Need help with design for diy lab psu from parts lying around - Page 1
Forum section: electronics / beginners

author: DocRaptor
author_rank: Contributor
timestamp: August 04, 2023, 03:32:48 pm
content: Hello everyone.I want to build a linear lab psu.I already have a adjustable switching lab psu, but it is a really cheap one with four potentionmeter.The model is called KPS305D.Adjusting current and voltage is not very nice.So I want as mentioned build a lab psu as an extra one and I want it to be a linear one.I also want it to be controlled over a keypad and not a potentiometer.And I want to use mainly parts I have laying around.I have ton of LM324 opamps, TIP142 and BC547 Transistors, LM7805 5V voltage regulator and bunch of resistors and caps (mainly because one of my hobbies are old computer and game consoles, so I have some spare parts to fix them).I also have a transformer lying around (It is from an old bell-system as a spare part).And a couple of Arduino Megas (from the time, when 3D Printer used Arduino Megas with RAMPS-shields on it) to make the digital part possible.I looked at some schematics, but most of them are also using a negative voltage.My transformer is only delivering a single voltage and I want to use a voltage doubler, so negative voltage is not realy available.So in the end I tried to make my own design based on some other schematics and from what I read and this came out.But I don't know if it would work.For information: Im not an electrical engineer, I'm a computer science student (from germany).So there can and will be errors.My question is: would this circuit in some way work?How far would it work?1: totaly fine, only set voltage and current once and it will hold it.2: somewhat, MCU needs to monitor and adjust it sometimes.or3: absolutly not, maybe only magic smoke comes out.

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: August 04, 2023, 05:28:49 pm
content: Back in 1976 I built a linear bench supply from the magazine Electronics Today International. It’s still in regular use today and works perfectly.The project is ETI 131 and there is a couple of zip files of the article here:http://www.thebackshed.com/forum/ViewTopic_mobile.php?FID=4&TID=4422&PID=0&LastEntry=The supply is basically a 0-20 or 0-40 Volt, 0-2 Amp linear supply that goes down to close to zero Volts and current.Sorry it’s not keypad controlled…SJ

author: themadhippy
author_rank: Super Contributor
timestamp: August 04, 2023, 05:49:14 pm
content: Might be worth a look at the first few videos dave did on designing a bench power supply ,.It covers the basics including how to use a pwm signal to control the output.

author: Kim Christensen
author_rank: Super Contributor
timestamp: August 04, 2023, 05:58:11 pm
content: Well, a couple of really obvious flaws:R11-R14 are all in parallel meaning they are effectively 2K5... Both the transistors (Q5&Q6) have 1K resistors (R1&2) on their collectors, so when either are fully turn on (Trying to limit current or voltage) they form a voltage divider with that "2K5" resistor. So, if Vcc was 20V, then the lowest voltage you'll ever see on the bases of Q1-Q4 is 5.7V. Perhaps those two 1k resistors (R1&2) should be in the base leads of the transistors instead.Your voltage and current feedback paths arepositiveinstead of negative feedback!Instead of pullup resistors (R11-R14) on the bases of Q1-Q4, you should be using a constant current source so that the output voltage can go higher under load.I haven't looked at the transient and frequency stability of the overall circuit.

author: Eraldo
author_rank: Regular Contributor
timestamp: August 05, 2023, 09:33:23 pm
content: So you ant to make it digitally controllable with arduino? That wouldn't be possible without an external adc and dac (the stock ones are 10 bit for the adc if I'm not mistaken and 8bit for the dac). Making a digital psu is too much of a hassle so it's easier for a fully analog one with multiturn potentiometers. There is a thread in eevblog discussing this topic:https://www.eevblog.com/forum/beginners/lm324-power-supply-with-variable-voltage-and-current/msg4973776/#msg4973776

author: BillyO
author_rank: Super Contributor
timestamp: August 05, 2023, 11:20:49 pm
content: The ATMega328 has built in ADC capability that is probably just fine for the purpose. The DAC situation is different though. Technically you could use PWM but that would be adding a switching layer so you'd probably need to create or add a DAC. Using 74HC595s with R2R ladders would do a fine job.

author: xavier60
author_rank: Super Contributor
timestamp: August 06, 2023, 01:32:51 am
content: The TIP42 can still be used in a Sziklai pair. The CV compensation might need some tweaking.https://www.eevblog.com/forum/projects/linear-lab-power-supply/msg2028295/#msg2028295https://en.wikipedia.org/wiki/Sziklai_pair

author: Kleinstein
author_rank: Super Contributor
timestamp: August 06, 2023, 07:47:14 am
content: A transformer from an old bell system tends to be relatively small. So I somewhat doubt one would need multiple power transistors in parallel.The solution with the transistors Q5/Q6 to combine the 2 signals adds nonlinear gain to the loop and makes the compensation / choice of BW a bit tricky. The capacitors C4 and C5 only work if there is an extra resistor in front of the input to the amplifier stage. Chances are one would want a bit large capacitors, especially with the relatively slow LM324.It helps to reduce the number of amplifiers in the loop, so the buffer U1D is better skiped.The circuit as shown does not even try to compensate for the drop at the shunt R7.For the set votlages PWM and filter should be OK. This is kind of a seprate parts from the regulator part.

author: MathWizard
author_rank: Super Contributor
timestamp: August 06, 2023, 07:17:13 pm
content: What sort of voltage and current can your transformer deliver ? Reducing the voltages to the ones you need, gets pretty annoying sometimes, especially if just using random parts. Pay close attention to the max power rating and safe temperatures of things.With an old stereo transformer, I made a linear 0-20V, 1A, CV/CC PSU with 4xTIP41 or maybe TIP31. I'm sure that transformer could do a lot more current too. That used another winding or transformer, for control voltages, and another winding or transformer for powering the digital V-A meter.In the end it worked pretty good. It probably could be hooked up to an MCU without much modification. If I had perfect 8-bit control, that would be like 78mV /step.

author: DocRaptor
author_rank: Contributor
timestamp: August 07, 2023, 06:30:37 pm
content: Quote from: MathWizard on August 06, 2023, 07:17:13 pmUnfortunately I can not tell what it can deliver. I had been looking for the transform on the weekend it is not in the basement anymore. I don't know why. Very likely my father has thrown it away a couple weeks ago. So now I have to look for a transformer. I will look around my partbins, maybe I have something to make constant current source like Kim Christensen recommended. Or better Op-Amps.With the posetive and negative feedback. Isn't it to get negative feedback op-amp you connect the output with inverting output?

author: Kim Christensen
author_rank: Super Contributor
timestamp: August 07, 2023, 07:52:14 pm
content: Quote from: DocRaptor on August 07, 2023, 06:30:37 pmYes, but you need to think about the entire feedback path.If you look at the voltage feedback path, you'll see that you've got 4 "amplifier" stages in series. Two are inverting and the other two are non-inverting. Therefore it's a positive feedback path (non-inverting) overall.

author: xavier60
author_rank: Super Contributor
timestamp: August 07, 2023, 09:00:30 pm
content: It will be easier to drive a PNP Sziklai pair.

author: DocRaptor
author_rank: Contributor
timestamp: August 12, 2023, 05:55:03 pm
content: I looked in some of my parts-bins and found some mpsa56 PNP-transistors.I look around how to make constant current source.It should make around 8mA (I read (and saw) that some PSU using four TIP142 have a constant current source delivering 8mA.For example here is someone building a linear psu and his constant current source is delivering 8mA youtube.com/watch?v=_CFIovMkRyg&t=1296s)I hope I looked up the right formular and values for calculating the needed values:(2.5--4)/820=0.007926(-4 is the emitter-base-voltage from this datasheet:https://www.mouser.com/datasheet/2/115/mpsa56-1164826.pdf.And 2.5 is just arbitrarily set by me, because two times the same resistor is simpler)

author: Kim Christensen
author_rank: Super Contributor
timestamp: August 12, 2023, 06:40:00 pm
content: The current source needs to connect to the higher voltage supply (VCC) and not 5V and you need to eliminate R11. Plus, you'd build it as shown below so that it's capable of less voltage drop across it when needed. You can use either circuit, though I prefer the 2 transistor version myself, but either will work fine: