Total newbie's first power supply - Page 1
Forum section: electronics / projects

author: technodruid
author_rank: Newbie
timestamp: December 22, 2019, 09:38:39 pm
content: Hi all, I've followed Dave for years now, and I've been reading up on the forum for a few weeks now trying to figure out a nice power supply design for myself.Given that I have never designed anything electronic in my life, there were a few moments where I looked at my design and said to myself "WTF did I do this for?" and many other things like that. BUT I think it's about ready to go.My design requirements were:-Linear-Two positive 1.25V-16V outputs with independent current/voltage adjustment-AC Mains voltage input (120VAC @60Hz)-Modular (each functional group would be build on their own boards, so I can re-design at a later time or re-purpose)-Beefy external heatsink to save me putting in a fan circuit.I'm pretty well ready to purchase my parts, but I'm still out on the fence about a transformer. Given I have ~4.5V drop from my LM317s and rectifiers on each rail. Any suggestions? And any pointers or recommendations about my current design?Thanks all!my power supply.pdf(45.83 kB - downloaded 200 times.)

author: H713
author_rank: Regular Contributor
timestamp: December 23, 2019, 12:47:32 am
content: Take a look at how most +/- power supplies are done- they use a center tapped transformer and a single bridge rectifier.Also, it's worth deciding what sort of voltage range you need. As an example, there's no use in having +/- 45V @ 5A if all you do is 5V logic, and if you're going to be servicing vintage Neve mixing consoles, you're going to need a +24V rail. The point is, what you do will dictate what kind of power supply you need.For me, I do a lot of audio work, and I like to be able to do at least +/- 18V, but +/- 24V is extremely useful. Also, current limiting is really nice so that if you get the plus and minus rails reversed you have a small chance of not completely destroying every op amp and transistor in a channel strip.That said, I assume you know what you need. If you don't plan to load the power supply down at the full 1.5A for long periods of time, this will work fine. Just connect the two secondaries in series to get a center tap (make sure you get the phasing correct!) and you should be good to go.https://www.antekinc.com/as-0518-50va-18v-transformer/If you DO plan to load it down at maximum power for long periods of time, either put a fan on the 50 VA transformer or go for a 100VA transformer.I really like the Antek transformers. Great quality and price.

author: mariush
author_rank: Super Contributor
timestamp: December 23, 2019, 01:29:53 am
content: I've posted a few schematics in the past on this forum. I'll add them again as attachments to this post.Here's an example of a simple 18v 2A max linear power supply. Other linear power supplies use similar designs.The below one in the pictures uses a transformer with two secondary windings to switch between one winding or two windings... let's say 10v AC and 20v AC ... so you'd have 12v DC / 24v DC peak voltages after the bridge rectifier and bulk capacitor.... then the pass transistor gets you up to 18vThere's a separate winding that creates 12v for the relay, but if your design has no relay you won't need it.There's two low current windings for +8v for the LM358 and to get +/-5v two separate windings Also as you see below, they use two separate windings for +/-5v for the lcd displays that show the voltage and current (not shown in pictures) and to power LM358.

author: Kawakneurder
author_rank: Contributor
timestamp: December 23, 2019, 02:07:40 am
content: Quote from: technodruid on December 22, 2019, 09:38:39 pmFirst of all, nice and clear circuit diagram.I have a question, Do you have any design requirement about output current? Or are you going up to the maximum output current of the LM317 (1.5A not considering power dissipation)?My biggest gripe with your current circuit is the potentiometer in the current circuit. The LM317 works such, that it always tries to get 1.25V between the adjust and output pin (i.e. it drives the output to get 1.25V drop towards the adjust pin). This means that there is always 1.25V voltage drop over that potentiometer. Power is calculated as P = U * I, so with the constant 1.25V, at 1.5A, it would give a nice and toasty 1.875W of power in the resistor! Potentiometers are usually limited to a couple of hundred milliwatts of power, and it is usually not a good idea to run any substantial current through them. I have seen this particular circuit floating around on the internet before, but I do not think it is a very good one. This circuit would be okay though, I think up to 80 or so mA.The problem is, having a current limit requires a bit more complex circuitry. Here is an example:https://www.circuitsonline.net/downloads/schakelingen/lm317_labvoeding_v2_b.pdfNote that the P1 current adjustment potentiometer is not in the direct power path.Anyways, your output voltage adjustment section seems quite good. The only remark I have is that it might be hard to find 3K potentiometers. Generally standard values are 1, 2 and 5 in multiples of 10 (i.e. 1k, 2k, 5k, 10k etc).Don't be discouraged though! It is a lot better than my first circuit (and I am sure for a lot of others too!). If you have a breadboard, you can just try and build it and see what happens. Even if it fails in some way, try to figure out why it failed and improve on it. That is the best way to learn, and you will learn fast!

author: technodruid
author_rank: Newbie
timestamp: December 23, 2019, 02:23:45 am
content: Thanks for the transformers link! I do actually want two positive outputs, as some of the projects I want to do to start out with require +4, +5, and +12 volts.I take it with those transformers I can choose not to wire the secondaries together and keep them separate?

author: not1xor1
author_rank: Frequent Contributor
timestamp: December 24, 2019, 06:56:46 am
content: Quote from: technodruid on December 22, 2019, 09:38:39 pmDo not attach PDF files, but image files.That circuit is not 0-1A but 10mA to the maximum current of LM317 with the fringe benefit of roasted potentiometers (I hope you have a huge amount of 125pots)Just look around for LM317 datasheets. Some manufacturer provides the schematic for proper variable current limit and variable current/voltage PSU with multiple LM317.They have some drawback (like high dropout voltage) but may be suitable for a beginner.You can replace the diode pairs and JFET current sinks of the circuit with LM385 and resistors.If you are interested I can provide a full schematic (later).