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Topic starter
2023-02-20 6:06 am
I started to build a linear power supply by searching my junk for a transformer. I found the 24VDC 4.8A supply in the attached photos. The 14 pin IC on the circuit board is a LM723CN voltage regulator. Bolted to the chassis is a U30D20A "superfast rectifier" and a pair of MJ10012 Darlington transistors. Is this enough info for someone to interpret the device?
It seems too nice to scavenge for the transformer. But I do not know the best way to turn it into a variable supply. I think I could install an LD1084V, some caps, a resistor and a pot downstream but I wonder if there is a more elegant way to use the circuit that already exists.
Or maybe I am too impressed by the large cap.
2023-02-20 2:35 pm
Hi @cecil,
Others may have more expertise, but I have just had a quick Google to see what I can find.
I would normally associate the 723 voltage regulator with a linear supply, though the data sheet I looked at mentioned it could also be used in as a switching regulator, but I think this is unlikely. So to answer the headline question, you probably do have a linear supply.
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Looking at the MJ10012 Darlington's data sheet at https://www.nteinc.com/specs/original/MJ10012.pdf, I note they have a surprisingly high voltage rating (400V) for a 24Vdc supply, but perhaps they were the devices closest to hand at the time. 🤨
The 723 can only output a low current on its own, so I am guessing that its output is the input to the Darlingtons in an emitter follower fashion.
If so, to change the voltage you would need to adjust the 723 voltage to something like:
(desired output voltage + Darlington Base-emitter voltage drop)
..... where the data sheet suggested voltage drop might be 2.5-3V.
Of course you would need to verify/modify these wild guesses with some measurements and circuit tracing...
Looking at the 723 data sheet, changing the output voltage over a range (of more than 5% variation), meant changing the values of R1 and R2 ... you may wish to study the data sheet to see if it is amenable to using a potentiometer ... sorry I haven't read it through carefully enough, but the description complexity suggests it might be a bit tricky.
I would beware of centring your entire strategy on the basis of the electrolytic capacitor. It may be perfectly fine, but they do have an unfortunate habit of 'drying out' and losing their capacity with age. It might need replacing, or not smoothing the supply as well as it used to.
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In summary, it probably is possible to cajoule the existing circuit into providing a variable voltage output ... whether it is worth the bother and do you have the knowledge to do it, are both questions only you can answer.
If I were in your position, I doubt if I would bother...
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That leaves the other possibility, which is to add a post regulator ... you mention LD1084V, for which, a quick glance at the datasheet looked promising.
I would query, but not dismiss, your desire to have a linear regulator ... because there are applications in which the lower noise output is beneficial. However, they can produce a lot of heat which must be disposed of. For example, assuming the present supply is used to provide 24V to a linear regulator, and the output of the linear regulator is set 5V with a load current demand of 3A.
Then the voltage regulator would need to dissipate (24 - 5) * 3 = 57 W
Switchmode regulator modules are presently available at very low cost ...They typically have an onboard potentiometer to preset the voltage, and in some cases also a current limit ... but it shouldn't be too hard to replace the onboard potentiometers with some panel mounted ones.
To help you design your power supply, in case you haven't seen them, I would reccomend looking at some of
Bill @dronebot-workshop 's excellent videos + description on the power supply subject, including linear and switchmode options e.g.
https://dronebotworkshop.com/linear-dc-power/
https://dronebotworkshop.com/atx-bench-supply/
https://dronebotworkshop.com/simple-supply/
https://dronebotworkshop.com/powering-your-projects/
Hopefully, you can find some good ideas from these examples.
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Good luck with your project and best wishes, Dave
2023-02-20 2:42 pm
Yes that is a linear supply. The big transformer is the dead giveaway. I'm not the one to give you advise on turning this into a variable output power supply though. Good luck!
SteveG
Topic starter
2023-02-20 7:51 pm
Thank you for your thoughtful answer. I don't have the knowledge to proceed with anything but undeserved confidence. If there was a real need I might hack away but since this supply already has a purpose (powering the exhaust fans at my welding bench) I think I'll keep modifications to a minimum.
I have a buck board that will run from this supply and provide all the variability I need--which is little and seldom. That will reduce the project to sheet metal and panel machining work where I am fully confident.
I had read through the power supply construction articles on the dronebotworkshop website. I chuckled about converting a PC PSU--I did that more than 30 years ago, before the ATX standard so I only got 5 and 12V positive rails, but that box still serves me to this day. No trouble from the caps in all these years.
But I will check the big cap in my 24V supply before pressing it into service. It is easy enough since the connections are bolted.
Again, I very much apprediate your insight.
DaveE reacted