DIY EGOR plans

Peter Gargano peter at techedge.com.au
Wed Nov 15 06:29:25 GMT 2000


NickG wrote:
> 
> 1) it looks like the circuit needs a positive AND negative voltage supply
> (+2.5v and -2.5v relative to ground). I'm thinking of either using a 555
> timer or an astable oscillator to make a switching power supply using a
> small transformer. Comments? Easier methods?

Nick,  If you look closely at U1A pin 1, you'll see that it's output 
is "grounded" and the schematic indicates it's at 0 Volts - What's
really happening is that the U1A op-amp is "dividing" the voltage
between pins 2 and 3 of the 7808 (perhaps this should be a 7805?)
regulator, into a "positive" and "negative" voltage of magnitude that
depends on where the wiper of R46 is. The Voltage shown as V-2.5 is
really at the "gnd in" potential and is presumably the vehicle's GROUND,
and the V+2.5 voltage is at the potential of the regulated five volts.

Put another way, U1A produces a "ground reference" that other signals
are measured from. You don't need a negative voltage generator!

Peter.
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