My lm324 based power supply project - Page 1
Forum section: electronics / projects

author: endorfin35+
author_rank: Newbie
timestamp: August 09, 2022, 10:36:10 am
content: Hello to everyone,I wanted to design a power supply that I could control digitally if needed. Of course, this project had to consist of affordable and easily available parts. With a little work, I prepared the following design. I am an amateur who is an electronics hobbyist. I request you to check my schematic. Any of your feedback would be very helpful. Thank you.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 09, 2022, 01:36:29 pm
content: The 2nd rectifier part with the fixed lower voltage tap looks suspect. Chances are that when the relay is switched to the 30 V tap, the input to the 7812 would also see the rectified 30 V, at least for half waves.One usually can't use 2 rectifiers with different but connected AC level and than also connect a DC side. With only relatively low voltage, chances are one could get away with just half wave rectification.For the negative supply one may be able to use a chanrge pump off the transformer instead of the extra NE555. Limited size capacitors would lower the voltage, kind of like a capacitive dropper.As shown the low side sense part does not work well with the digital option. The low side sense part, when used externally should have some extra protection.Using external sense often also needs a somewhat slower compensation, as there can be unwanted phase shift or inductance that make the regulation with external sense more tricky.The sense circuit looks more like OK for inside the case (e.g. directly connect to the terminals), but not as external terminals.With the output stage with gain the compensation can be tricky. One should check this with a simulation.

author: LM21
author_rank: Regular Contributor
timestamp: August 09, 2022, 01:40:13 pm
content: LM324 is very old opamp, isn't there anything better or modern. DACs or digital analog converters with opamps are made for creating a voltage from binary data.

author: endorfin35+
author_rank: Newbie
timestamp: August 09, 2022, 04:15:51 pm
content: @Kleinstein ,First of all, thank you for your comments. I reviewed your findings.Quote from: Kleinstein on August 09, 2022, 01:36:29 pmI had a hard time understanding this. But when I tried it with simulation, I saw that it was really like you said. I only have 2x15 transformers and I want to use it. In this case I can use the 7812 with 30V but it will get hot. However, I am undecided on what to do about it.Quote from: Kleinstein on August 09, 2022, 01:36:29 pmI made some fixes in the low side detection part. My plan is to connect the ends to the connector of the case, I do not think external usage.Quote from: Kleinstein on August 09, 2022, 01:36:29 pmI actually tried the basic parts of the circuit on bread board. 3mV (p-p) ripple occurs in CV mode and slightly more ripple occurs in CC mode. I thkink its will work fine.------Quote from: LM21 on August 09, 2022, 01:40:13 pmYes. I know that. The lm324 is an old product. However, it is still fairly common and cheap. That's why I'm using the Lm324.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 09, 2022, 07:22:47 pm
content: For the transformer tap for the 7812, I forst though that half wave rectification could be an option, but I am afraid it does not wrok too. There could be the option to more the relay to behind the rectifier, but before the filter capacitors. So the relay would switch pulsed DC instead of AC, which should not be a big deal.The rectifier would be wired on the outer windings and the center winding would be half the voltage, like it is done for a +- supply. One would likely need an extra diode for the 7812 supply from there to isolate the filter capacitor.The main part with the simulation / testing is if the circuit would oscillate, which would be more on the 10-100 kHz range.Getting quite some ripple is unusual. Normally, if there is no ground loop at the filter caps, there should be essentially not ripple (e.g. << 1 mV), at least for the CV mode. The CC mode may show a little due to the relatively low voltage at the shunt.Another small note: the reverse diode for the outpue should be more like sized for the supply current, mainly in case a 2nd supply is used in series, that may be of similar size. So at least a 1N540x or P600.For internal use the sense concept look good enough.

author: endorfin35+
author_rank: Newbie
timestamp: August 09, 2022, 08:06:19 pm
content: I gave up on using the 7812. I made some adjustments to the Sense lines. I'll try again on the breadboard tomorrow. Then I write the Output values.

author: endorfin35+
author_rank: Newbie
timestamp: August 11, 2022, 07:49:50 pm
content: I'm still working on the circuit. I have done the power needs of the opamps separate with second transformer. Output ripple voltage dropped to 1mv. Voltage compensation works fine.The problem is the low side sense is not working properly. The feedback on the high side is easy, but on the low side where should sense connect? From which point is the negative feedback reference applied to this circuit?

author: cellularmitosis
author_rank: Unknown
timestamp: August 11, 2022, 08:33:23 pm
content: Quote from: endorfin35+ on August 11, 2022, 07:49:50 pmLooks like you just need to move it to the other side of the shunt.

author: cellularmitosis
author_rank: Unknown
timestamp: August 11, 2022, 08:35:30 pm
content: Like this

author: cellularmitosis
author_rank: Unknown
timestamp: August 11, 2022, 08:40:58 pm
content: oh wait, your current setpoint is based off of the negative rail, I'm not sure.

author: xavier60
author_rank: Super Contributor
timestamp: August 11, 2022, 10:17:32 pm
content: This similar design might provide some useful ideas,https://www.eevblog.com/forum/beginners/lm324-power-supply-with-variable-voltage-and-current/msg3582664/#msg3582664Any extra circuitry such as tap switching and digital control should be powered from an auxiliary supply referenced to the 0V output terminal.

author: profdc9
author_rank: Frequent Contributor
timestamp: August 11, 2022, 10:47:06 pm
content: I designed something similar that allows an external control of voltage or current.I did both a linear power supplyhttps://github.com/profdc9/LinearPSand a TL494 based power supplyhttps://github.com/profdc9/FlexibleChargerThe linear power supply uses several LM358 ICs, which is the same op-amp as the LM324 but with only two op-amps per IC.I intend it to be driven by an optoisolator with a PWM simple which can generate an isolated analog signal.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 12, 2022, 08:50:16 am
content: The circuit with the shunt at the low side is nice, when one has onely 1 transformer / supply voltage. If one has a 2nd transformer / isolated supply, one could as well use the classic floating regulator with the extra supply for the regulator, reference and if wanted a digital display.There is still one resistor (R30) going to the drive side of the low side, instead of the sense side.The NE555 charge pump can cause quite some ripply on the low votlage supply, that may result in ripple.At the relatively low ripply level like 1 mV it also needs care with the layout. A common mistake is to have a ground loop, so that the ripply current to the main filter capacitors couples to the DC voltage. So there should be separate line going in and out from the filter caps.

author: endorfin35+
author_rank: Newbie
timestamp: August 13, 2022, 03:55:57 pm
content: I decided to make the AC tap selector section optional with a separate card. This is the latest for now. Thanks for your supports.