Is this variable PSU design any good? - Page 1
Forum section: electronics / beginners

author: McClane
author_rank: Newbie
timestamp: March 02, 2023, 07:28:14 pm
content: Hi everyone,I was looking for a fun tinkering project and stumbled across this variable power supply design:https://www.eleccircuit.com/cheap-adjustable-0-30v-3a-laboratory-dc-power-supply/My main goal is to learn some KiCad, soldering and electronics in general on the way and not to build the greatest power supply ever. Do you think this design serves this purpose or is this a total waste of time, or worse, maybe even dangerous (provided one is careful and knows some basic electronics)?To me, the circuit seems pretty simple for a variable PSU. What might be some disadvantages compared to more complex or commercial variable PSUs?Thanks for some enlightenment!

author: coromonadalix
author_rank: Super Contributor
timestamp: March 02, 2023, 08:11:42 pm
content: you get variable voltage, no current variable, just a max current limit / protection / overloadpls note lm723 based psu by design doesn't get to zero volts, around 1.2 volts, you need to add more circuitry to null this voltage

author: mariush
author_rank: Super Contributor
timestamp: March 02, 2023, 08:24:08 pm
content: Have a look at the attached power supply article, it's something you could build on your own.Also attaching the 18v 2A schematic - you can make it only with the bottom half, the top half with the two ICL7106 meter chips is just to display the voltage and current on lcd screens.This one requires either a transformer with more windings or 1 big transformer and 1-2 smaller transformersThe schematic has 2 big secondary windings (to allow switching between low voltage and two voltage and reduce losses on the transistor, one low current winding to get 12v for the relay that switches those two windings and 2 low current windings to get +8v, +5v and -5v for LM358values are in that notation ... ex 472 is 47 x 10^2 = 4700, 103 = 10k potentiometer/resistor , 511= 51 x 10^1 = 510 ohm ..

author: McClane
author_rank: Newbie
timestamp: March 02, 2023, 09:30:48 pm
content: Quote from: coromonadalix on March 02, 2023, 08:11:42 pmThanks for pointing that out!That would be a disadvantage which I was not aware of.Quote from: mariush on March 02, 2023, 08:24:08 pmWow, thanks for the article!That’s probably the better alternative compared to the one I found.Does the 18V 2A have any advantages over the other one? If not, I think it would be quite nice to have the 24V, so I’d probably prefer the one from your article.

author: Zenith
author_rank: Frequent Contributor
timestamp: March 03, 2023, 03:36:02 pm
content: If the intention is to create a practical unit you are going to use routinely, something you might factor in is the cost of the case, meters and heatsinks. That can be surprisingly expensive. Of course if you already have these to hand, that's different.When I went this route, years ago, I eventually just bought a commercial PSU, which I still use.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 03, 2023, 07:09:17 pm
content: There are 2 main concepts for a power supply.One is like the initial post with the LM723 based one: Essentially a single supply and a transistor as emitter follower behind an amplifier. This version can be simple but in the simple form is limited to the votlage range of the amplifier. This makes this version tricky for more than some 25-30 V and also adds a bit to the voltage drop.The 2nd is like in the 18 V example with an extra supply for the regulator and often 2 transformers (or a special one with more secondaries). The main power is from a separate transformer.This version can be quite universial - up to the point of using the same PCB for different combinations like 5 V 10 A or 100 V 0,5 A with just exchanging a few parts and resistor values.Many commercial linear lab supplies are of this type. The linkes plan looks a bit the plan for a commercial one - though in my oppinion not a very good one.A good lab supply can be a surprisingly complex project. The more basic part is getting the power / current ratings right. The tricky part is getting the compensation for a fast and still stable control loop that can handle different loads over a large range. For some reason there are lots of poor plan out in the internet - like beginners solutions that kind of work OK for well behaved loads and likely fail with the more tricky ones. Ideally there is quite some math behind the design and one would use quite some spice simulations to get details, like a CV CC cross over with little overshoot right.Price wise the commercial supplies from China are hard to beat with a DIY solution. They however tend to be a bit optimistic with the maximum current and may run a bit hot and may fail a bit early.One may get away OK with a DIY solution if one can resue some parts, like a case, transformer and heat sinks from an old broken Hifi amplifier.

author: rstofer
author_rank: Super Contributor
timestamp: March 05, 2023, 09:39:35 pm
content: When you think about the 2N3055 and its heatsink, your problem is not Vmax at Imax it's Vmin (say 1V) at Imax where the transistor has to dissipate the maximum heat. Suppose the input voltage is 40V and the output is just 1V at 3A. The power dissipation will be 39V * 3A or 117W. The solutions are parallel transistors or tapped transformers. On commercial supplies you can hear the tap selection relay(s) clicking as you change voltage.

author: McClane
author_rank: Newbie
timestamp: March 09, 2023, 07:11:51 pm
content: Thanks for all the helpful replies!I’ll see how far I get and will come back if I have more questions (or with the result).

author: MathWizard
author_rank: Super Contributor
timestamp: March 15, 2023, 03:02:16 pm
content: I made a 20V 1A CV-CC PSU with a transformer from a stereo, with Vcc on the pass transistors about 21-40V depending on load. I split the load between 4 TO-220's sized BJT's, on about a >1 pound heatsink, small finned. And with a 12V computer fan, it stays just about hot, at around 30-40W dissipation. IDK how much more that heatsink could do if I had the fan on max, but it would be a lot nosier.I have one heat-sink from that stereo receiver. I can't remember what the stereo was rated, but I can't see that HS taking 200W, maybe 3x my little 1 above. Sometime soon I'll have to test it and see.I'm glad they were all salved parts tho, heatsinks and transformers must cost a lot, just for shipping, never mind their own price.