[sdiy] EFM - Madmouse

[Scott Bernardi]

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