[sdiy] wall wart confusion
Bob Weigel
sounddoctorin at imt.net
Mon Feb 27 23:36:16 CET 2006
Who is Laurie? THought that's what I said. Must have missed someone
else saying the same thing... anyway there are several types of
'ground'. Please endure through this tutorial as it contains some hints
that will probably revolutionize your understanding on this matter.
This little journey contains many useful tidbits and is well worth the
time to absorb for people who don't have a fairly high level
understanding of these things even.
Earth ground means there is a connection to earth potential.
"Common ground" could be earthed but not necessarily. In fact it could
be totally isolated from earth and there may or may not be an earth
ground also that for intsance shields the casing for safety reasons or
for ES shielding.
For instance old fender amps and the like often have a ground switch.
Yet..they had a two prong cord. ie.. not directly earth grounded at
all. The switch let you select which ac line coming in was touching a
coupling capacitor (usually .022 - .068uF with .047 the most common I
believe) whose other leg was touching the case ground. Case ground was
used as a common tie point then for all lowest potential points in the
amp.... since THESE amps used only single sided voltages except for bias
voltage I guess you could say.
In other words the power supply would have 440V- 490V to feed the high
voltage needs of the power tubes. This was often filtered again after a
choke to feed the screen grids of the power tubes.. This was dropped
through resistance to attain another refiltered 350V ish supply for the
plate resistors of the inverter tube. Then dropped some more and
refiltered again for an even lower 200 some voltage supply for the
pre-amp tubes.
Meanwhile a separate winding fed the heaters of the 6V tubes, and a
separate winding for trhe 5V rectifier in amps that used that method of
rectifying. This winding had HV insulation since the DC voltage would
accumulate right on that heater! (as electrons rushed from the heater
to the plates powered by the HV windings of the power transformer; the
center tap of which is case grounded!)
Anyway these 6.3V windings were, in the old days just left to float
often. (Eg. early Gibson EH-150 guitar amp). Today this is a source of
GREAT NOISE because we have so much more 60hz floating around than back
then. Quickly mfgs figured out that they need to put a low impedence
from this winding to ground. However, in event of power tube failure
it's not a great idea to totally ground them as it can destroy the HV
windings of the transformer possibly especially if someone sticks the
wrong fuse in. :-) So Fender put a 100ohm 1/2W resistor from each leg
of the heater and in case the HV shorts, they will actually burn up
before damage occurse to more expensive stuff. (100ma even going through
100 ohms, which all power transformers can supply, =1W of power.)
Anyway, THE BIAS voltage in the bigger fender amps is designed to feed
a small amount of current BELOW CASE GROUND to the control grids of the
power tubes. This allows operation with the cathodes grounded directly;
rather than running through a resistance to create a positive voltage
there (which limits the clean headroom of the amp even with a paralell
capacitance to feed signal by the resistor; a common failing part always
also..). TUBES..work this way. If you allow the grid and cathode to
be at the same voltage, they run pretty much wide open and the tube
glows and burns out quickly. (Not to mention it's already at saturation
so no signal dynamics are possible..) So the control grid must have a
voltage below that of the cathode. ~-14V for EL84 tubes... ~-40V for
EL34 tubes and ~-50V for 6L6 tubes... etc.
To produce this voltage fender used an appropriate voltaged AC winding
and they fed it through a diode that points away from a filter
capacitance whose POSITIVE terminal..touches common case ground and
whose negative terminal of course attaches to the diodes anode. The
other leg of the winding is grounded creating a charge pump that will
give a negative voltage buildup on that capacitance.
So anyway... I dont' know what the circuit looks like that you are
working with. But if it's designed for a + and - voltage rail with a
common ground that is HALF WAY BETWEEN those voltages ...and you have
available something with +18, 9, and 0V...then yes treat the 9V leg as
the common ground and attach the other two with 18V on the + and 0V on
the - and you should be ok. -Bob
amokan wrote:
> Thanks. Would I just tie all grounds on the circuit to a common point
> in the case?
>
> On 2/27/06, *Aleks Clark* <aleks.clark at gmail.com
> <mailto:aleks.clark at gmail.com>> wrote:
>
> yea, just use an 18v wall wart. what would have been 0 would be
> -9, and what would have been +18 would be 9. Like Laurie said,
> it's a matter of relative potential.
>
> On 2/27/06, * amokan* <amokan at gmail.com <mailto:amokan at gmail.com>>
> wrote:
>
> So, are there wallwarts that will do +/- ? Am I better off
> using an AC wallwart and then building one of the many power
> supply circuits available online?
>
> I really just want something cheap and that only involves 1
> plug. I also want to avoid anything much more than a regulator
> in my circuit to keep it small.
>
> I've never really put much thought into this until now. All of
> my circuits are done on a breadboard with a bench supply and
> by the time I get them in a frac, they just plug into my main
> supply that "magically" works :)
>
> Thanks again everyone. Sorry to drag this thread on.
>
>
>
> On 2/27/06, *Laurie Biddulph* < elby_designs at ozemail.com.au
> <mailto:elby_designs at ozemail.com.au>> wrote:
>
> If the outputs are truly isolated from the mains power
> (specifically that is not connected to the Earth pin -
> which they should be) then you can connect the positive of
> one wallwart to the 0V of the other. This (double) 0V
> connection is connected to the 0V connection of your
> module and you then have +ve (from the free positive lead)
> and -V (from the free 0V lead).
>
> What you won't get is tracking between the outputs so one
> might go up while the other droops etc. If you are using
> these to power regulators in your circuit then this will
> be less of a concern. You will also need to power outlets
> and if your wallwarts are large then it may not be
> possible to get 2 on to them (double) outlet.
>
> The size of a wallwart is a possible indication of wether
> it is a transformer or switching design but not always. A
> low power transformer unit will be quite small. Usually
> there is next to no weight in the switching designs. Above
> should hol true for these as well as the outputs should be
> totally floating.
>
> Best Regards
>
> (Mr) Laurie Biddulph
> Mobile: 0404 846 943
>
> Elby Designs
> ABN: 70 022 727 605
> http://www.elby-designs.com
>
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> ----- Original Message -----
> *From:* amokan <mailto:amokan at gmail.com>
> *To:* synth-diy <mailto:synth-diy at dropmix.xs4all.nl>
> *Sent:* Monday, February 27, 2006 5:20 PM
> *Subject:* [sdiy] wall wart confusion
>
> I didn't pay attention back in high school electronics
> when it came to power supplies so please forgive me if
> this is a newbie question...
>
> I've built bipolar supply kits for modular use but
> never really understood _how_ they work. Negative
> supplies are a mystery to me. I know there is a
> difference between ground and the negative rail, but I
> don't know what the difference is. A 9v battery is
> in-fact bipolar, yes?
>
> Long story short, I'm working on modules that will run
> in a stomp box format and I'm trying to figure out the
> cheapest & easiest way to power them. I know I could
> use the dual 9v battery solution to power them
> (assuming the circuits could run at 9v) but I'd really
> like to just use a standard 9v or 12v dc wallwart that
> I have laying around the house. How do I get a
> negative voltage off of a standard 12v wall wart? Is
> it already there normally, or just a positive and a
> ground?
>
> Sorry for rambling. If there is a web resource
> explaining this, please let me know.
>
> Thanks!
>
>
>
>
>
> --
> http://www.nomesh.net <http://www.nomesh.net>
> http://www.radioresponse.org
> http://www.freebsd.org <http://www.freebsd.org>
>
>
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