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Dual Power Supply

R

Ramendra S Roy

Jan 1, 1970
0
Hi!
I have been trying to make a +15 0 -15 regulated power supply. I have
been using a pair of LM 723C with TIP 3055 and TIP 2955 (I think I have
the correct number here) as the pass transistors (NPN and PNP) for the
+ve and -ve sides repectively. I am interested in a dual supply which
can supply up to 2 amps with high regulation and as low ripple as possible.

The question is - are there resources which explain the use of the 723
in detail? Can I make sure that the +ve and -ve rails are at the same
level from 0? Do I really need 2 723s? Lots of questions I know. Many
thanks to anybody who can help me even on one of the questions here!

Thanks
roy
 
J

John Popelish

Jan 1, 1970
0
Ramendra said:
Hi!
I have been trying to make a +15 0 -15 regulated power supply. I have
been using a pair of LM 723C with TIP 3055 and TIP 2955 (I think I have
the correct number here) as the pass transistors (NPN and PNP) for the
+ve and -ve sides repectively. I am interested in a dual supply which
can supply up to 2 amps with high regulation and as low ripple as possible.

The question is - are there resources which explain the use of the 723
in detail? Can I make sure that the +ve and -ve rails are at the same
level from 0? Do I really need 2 723s? Lots of questions I know. Many
thanks to anybody who can help me even on one of the questions here!

Thanks
roy
The LM723 data sheet provides the basic circuits for several regulator
configurations. Are the specs on any of these close to what you want
to build?

http://cache.national.com/ds/LM/LM723.pdf
 
C

Chris

Jan 1, 1970
0
Ramendra said:
Hi!
I have been trying to make a +15 0 -15 regulated power supply. I have
been using a pair of LM 723C with TIP 3055 and TIP 2955 (I think I have
the correct number here) as the pass transistors (NPN and PNP) for the
+ve and -ve sides repectively. I am interested in a dual supply which
can supply up to 2 amps with high regulation and as low ripple as possible.

The question is - are there resources which explain the use of the 723
in detail? Can I make sure that the +ve and -ve rails are at the same
level from 0? Do I really need 2 723s? Lots of questions I know. Many
thanks to anybody who can help me even on one of the questions here!

Thanks
roy

Hi, Mr. Roy. The data sheet of the LM723 (referenced in Mr. Popelish's
post above) shows a lot of what you need. Look at the schematic in
Fig. 4 for your basic circuit.

Unfortunately, the current limit and current foldback don't work with a
negative supply. So your best shot might be to use two 16VAC, 4 amp
transformers, and have both of them feed bridge rectifiers and mongo
filter caps (think at least 20,000uF). You can then have two separate
trimmer pots to trim voltages to be identical, and tie the negative of
one LM723 15V regulator to the + of the second 15V regulator. You can
call the junction 0V, and there's your +/-15V supply. Use two 1N5402
barrel diodes at the outputs to protect against reversing voltage.

The term describing a dual +/- power supply where the magnitude of the
negative voltage stays the same as the positive voltage is "tracking
regulator". A good tracking regulator will have both supplies tracking
(usually the positive supply is the master, and the negative the
slave), so the negative supply will even compensate for output
variations in the positive supply with loading or line power
variations.

If you're interested in doing this, It's possible to make a +/-
tracking regulator with two 723s and several op amps, but it's pretty
complicated.

A very simple home-brew tracking regulator can be made with an LM340-5,
an LM320-5 and an LM1558 dual op amp as shown in Fig. 17 of National
Semiconductor AN-103

http://www.national.com/an/AN/AN-103.pdf#page=10

This is good to the current limit of the LM340 and LM320. You can use
the tricks in Fig. 6 and Fig. 10 to bump up the maximum current on one
or both supplies with a pass transistor, and to set short circuit
current limit with a current sense resistor. It tracks from +/-5V to
+/-15V. You can use two LM741s instead of an LM1558 if you want, and
tweak the offset adjust on one of the op amps, or better, introduce a
DC offset voltage to the feedback to compensate for differences between
the + and - voltage. Post again if this is of interest.

A very simple ersatz 1 amp +/- 1.25 to +/-15V regulator can be made
with an LM317, an LM337 and a dual 1.5K ganged pot. Tweak differences
in reference voltage by adding a few ohms to one of the 120 ohm
resistors. Also, you can make an ersatz 2 amp dual regulator with two
ST Semi L200s and a dual-ganged 10K pot. These aren't technically
tracking regulators, because the negative supply won't track voltage
droop caused by line or loading, but it should be OK for most
applications. The latter is described in Fig. 21 of the ST appnote "A
Designer's Guide to the L200 Voltage Regulator".

http://www.st.com/stonline/products/literature/an/1678.pdf

The L200 can crank 2 amps, and no extra op amps are required. Later in
the appnote is a more complex circuit that will track for you. This
circuit goes from +/-2.85V to +/-15V. Obviously, use TO-3 packages for
all transistors and power ICs.

The best thing is to make something easy that will do the job, and get
in the game. And be sure to get massive heat sinks, and get a good
fan. Two amps on both + and - linear supplies will mean up to 80 watts
of heat to dissipate.

Good luck
Chris
 
J

John Fields

Jan 1, 1970
0
20,000uF for 2 amps !

Good Lord.

Depends on how much headroom he needs.

20,000µF will give him about 1.2V of ripple with full-wave rectified
60Hz feeding a 2 amp load.
 
C

Chris

Jan 1, 1970
0
Pooh said:
20,000uF for 2 amps !

Good Lord.

Graham

Yup. You want the rails to be about +/-20V for a +/-15V supply. If
you use 16VAC transformers for the + and - power supplies , I'd guess
you'd have a peak voltage after rectification of 21V or so. A really
large cap (22,000uF) means there's less than a volt of ripple.

Big linear power supplies generate a lot of heat. A big cap is cheaper
than a bigger heat sink.

But I guess I was talking more about a variable tracking supply -- I
guess I got a little off track. If the OP chooses a design that
doesn't have overtemp shutdown (like the 723 designs in the data
sheet), he may be in a situation with a shorted output where the pass
element is cranking as much as 40 watts.

Thanks for the spot. Glad somebody's checking -- we couldn't have
incorrect information floating around the internets. If that happened,
it would be just like TV journamalism. ;-)

Chris
 
R

Ramendra S Roy

Jan 1, 1970
0
Thanks everybody! I have the answers I needed. Unfortunately it means
that I have to do a bit of redesigning - especially the short cicuit
protection. I have to make sure both rails are clamped down as soon as
the current exceeds a maximum of 2 amps on any one of the rails. As per
Chris's suggestions - I am going to improve on this cicuit later. Looks
like I need to look around and decide on which regulator (or maybe use
op amps) is the best for my application.

Thanks
Roy
 
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