics:
effective leakage inductance 2.5mH
effective capacitance 250pF
current out of probe 1 : 30nA
current out of probe 2: 100nA (assuming 10Meg DC resistance)
That is why the reference lines appear shifted.

10 megs of probe resistance can indeed shift the slow waveform at the
transformer secondary. AC coupling the scope can eliminate that.

The difference flows through the transformer secondary , builds up and
accumulates in the winding as a DC-current, until a huge spike is
created.

The snap happens immediately after the diodes have been conducting
hard, conducting amps. No scope probe current is going to "build up"
during the power diode forward conduction interval.

The scope probably has some auto-zero routine, which gets more and more
confused, until the

Sorry, it's not the scope that's confused.

common mode range is gone and some protection device inside the scope fires.
That instrument is crap. And Larkins explanation as well. Sorry Johns, had
to write that!


You deny the existance of pn-junction diode reverse recovery. Lots of
other people don't.

Have you ever seen a scope with a front-end "protective device that
fires"? I've studied schematics of maybe 100 oscilloscopes, and I've
never seen one, nor have I ever seen a scope generate a display spike,
whether it was overloaded or not.

And a scope with an auto-zero routine??!! Please.

For Pete's sake, do a little math. Compute the energy in the spike,
and see if scope probe capacitance can account for it.

John

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