AW: [sdiy] transformer question

[Czech Martin]

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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