AW: [sdiy] transformer question

[harry bissell]

Another reason the DC ratings are so poor (compared
to AC) is luck.

Most of the time... the AC contact will close with
much less than the peak voltage across it. Might even
get lucky and have it randomly close at zero volts...
no spark at all.

DC is worst case... every time !

H^) harry



--- Czech Martin <Martin.Czech at Micronas.com> wrote:
> Sorry, I forgot to mention the most important NoNo:
> 
> most  switches are designed for AC current.
> I.e. the spark will vanish automatically during
> reversal.
> This is of course not the case when dealing with DC.
> 
> So all relays etc. have a hard time switching DC,
> contact material will
> migrate,
> burn out, etc. etc. Did you ever notice that in
> switch specs the DC current
> value is a factor of 10 or more lower then the AC?
> 
> Of course the parasitic contact resistance (which
> may grow during life)
> will be turned down by the square of the trafo
> voltage ratio, if placed
> on primary side.
> 
> Another damn good reason to leave the secondary side
> allone...
> 
> 
> m.c.
> 
> > -----Ursprüngliche Nachricht-----
> > Von: Czech Martin
> [mailto:Martin.Czech at Micronas.com]
> > Gesendet: Freitag, 4. Januar 2002 11:51
> > An: 'Sowa Roman'; diy (E-Mail)
> > Betreff: AW: [sdiy] transformer question
> > 
> > 
> > basically : yes.
> > usually it is sufficient to treat the primary side
> in the manner you
> > mentioned.
> > pitfall here: relay may not come up (undervoltage,
> or whatever), so 
> > resistor will burn or circuit will be heavily
> undervoltaged.
> > pitfall 2: some relay circuits are not foolproof:
> fast 
> > toggling will again
> > trip the power fuse.
> > 
> > As for the switching on the secondary side I think
> that this 
> > has no real
> > benefit, 
> > it may be even dangerous if the wrong contact will
> close at 
> > the wrong time
> > (power supply sequencing). One always has to keep
> that in mind when
> > designing
> > circuits with several power and ground pairs. The
> failure of one pair
> > should not latch up something else.
> > Also there are no ideal swithes, contact bouncing
> may lead to very
> > disturbing
> > power supply transients.
> > So I like the idea of "soft ramp up" much more.
> > 
> > I've constructed a couple of power supplies
> with/without 
> > torroids, up to
> > 400VA,
> > one with 60000uF/60V, it seemed that they
> worked...
> > 
> > But I really would like to understand this more,
> I'll set up a little
> > circuit simulation
> > with/without choke coils...
> > 
> > btw.:
>
http://www.geocities.com/SoHo/Museum/4459/circuits/circuits.html
> > 
> > contains some circuits about power supply (some
> untested!)
> > 
> > m.c.
> > 
> > > -----Ursprüngliche Nachricht-----
> > > Von: Sowa Roman [mailto:Roman.Sowa at upc.com.pl]
> > > Gesendet: Donnerstag, 3. Januar 2002 16:31
> > > An: diy (E-Mail)
> > > Betreff: RE: [sdiy] transformer question
> > > 
> > > 
> > > just a thought:
> > > 
> > > wouldn't be usefull a circuit where transformer
> is powered up 
> > > via, say, 1k
> > > resistors in series with primary windings, and
> with the load 
> > > disconnected on
> > > secondary side, so only rectifier and capacitors
> are connected. When
> > > capacitors get charged to full working voltage,
> a relay 
> > > shorts those 1k
> > > resistors and connects the load to the power
> supply.
> > > Have anybody tried this?
> > > 
> > > Roman
> > > 
> > > ""-----Original Message-----
> > > ""From: jhaible at t-online.de
> [mailto:jhaible at t-online.de]
> > > ""Sent: Thursday, January 03, 2002 3:26 PM
> > > ""To: Czech Martin
> > > ""Cc: diy (E-Mail)
> > > ""Subject: Re: [sdiy] transformer question
> > > ""
> > > (..snip..)
> > > ""under load: A transformer which has a higher
> DC resistance
> > > ""in its winding has its rated voltage *under
> nominal load*
> > > ""(and it will goo higher without load). If you
> add an external
> > > ""resistor, the voltage under load will be
> smaller than the rated
> > > ""voltage of the transformer, of course.
> > > ""
> > > ""So the solution is: Select a toroidal with
> slightly higher
> > > ""secondary voltage, and use external resistors
> to fit your needs.
> > > ""There are also special rectifiers with
> considerable resistance,
> > > ""but I've never used them. I think Harry can
> tell us about these.
> > > ""
> > > ""JH.
> > > ""
> > > 
> > > 
> > >
>
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> > 
> 
> 


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