[sdiy] EFM - Madmouse

[Scott Bernardi]

Ah, well, wrong about the MadMouse, but what I said works for a "dual
complimentary bipolar supply". And when you're going for space savings
(like in the MadMouse) your method is better 'cuz you can use the
external wallwart. I'm just too used to building modulars in big cases. 

"tomg at efm" wrote:
> 
> Hey Scott,
> Good guess but no prize this time....;-) It does indeed need +/- 12VDC
> but this is accomplished by using a half-wave supply so... +/-V are both
> derived from the same winding.  It makes using a common 12VAC
> wall-wart possible so there is no need to expose anybody to 117VAC.
> 
> It's a safety thing! Those in the know.... who want to modify things...
> assume responsibility for exposing themselves to high current AC.
> Please be careful!...Current kills!
> 
> As to the way it works......The wall wart delivers about 12VAC to
> the +/- rectifier-pair a little more is better say (13-14V). after filtering
> the rectified voltage is about +/-16VDC which is just enough to
> regulate...;-) It's amazing how well this works. There is typically about
> 0.2VAC of unregulated ripple and 0.000001VAC on the supply side.
> You can actually build a audio-pre or high-gain distortion  this way
> and live with it..;-)
> 
> Tom
> 
> > My guess is the MadMouse needs +/- 12v DC. To do that, you need a center
> > tapped transformer whose AC rating is twice as much as the DC voltage. A
> > 24VCT will do, or a 25.2VCT which might be more common.
> > The AC rating of a transformer is an "RMS" rating; the peak voltage will
> > be (for a 25.2VCT)  12.6V * 1.414 = 17.8V peak. Take off a couple of
> > diode drops for a bipolar full wave rectifier, 17.8-1.4 = 16.4V. If you
> > filter this enough to get 10% ripple, you'll have a 1.6v ac riding on
> > top of a 16.4 - (1.6/2) = 15.6v average voltage going into the 12V
> > regulator. The 7812 and 7912 type regulators need about 3v of headroom
> > (ie, 12+3 = 15v), so you're right there with the 25.2VCT transformer. A
> > 24VCT would probably work, but it's cutting it kind of close, especially
> > if you run it near its rated load current.
> >
> >
> > "Reverend djbluE (Chromatest)" wrote:
> > >
> > > I'm still trying to figure out what rating my transformer needs to be
> > > for the power supply on the madmouse...
> > >
> > > for some reason, I get no responses from the EFM list, so I ask here.
> > >
> > > The instructions say It's a 12v AC input to the board.
> > >
> > > I'm guessing it's Center Tapped since it gives me 3 connection points.
> > >
> > > I currently have a 14V wall wart (not CT though) and am wondering if
> > > this will work. if not, I will buy a new transformer and mount in the
> > > bottom of my case.  Looking at transformers, I see that a 12V AC CT
> > > trasnformer gives off 6-0-6 and this seems like it would be too low,
> > > so I'm thinking that maybe I need a 24v AC CT that would give
> > > 12-0-12...
> > >
> > > any advice?  any ideas how I could figure this sort of thing out?
> > >
> > > thanks all!
> > > -ben
> >
> > --
> > Scott Bernardi
> > sbernardi at home.net

-- 
Scott Bernardi
sbernardi at home.net