723 the final insult.

Paul Schreiber synth1 at airmail.net
Wed Jan 28 23:36:53 CET 1998


Actually, a 723 can regulate down to 2.25volts. I did this on the TRS-80 (refrain from trash80 comments) power 
supply back in '76, when all you youngsters were in diapers. And if you can generate a -2.25V you can get it to
go to zero (did this on an "Archer Kit" power supply in '75). Pass the Geritol.

But it takes a special trick. Also, got 'free' adjustable foldback current-limiting. Takes 2 transistors, about 6 resistors.

As I said before, for most synth stuff (<100ma, which is about 15 TL074s) you can't beat a 723 for
price/performance ratio. The SGS 723's you can get for about $.25, and the TI uA723 (the best ones) are
about $0.38. The line & load regulation is a factor of 10 better than a 3 terminal, and the output noise
floor is about 100 times lower.

Paul Schreiber
Synthesis Technology


----------
From: 	The Dark force of dance[SMTP:batzman at gist.net.au]
Sent: 	Wednesday, January 28, 1998 8:12 AM
To: 	'DIY'
Subject: 	723 the final insult.

Y-ellow Y'all.
        Ok For those who were wondering. After many days delay for one
reason or another I finally got back to the PSU and finished it even. I
ended up not using 723s for the split supply and instead used an LM317/337
pair. Thanks Paul. :) They were for another project but as it happens I
no-longer needed that other project as I bought a proper programmer, once
again, thanks Paul, so I could appropriate them for this PSU. It was just a
lot easier all round. I used a pair of 2N3055/2955s as series pass
transistors to increase the current handling capacity of the basic
regulators. Ok so the main current limiting is in the form of a pair of
fuses but I stuck a relay across the supply after the fuse so that if one
blows, the supply switches off entirely.

I did use a 723 on the other supply. Originally a 5 volt, high current
supply for logic. I made that adjustable to about 12 volts. Hell why not?
The main split supply has a pair of analogue style meters on it and a 4
position rotary switch selects an amp meter in circuit with any of the supplies.

Nuff about that.

Conclusions. 723s are a lot of work compared to a 3 terminal regulator such
as the 317. It may give a lot of performance over a 3 terminal reg but the
performance needs to be a trade off against complexity. If the supply is
either a single supply or a dual with separate windings, the 723 makes an
excellent regulator. Essentially you build up 2 separate positive regulators
and join them at the hip. There still remains the problem of a far from
linear adjustment. You would need to obtain the fairly rare, _Anti-log_
taper pots to give a reasonable range of adjustment from maximum to minimum.
A partial trade off can be gained by putting a potential divider on both the
Vref and non-inverting inputs. Or put another way. Trim both inputs to the
op-amp. Having said that, this behavior could be useful in that the lower
the voltage, the more fine the adjustment becomes. 

There would appear to be no easy way to provide current limiting on a
negative supply regulator by simply using the 723. It would of course be
possible to do this with an external comparator or op-amp but this adds more
complexity. Likewise a second 723 could be used since it has all the
necessary bits and pieces on board to form an external current sensing
circuit. But once again, adding to the complexity. But it could be useful to
note that essentially speaking, the 723 is a buffered zener coupled with an
op-amp. Plus a pass transistor and another reference zener. The buffered
zener is set at about 7 volts so you can't use a supply source of less than
this voltage other wise the zener no-longer provides a reference. Which
means of course that 6 volts is right out. Which is kinda a shame for low
voltage battery stuff for which it would be otherwise idea. The 723 has a
very low quiescent current as compared to a 3 terminal reg.

It would be ideal as a split supply regulator in a synth. In this case you
know you are not going to exceed the maximum current because presumably
you've designed it that way.

It's drawbacks however are.
Non linear adjustment.
No current limit as a negative regulator.
Unable to regulate below 7 volts.

I don't know if anyone was/is interested in this stuff but this is what I've
observed. It may be handy to know in the future.

Hope this helps.

Be absolutely Icebox.
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