Reviewing the LM723

KL7AJ

Joined Nov 4, 2008
2,229
Sure, it is ancient. You can stock a bunch of more modern voltage regulators or you can just stock one that does positive or negative, low current or high current with just adding a cheap power transistor. Nonetheless the LM723 has some great lessons on how voltage regulators work. Operating voltage goes up to about 37 V. Not a bad deal for about $0.50 US. Old, yes,, but still very educational and useful.
The LM723 has two basic sections. It has a voltage reference section and a regulator section. The reference section puts out about 7 Volts. Absolute Maximum Rating on the spec sheet says 15 mA, maximum to be drawn. From 1 mA to 15 mA this output probably won't waver even 1 mV. The voltage regulator section can tolerate about 150 mA being pulled from it. Adding an external transistor adds to this. Most any transistor will do.
For operation below 7 Volts a voltage divider is added to the output of the voltage reference. The Voltage regulator section compares this voltage to the output voltage. For operation above 7 Volts you put the voltage divider on the output and compare the 7 V reference against a tap at 7 Volts on the divider.
The reason it's still around despite being ancient is that it's very versatile, and just plain works.
 

MrAl

Joined Jun 17, 2014
11,396
Hi,

I know how it feels to 'like' a chip of a certain part number. Mine is probably the lowly LM358 that many people complain about. It is certainly not good for everything, but i've been able to get it work in applications that everyone else would swear it would not work in. Even the crossover noise can be eliminated for example with a simple little trick.
Cant remember when i first starting using it, maybe around 1980, but it's still used in new designs today.
 

crutschow

Joined Mar 14, 2008
34,285
They hired a, "real" engineer! Real college degree = State of the Art Design, don't you know? Real engineer, big oven, and not a fail-safe in sight...not even a human watching the first full load, 24 hour run. Better to hire a college graduate than a blue collar person with 15 years experience and a State Certification to prove it. Real Engineers don't make mistakes like that, so there is no need to pay anybody to monitor the progress.
I come from a military/satellite design and test background so it blows my mind that they could do a first run like that without someone baby sitting it. :eek: That is so incompetent as to border on the criminal. But, of course, the engineer gets the blame, not the idiot managers who hired him.
Any long term temperature tests of our production products were always monitored 24 hours a day, even with ovens that had a separate fail-safe.
 

#12

Joined Nov 30, 2010
18,224
the engineer gets the blame
I can tell you the story from my lawyer's point of view: If I did it, it was my design and my construction, so it would be my License, my Corporation, and my Insurance that would take the $3M hit. If they hired a PE, it would work the same for him. The part I'm not sure about is: If they used an in-house engineer, it seems nobody but the shareholders would bite the big one. A bunch of suits would, "blame" the engineer, but he wouldn't be paying for the fried equipment.

Am I right?
 

KL7AJ

Joined Nov 4, 2008
2,229
FWIW, I'm partial to the LM723 because that's what I cut my teeth on. Those years of experience allowed me to develop some versatility and I especially admire the ability of the chip to protect itself from excessive loads. When the company hired a, "real" engineer, his three pin regulator designs sounded like popcorn when subjected to the tests I used on my LM723 designs. Of course, that was AFTER I showed him how to find the limit on how many turns you can fit inside a torroid.:D

Some ten years later, I was denied a job designing an oven to temperature cycle new products because I was only a State Certified designer of heating and cooling machines. The, "real" engineer who got the job designed a machine that melted $3 million worth of products on its first run. I have the photograph.;)

I have virtually worshiped some great engineers during my career, but claiming you're a, "real" engineer doesn't impress me; your designs do. Our colleges are presently turning out thousands of, "real" engineers that couldn't design a clothes dryer without an Arduino in it, and they often struggle to do that because they don't know how to use a bjt as a switch or how to get a microcontroller chip to fire a triac!:mad:

/end rant
http://www.paulgraham.com/taste.html
Every designer needs to read and head this.
 

#12

Joined Nov 30, 2010
18,224
Every designer needs to read and head this.
A thousand words about aesthetics and not one word about BSEEs that have to ask AAC how to find the collector lead on a transistor? Or is it the, "not one word about financial liability" that I was supposed to notice?

Maybe I'm in the wrong Thread. Glad to hear about -2F in Alaska. If you're acclimated to that and the wind isn't blowing, it can by frankly beautiful to stand in the sun...hang on...this is December...to stand in the dark and admire the Aurora Borealis...Borrealis...light show in the sky.
 

dl324

Joined Mar 30, 2015
16,846
I know how it feels to 'like' a chip of a certain part number. Mine is probably the lowly LM358 that many people complain about. It is certainly not good for everything,
When I started in electronics 40 years ago, LM723 was state-of-the-art and I still use it from time to time; though I often prefer the convenience of a 3 terminal regulator. LM358 is my go to opamp because I always have them on hand. I have a small number of "favorite" opamps (LM308, LM318, LF356), but most of my circuits are fine with LM358 or LM324.
but i've been able to get it work in applications that everyone else would swear it would not work in. Even the crossover noise can be eliminated for example with a simple little trick.
with a constant load to the negative supply...

A true craftsman will have a will have a wide range of "tools" to select from and know how to use them.
 

MrAl

Joined Jun 17, 2014
11,396
Hi,

Yes, a constant resistive load to the most negative supply keeps the output biased into a non crossover type amplifier class...good of you to notice that as not everyone is aware of that, and there is a not very well known app note too which is seldom brought up in forums like this one.

I think i first encountered the 723 back around 1975 maybe. It's a good chip, but i forgot what the error amp basic bandwidth was as that could make a big difference on the response of a voltage regulator made with it. If it is fast by today's standards then i would use a 723 again. Maybe i'll look it up and throw a few circuits together with it provided the error amp unity gain bandwidth is good enough (better than 1MHz i hope).
I'll look it up or if someone else wants to and post that info here too.
 

#12

Joined Nov 30, 2010
18,224
Yes, a constant resistive load to the most negative supply keeps the output biased into a non crossover type amplifier class
You posted this information some time in the last month, and some of us have good memories.;)
 

dl324

Joined Mar 30, 2015
16,846
You posted this information some time in the last month, and some of us have good memories.;)
I didn't happen to see that post.

I was breadboarding a circuit recently, in my living room of all places, and didn't feel like going to my bench to get a "dual supply" opamp; so I used an LM358 with split supplies. When I was finished, I decided to experiment with it to see if I could see any crossover distortion. Even without the load to the negative supply, it wasn't noticeable (a spectrum analyzer is on my wish list).
 

MrAl

Joined Jun 17, 2014
11,396
Hi,

#12:
Oh ok, ha ha.

dl324:
You can hear it though as it sounds like a slight 'buzzing' sound with headphones. It goes away with the right load however. Some people REALLY dont like that crossover distortion and dont know you can get rid of it because the technique never appeared on the data sheet.
 

#12

Joined Nov 30, 2010
18,224
I've just about read every application note and linear brief in my copy of National Semiconductor's Linear Applications book. Same for Signetics...
That's what I did instead of, "formal" schooling. After understanding every bit of about 50 app notes/Linear Briefs, you just about have the equivalent of taking EE 101. If you want to thank someone, @ramancini8 is a member here.
 

MrAl

Joined Jun 17, 2014
11,396
Hi,

Why thank him?

I have the old NS Linear book, from 1980 ha ha. Some cool stuff though.
Nice to hear other people enjoy that stuff, i was beginning to think i was the only one left :)
 
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