A dead simple power supply - Page 1
Forum section: electronics / beginners

author: riverrun422
author_rank: Newbie
timestamp: October 24, 2022, 05:20:59 am
content: I'm having a go at making my very first power supply. To keep it simple, its a linear, unregulated supply. Looking for any tips, improvements, or warnings before I build it and put it in a boxIt will be used to power a piezo speaker (high voltage, unknown current), but would be nice to be able to reuse it. I've attached a picture of what I have so farThanks!

author: geggi1
author_rank: Frequent Contributor
timestamp: October 24, 2022, 05:26:01 am
content: You should have some fuses to protect the transformer form shorts

author: james_s
author_rank: Super Contributor
timestamp: October 24, 2022, 07:21:00 am
content: A single fuse on the primary will protect everything.Seems like a power indicator would be a nice addition too, and frankly I'm not sure there's a lot of use in an unregulated supply, a regulator with current limiting would make it more useful but maybe there's a specific application in mind.

author: Dave
author_rank: Super Contributor
timestamp: October 24, 2022, 07:44:55 am
content: Quote from: riverrun422 on October 24, 2022, 05:20:59 amLinear implies that the element that does the regulation is operated in the linear region. You don't have that.Make one side of the primary winding grounded in the LTspice simulation. Every piece of the circuit needs to be somehow referenced to ground to be able to properly simulate it.

author: smd_hunter
author_rank: Newbie
timestamp: October 24, 2022, 10:02:12 pm
content: I think I would put:A MOV on the primary after the fuse to restrict input peaks.A PTC in series on the secondary before the bridge rectifier to protect the transformer if there is a short at the output.But I don't know what else while keeping it simple

author: Benta
author_rank: Super Contributor
timestamp: October 24, 2022, 10:19:32 pm
content: Looks fine to me, depending. I suggest:1: primary fuse (fire proofing, mandatory in the EU, don't know the US codes).2: indicator (neon lamp plus resistor on the primary).The MOV is normally not necessry, unless you have very dirty mains power. Transformers are extremely robust.Now, you write nothing about voltage, current and power. The cap size makes me think a couple of A output current.Power is interesting:1: <10 VA transformers are usually inherently short-circuit proof.2: >300 VA transformers plus big caps mostly need soft-starting.

author: radiolistener
author_rank: Super Contributor
timestamp: October 25, 2022, 09:58:27 am
content: You can put 10 nF capacitors in parallel to each rectifying diode. It helps to reduce noise from diodes.

author: JohanH
author_rank: Frequent Contributor
timestamp: October 25, 2022, 10:28:41 am
content: Quote from: radiolistener on October 25, 2022, 09:58:27 amThis is mostly not necessary and doesn't help a lot. But sure, you could do it. But if you really want to reduce noise ("ringing") from diodes (which is caused by transformer leakage inductance and capacitance), in the correct way, you have to add a snubber before the rectifier, tuned for the specific transformer. But this is kind of too advanced level for this application (and not necessary in a simple power supply).

author: Vovk_Z
author_rank: Super Contributor
timestamp: October 25, 2022, 06:35:53 pm
content: I may add that capacitors in parallel to each rectifying diode help in a case when it is a power supply for a radio. Otherwise, I agree that RC-snubber works better.

author: Benta
author_rank: Super Contributor
timestamp: October 25, 2022, 06:39:56 pm
content: Totally academic discussion IMO. We're talking 50/60 Hz sine here, so discussing turn off phenomena in really slow rectifiers is a bit over the top.

author: bdunham7
author_rank: Super Contributor
timestamp: October 25, 2022, 07:18:36 pm
content: Quote from: Benta on October 25, 2022, 06:39:56 pmI just built a low-noise linear LED driver PSU and the 1N400x diodes were the primary source of conducted and output noise. A resistor and two capacitors for each diode pair (mine was FW doubler, so only one pair) with the resistor selected for best damping when looking at it on a scope will reduce this noise to a nearly undetectable level.

author: Benta
author_rank: Super Contributor
timestamp: October 25, 2022, 07:44:32 pm
content: If that's really an issue (never experienced it myself), just use Schottky rectifiers.

author: radiolistener
author_rank: Super Contributor
timestamp: October 26, 2022, 02:03:17 pm
content: if you're using power supply for a digital circuit, it doesn't matter. But if you're planning to use it for analog circuit which is sensitive to a noise then it worth to implement.

author: JohanH
author_rank: Frequent Contributor
timestamp: October 26, 2022, 05:02:26 pm
content: Quote from: radiolistener on October 26, 2022, 02:03:17 pmYes, in that case I've used this method to experimentally get correct component values for the snubber:https://www.diyaudio.com/community/threads/simple-no-math-transformer-snubber-using-quasimodo-test-jig.243100/By selecting snubber capacitor and resistor manually with the help of this "ringer" circuit, you quickly get a very good result.Here are before and after pictures for a specific transformer. Every transformer (with different specifications) ends up with different snubber component values.

author: TimFox
author_rank: Super Contributor
timestamp: October 26, 2022, 06:04:22 pm
content: The topic of rectifier noise in analog low-noise applications is covered nicely in pp 374-377 of Merlin Blencowe, "Designing high-fidelity tube preamps", Merlin Blencowe 2016.He discusses fast-recovery diodes, Schottky diodes, "snubbing", and "slugging".The problem is not exactly the switching time of the diodes itself, but the excitation of the parasitic resonant circuit formed by the transformer's leakage inductance and stray capacitance (including diode capacitances)."Slugging" is a brute-force approach of adding a capacitor across the secondary to lower that resonant frequency, possibly using a capacitor across each diode to form that capacitance."Snubbing" is a more elegant approach of adding a series R-C network across the secondary: the capacitor is made large enough to swamp out the strays and diode capacitances, but the resistor has an optimal value (not too large nor too small) to damp out the resonance.His example, with 1N4007s in a full-wave bridge, used a 10 nF capacitor and 1.5 kohm resistor for a practical 12 V, 6 VA transformer.There was an additional slight improvement when substituting the faster UF4007 for the original 1N4007s.The apparent leakage inductance of that transformer was approximately 20 mH.He mentions that the "quiet" reputation of tube rectifiers is mainly due to their higher on-resistance.

author: bdunham7
author_rank: Super Contributor
timestamp: October 26, 2022, 07:48:39 pm
content: Quote from: TimFox on October 26, 2022, 06:04:22 pmWhen I breadboarded my PSU, I found both a spike and ringing. The spike has more effect on conducted emissions than the ringing, so my goal was to eliminate the spike and reduce the ringing. The lower bandwidth of the ringing is also more easily absorbed in the filter caps and regulator. The PSU has a16Vx2 20VA transformer and a FW voltage doubler setup. After some experimentation, I settled on 68nF caps and a 47R resistor--a bit different than what an RC snubber across the transformer would look like I think. I didn't try any faster diodes, I think I actually just used 1N4007.Here are the results, first the output of the bridge and then the voltage measured directly across one of the diodes. I think that is pretty close to critical damping and the PSU is electrically very quiet.

author: Benta
author_rank: Super Contributor
timestamp: October 26, 2022, 08:40:35 pm
content: It seems the OPs "deead simple" wish has been squashed completely in favour of internal discussions on fringe aspects.I have a feeling he/she won't return.

author: bdunham7
author_rank: Super Contributor
timestamp: October 26, 2022, 08:50:37 pm
content: Quote from: Benta on October 26, 2022, 08:40:35 pmThat would be a shame. All we're talking about is eliminating a possible source of audible annoying noise in his piezo driver circuit using a common method involving a few simple components. How is that not simple?