Trying my hand at building a PSU - Page 1
Forum section: electronics / beginners

author: antolanca
author_rank: Contributor
timestamp: June 24, 2020, 12:34:06 pm
content: Hi everyone. I'm going to try my hand at building a PSU from my old computer PSU. However, I don't want to be fiddling about with cables and different connections or completely modifying a perfectly usable PSU like many examples online so I designed a circuit that allows me to choose what voltage and how many amps by flicking some switches (12V, 5V and 3.3V with the choice of 1A, 2A and 3A).Now I know there are other alternatives out there such as the LM317 which I can use to limit voltage or current but I'd like to just work on those voltages so regulation or adjustment isn't important for me as the PSU has already done it for me.Also, this would be mainly for my Raspberry Pi projects and for board repair learning.Below you will find all the files I've made for planning plus the parts list. The power tracks are 0.6mm wide and the rest are 0.3mm.The circuit does work when simulated in Yenka (or Crocodile Clips if you're old school) but I'd like to know the opinion of someone that understands things better than me plus recommendations on better parts.Also, can anyone recommend a switch that can deal with a max of 3A (preferably more) that can switch between the 3 voltages such as a rotary switch please?Thank you in advance and keep tinkering.Parts List:LSR-120/20 - UNBRANDED Cement Resistor, Audio, 20W, 12RPELR0240 - PRO ELEC Switch, Rocker, 12V, RedW22 5R JI - TT ELECTRONICS / WELWYN Resistor, Wirewound 7W 5% 5RMCKNP05SJ033JAA9 - MULTICOMP PRO Resistor, Wirewound, 3R3, 5%, 5WsPPW01080 - PRO POWER Switch, Mini Rocker, SPSTMultimeter display to wire in:https://www.amazon.co.uk/ARCELI-Digital-Multimeter-Display-Voltmeter/dp/B07J2Q47LW/ref=sr_1_14?dchild=1&keywords=multimeter+display&qid=1593001512&s=diy&sr=1-14

author: MosherIV
author_rank: Super Contributor
timestamp: June 24, 2020, 09:34:54 pm
content: Your design will not give what you think.Using resistors as current limit means that the voltage will vary depending on the load.In general using ATX psu as a bench power supply is not a good idea.They are designed to power computers. They have no (useful) current limit for a bench psu, they will just supply max rated current, shut down or blow up.They are also smps, meaning that they can be noisey, especially for analogue circuits eg op-amp circuits.

author: wizard69
author_rank: Super Contributor
timestamp: June 24, 2020, 10:00:51 pm
content: As already mentioned the design will not give you what you seem to expect.For simple usage, at fixed voltages, I would suggest using 3 terminal regulators in either switch mode or linear. Those are simple devices and most have short circuit protection. They would take care of the 3.3 and 5 VDC supplies when connected to a higher voltage supply.Quote from: antolanca on June 24, 2020, 12:34:06 pmWhile this might turn heads the wrong way, I don't see current control as mandatory for a entry level power supply. You could use fuses if you are extremely concerned or simply choose a 3 terminal regulator that limits current to an acceptable level.QuoteI have no idea what Yenka is and in this case don't care. The resistors you are adding inline create a voltage divider so you have no idea what the circuit you will be hooking up to will actually see voltage wise.QuoteThere are literally hundreds of switches in stock at distributors. Which one is best really depends on many factors specific to your project.IN any event I have nothing against a fixed voltage power supply. That is in fact how I got started in electronics. Such a supply though needs to regulate the voltage to whatever is being powered, this circuit does nothing of the kind.

author: antolanca
author_rank: Contributor
timestamp: June 25, 2020, 12:19:05 pm
content: Thank you for your replies.QuoteI don't understand. How are the resistors acting as a voltage divider if they have different outputs that only join after the resistors? The 12V circuit per example has a maximum of 18A input with 1 resistor that goes straight to output with 12R giving 1A output then 2 more in parallel with a switch on each. So I flick the switch and each resistor will add 1A giving the maximum total output of 12V 3A.And a voltage divider has the resistors in series and their output between R1 and R2 and before R3.Please do advise on this as it would be of great help to me in the future.QuoteQuoteBut I do need to regulate the amps as if I need to find a short circuit on a board, I'll need to use voltage injection with a limit for the current so I can look for a heat source. Or when I need to power a circuit and I want to limit the amps being used hence the 1A ,2A, 3A scale since the voltage has already been adjusted.So it would be best if I use 2xLM338 in series to regulate the voltage and the amps since it can handle 5A maximum with a current limiting resistor (so it doesn't go above 3A) and use 2 potentiometers to adjust it? If I do it like this would I still be able to use my PSU or would I need to build everything from scratch?

author: tunk
author_rank: Super Contributor
timestamp: June 25, 2020, 01:30:46 pm
content: Disclaimer:I have no formal background in electronics and not much experience.What will happen if you set it to 12V/1A and your device draws 0.5A?Also:- the power resistors will not fit on your PCB- the LED will draw ~60mA- are you sure the PCB traces can handle 3AEdit: I don't know the answer to this: What will happen if you(accidently) switch on 2 or 3 of the 3.3/5/12V switches?

author: kallek
author_rank: Regular Contributor
timestamp: June 25, 2020, 05:41:08 pm
content: If I understand your current circuit correct, your current-limiting resistor and load resistance together acts as voltage divider. So voltage drop when load increases.If you want very simple current limited voltage regulator circuit, watching this RSD Academy video series would be good start. Simple but effective.

author: antolanca
author_rank: Contributor
timestamp: June 26, 2020, 08:47:19 pm
content: You just given me a treasure chest with that RSD Academy video. I am going to do their course and follow their linear power supply instructions. Thank you so much. No one does electronics courses around where I live in the UK.