LM723 first attempt - Page 1
Forum section: electronics / beginners

author: davelectronic
author_rank: Frequent Contributor
timestamp: December 16, 2020, 03:44:10 pm
content: Having built various 3 pin voltage regulator power supplys, I was thinking of giving the LM723 voltage regulator a try. I've found a 3 Amp version with a single pass transistor. Sometimes you find circuits doing searches that aren't always very good, I have looked at the Texas instruments data sheet, but didn't find a circuit that is adjustable voltage with a potentiometer. The circuit I would try is in the image below, I'm just asking if this example looks right before trying this circuit. I have come across other LM723 circuits of greater power output ratings, but for a first attempt wanted to keep it reasonably simple. If I manage a good result from this LM723 circuit, I would attempt a second build, but of greater power. Do you think this circuit looks right in this schematic.Thanks for reading, any thoughts appreciated.

author: Wimberleytech
author_rank: Super Contributor
timestamp: December 16, 2020, 05:32:14 pm
content: The 723 comes in three package styles. Pin numbers are different in each. It would help us to help you if you would label the pins.Since this is your first attempt, eliminate R3 and set R4 to 0.Drawing schematics with four-way connections is a bad practice IMO.

author: davelectronic
author_rank: Frequent Contributor
timestamp: December 16, 2020, 07:38:47 pm
content: I didn't know its availability was as spars as that, i found a couple of credible sources. Its the LM723CN. It was available from a UK supplier i often used, but it looks like they have dropped selling it. Hopefully this is the regulator IC i need.

author: Wimberleytech
author_rank: Super Contributor
timestamp: December 16, 2020, 07:50:07 pm
content: Quote from: davelectronic on December 16, 2020, 07:38:47 pmYes, that is the package consistent with your your schematic.

author: srb1954
author_rank: Super Contributor
timestamp: December 16, 2020, 09:38:20 pm
content: Quote from: Wimberleytech on December 16, 2020, 05:32:14 pmI wouldn't alter R3 and R4 as these provide a measure of fold-back current limiting to protect the series pass transistor if the power supply output is shorted.Without the fold-back current limiting and in the event of a short circuit the transistor safe operating area (SOAR) could be exceeded causing failure of the transistor. However, in the absence of any datasheets for the MJ3001 showing a SOAR area diagram, it is not possible to determine exactly how much SOAR protection is required.Quite apart from the SOAR issues the fold-back current limiting also eases the heatsinking requirements for the transistor. Without this the heatsink requirements would be very difficult to achieve for thermally protecting the transistor in short-circuit conditions.

author: davelectronic
author_rank: Frequent Contributor
timestamp: December 16, 2020, 11:49:26 pm
content: Thank you for your replies, I was going to add a crowbar circuit I found from an amateur radio users site. But good to know the circuit is achievable, I would like to build a 30 Amp plus at 13.80 Volts, but I would sooner try something more modest before embarking on a big linear power supply circuit. Up until now I've only built 3 terminal regulator power supplys. They work fine, I just wanted to try something more challenging.

author: Yansi
author_rank: Super Contributor
timestamp: December 17, 2020, 01:19:27 am
content: Quote from: Wimberleytech on December 16, 2020, 05:32:14 pmIt depends. In EU, this is pretty normal to do. (in fact, this is an official normalised way to draw schematics). But in the US, it is I guess not, as you have not used the junction dot and crossed wires with little arcs.To the OP: The schematic looks about okay to me. (Maybe I'd move the anode of D2 to the +V output node directly. Also I'd add an anti-parallel diode across the output terminals!)

author: Wimberleytech
author_rank: Super Contributor
timestamp: December 17, 2020, 01:24:15 am
content: Quote from: Yansi on December 17, 2020, 01:19:27 amHere is a question to ponder. Why does Europe use a comma to separate the fractional part?

author: davelectronic
author_rank: Frequent Contributor
timestamp: December 17, 2020, 02:43:45 pm
content: I'm not sure what is meant by four way connections, I see a positive rail and a negative rail. And interconnected components between the IC and the two voltage rails. I'm sure I could follow the schematic to lay the circuit out on copper stripboard. Why would the anode of D2 be better connected to + Volts rail directly ? I chose it for a first attempt as it looks doable at my skill level. There are higher current circuits I would really like to do, and skip this one. But I'd be more confident if I managed the circuit above first.

author: Yansi
author_rank: Super Contributor
timestamp: December 17, 2020, 03:08:35 pm
content: I understand he means to not connect together 4 wires in one junction point in the schematic. As obviously, you will offend some Americans, same way they offend us Europeans (or those following the DIN standards or similar) with their strange looking wiggly line resistor symbols.On a more serious note, this is likely just a matter of taste or about what standard you need to follow (if you have for example such set by your employer or even yourself). I am fine with 4 wires in one junction in a schematic and never had a problem telling apart which is just a wire crossing a 4way junction. Even looking at some very old shitprint schematics. (A common argument, why not to do so). And this is the way our nation has drawn schematics for many decades.Why we use commas as a decimal "point" and space as a thousand separator, I don't know. But all this and above is a topic for a different thread. Now back to the 723 power supply.

author: Wimberleytech
author_rank: Super Contributor
timestamp: December 17, 2020, 03:47:52 pm
content: Quote from: Yansi on December 17, 2020, 03:08:35 pmGeez Yansi! This is not about borders. I have reviewed tens of thousands of schematics over my career and have observed the four-way error too many times to count. If you have any training in engineering, you can understand the concept of minimizing error. When two wires cross and a dot is used to indicate the connection, many times the dot is forgotten or lost in the quality of the copy (e.g., photocopy). If, instead, you offset the connections THERE IS NO AMBIGUITY! That is why it is a superior method for drawing schematics regardless of your native language. See the diagram attached.QuoteShitprint schematics are precisely why dots should not be used to indicate a shorted four-way.QuoteThe comma separator is a superior way to separate the fractional part of a number. Here is why. The number 3.124 (three and 124/thousands) has a dramatic change if the "dot" is lost due to handwriting or poor copy. The number 3,124 (three thousand, one hundred and twenty four) does not change in value if the comma is lost. This is why the European system is better than the American system. See there buddy...it is not about your native language but using your brain.

author: Yansi
author_rank: Super Contributor
timestamp: December 17, 2020, 03:53:04 pm
content: Quote from: Wimberleytech on December 17, 2020, 03:47:52 pmI've said enough with this offtopic!

author: Wimberleytech
author_rank: Super Contributor
timestamp: December 17, 2020, 04:40:34 pm
content: Quote from: Yansi on December 17, 2020, 03:53:04 pmSuffer me this bud...see what Dave says:https://youtu.be/R_Ud-FxUw0g?t=4117

author: davelectronic
author_rank: Frequent Contributor
timestamp: December 18, 2020, 03:03:41 pm
content: Thank you for clarifying the 4 way connections in schematics. I watched the video from the link you posted, I hadn't realised the LM723 is considered obsolete. At least from a UK supplies, I can still find a reputable small electronics supplier in the UK. I don't think I would go for eBay purchase of the IC. I've been bitten once with fake LM337K regulators from the far East. Might buy a few, as I can see the IC becoming harder to source in the future.

author: HB9EVI
author_rank: Frequent Contributor
timestamp: December 18, 2020, 04:36:59 pm
content: the LM723 is still available as new e.g. over Mouser, but for quite a price and in the metal TO canit's still a very good device which allows real low noise regulation.

author: Ian.M
author_rank: Super Contributor
timestamp: December 18, 2020, 05:23:18 pm
content: Fairchild's original µA723 was extensively second sourced by National Semiconductor as the LM723, Motorola as the MC1723CP, Texas Instruments as the µA723 and various others.T.I. still has it in current production, including in the PDIP package:https://www.ti.com/product/UA723so you shouldn't have any problem finding it under that part no. at major distributors.

author: HB9EVI
author_rank: Frequent Contributor
timestamp: December 18, 2020, 06:28:52 pm
content: perfect! thanks for the hint; if you search for LM723 you just find the expensive metal can version

author: Wimberleytech
author_rank: Super Contributor
timestamp: December 18, 2020, 09:01:12 pm
content: I thought I had some in my ancient stock (this from the mid 70's). This is what I found.I should build a supply just for grins.

author: davelectronic
author_rank: Frequent Contributor
timestamp: December 20, 2020, 03:36:45 pm
content: The uA723 is the same as the LM723CN ? It looks like it is. The best price in the UK I can find it for is £2.00 per IC, bit expensive, but it's a good source. eBay have got them, but it's a gamble whether it's a fake or not.

author: HB9EVI
author_rank: Frequent Contributor
timestamp: December 20, 2020, 03:53:06 pm
content: Mouser has them for under 1 buck; uA723 and LM723 are the same. I'd waste my money with chips of unknown origin and quality for sellers on ebay or ali if I can get genuine articles on Mouser or Digikey.after getting more and more fake chips, if the order was delivered at all, I entirely stopped ordering such articles on the china market; it's not worth the pain

author: Wimberleytech
author_rank: Super Contributor
timestamp: December 20, 2020, 03:58:15 pm
content: Quote from: davelectronic on December 20, 2020, 03:36:45 pmuA was the prefix used by Fairchild Semiconductor. LM used by National Semiconductor (now TI). They are the same part. The "C" designation is for temperature range. Offhand, I don't recall what the "N" designation means. Doesn't matter for you. Any will work.

author: HB9EVI
author_rank: Frequent Contributor
timestamp: December 20, 2020, 04:03:29 pm
content: N is the mark for many IC in the DIP package; the 74-series is N too for DIP

author: BravoV
author_rank: Super Contributor
timestamp: December 20, 2020, 04:07:06 pm
content: Talking about 723, there is member here who butchered many-many of it's variants ->723 slaughter house