Transistor only power suply - Page 1
Forum section: electronics / projects

author: Enrico
author_rank: Newbie
timestamp: May 26, 2019, 05:41:14 pm
content: Hi, i'm new to the form(but not new to eevblog).I've been in to electronics as a hobby for 5 years more or less, starting with arduino an then moving back to the bare bone analog stuff. Now i'm in need of a bench power supply for my rather small lab. I was thinking of a simple linear one. I've already got from an old gate control board a 24v main transformer that i want to use as my power suorce. Now i found this little schematics that i tested with some savaged components and it works great, only thing i miss now is a current limiting mode. I already have a R020 resistor that i'd like to use as shunt but i can still make my own from some nicrome wire (just under 2 ohms). Now i have no experience in actually working with logics with transistor and now i need your help: how would you modify the schematics so that i can ad a variable cc mode, but without adding any extra main switching transistor?thanks in advanceEnricoPs: just to let you know i'm pretty nooby

author: Kleinstein
author_rank: Super Contributor
timestamp: May 26, 2019, 06:58:01 pm
content: The circuit shown is a very crude voltage regulator, with quite some temperature dependence. Also the exact of capacitor for C2 can be important, it needs to have quite some ESR as this helps with loop stability.While one can build a reasonable lab supply just based on transistors, it's rather difficult and not popular anymore since operational amplifiers got affordable. There are quite some plans around using 2 OPs - one for the current and one for the voltage regulation. That would be about the typical way to build a supply today.In linear power supply most of the transistor are not working as a switch, but more in a linear mode. For learning something like a small audio amplifier (e.g. head phone) might be a better circuit to learn.If really needed with transistors and very simple, it would be a transistor working similar to Q2, but for the current. However the capacitor C2 makes thing tricky, as the current limit would not be fast enough to protect the output stage. So it would need a different circuit to start with.

author: Enrico
author_rank: Newbie
timestamp: May 26, 2019, 09:09:33 pm
content: First, thanks a lot of the reply.I have been salvaging speakers amplifiers for a lot since i like listening to hardbass during my lab time.Now back on the power suply, i know the circuit isn't the best, but it's like everything my strating point, some time in the future i will buy a proper power suply but now i can't afford it and my mum(i'm 15) won't let me buy one anyway. Still i need one to test lasers and stuff, so i need a very basic one now. Still i'm following an old magazine that here in italy was very famous and now fallied so now all his versions are free too download.Getting back to the circuit, i have the capacitor with the high esr(mesured it with the diodegonewild esr capacitor tester from his website) and it works(according to my meter) . . . Soo i only miss the cc feature, no op amps for now because i don't have any on hands, i can't savage any from anywhere and can't buy any. Hope you can still help! It would be pretty great!Enrico

author: xavier60
author_rank: Super Contributor
timestamp: May 26, 2019, 10:07:49 pm
content: This should work. How well, I cant be certain.Current limiting has to be fast enough to prevent damaging current spikes occurring. Some overshoot can be a good thing.Placing R6 in series with C2 will speed up the response but might also cause the voltage regulation to become unstable.If you cant check with a CRO, look for signs such as jumps in the voltage control range and poorer than usual load regulation.After all seems to be ok, lower values for C2 can be tried.EDIT: On second thoughts, something based on Kleinstein's suggestions is simpler and likely to work better. I have deleted my original,Use a lower value Pot for VR2 if you have something.

author: Enrico
author_rank: Newbie
timestamp: May 27, 2019, 07:25:33 am
content: thankyou so much, i will try that later