filtered linear ps take2 - Page 1
Forum section: electronics / beginners

author: look4awhile
author_rank: Contributor
timestamp: September 22, 2023, 05:30:57 am
content: Original threadhttps://www.eevblog.com/forum/beginners/noisy-linear-ps-design/25/Attached is take 2 of the design. I'm still trying to use 12v transformer. The design I have from the previous thread works well with 18v transformer, but drops voltage on 12v transformer.This time around its not noisy at all. The problem I'm having is that output voltage changes both from input voltage, as well as load. Non of this happens on the simulator. When potentiometer is properly adjusted it works with 12v transformer too.

author: xavier60
author_rank: Super Contributor
timestamp: September 22, 2023, 12:09:31 pm
content: It's likely to be oscillating. Scope the output of the opamp.Try a 100 ohm resistor between B-E of Q3. If that doesn't fix it, then frequency compensation will need to be added to the opamp.

author: DavidAlfa
author_rank: Super Contributor
timestamp: September 22, 2023, 01:56:16 pm
content: That will be still orders of magnitude worse than a LM317.Glueing together internet circuits and guessing what will happen is not the way.Also the amp you showed looked terrible.If an amp causes hummmm because there's 20mV ripple in the power supply, it's a simply a bad design.

author: look4awhile
author_rank: Contributor
timestamp: September 22, 2023, 10:27:13 pm
content: So the issue was TL431 is giving input-dependent voltage, instead of fixed (based on U6/R1 only). And I am not sure why, since the application seem to be by the datasheet. Adding small capacitor around its anode\cathode does not change anything.Replacing with say 7812 addresses the issue, but the voltage drop is still too high to use 12v transformer

author: xavier60
author_rank: Super Contributor
timestamp: September 22, 2023, 11:04:27 pm
content: If it were to work properly, the output voltage should be within a few millivolts of U5's Cathode voltage.

author: DavidAlfa
author_rank: Super Contributor
timestamp: September 22, 2023, 11:04:42 pm
content: A 12V transformer will give easily 15-18V with no or low load.So if you put something that gives 12V with no load, it's the wrong transformer.The 7812 needs +1.5V over the nominal output. So 13.5V at the very minimum.

author: look4awhile
author_rank: Contributor
timestamp: September 22, 2023, 11:33:07 pm
content: Quote from: xavier60 on September 22, 2023, 11:04:27 pmYes, that part works fine. The problem is that cathode voltage changes based on the input voltage somehow. And I don't understand how.

author: look4awhile
author_rank: Contributor
timestamp: September 22, 2023, 11:42:20 pm
content: Quote from: DavidAlfa on September 22, 2023, 11:04:42 pmI'm getting ~17-18v from the transformer. 7812 needs roughly 14. Rectifier puts it under 16. Low pass filter takes just over3 volts. Reducing resistors makes capacitors unpractically large. Everything works well with 18v transformer. The issue is obviously the low-pass filter.

author: vk6zgo
author_rank: Super Contributor
timestamp: September 22, 2023, 11:50:35 pm
content: You are still "trying to polish a turd".The first thing to learn about regulated supplies is that trying to get 12v out of an active regulator fed by a 12v transformer is an impossible task.Practical regulators all require some amount of "overhead" in terms of input voltage.Another thing you have learnt is why tube designs back when they were mainstream, used higher voltages.Running with 12v "HT" is fiddly at the best of times, & car radio designers had to use tubes designed from scratch for 12V HT, during the short "hybrid" period of such radios.As you are unlikely to need a lot of current capacity, you have an "out" which wasn't available to car radio designers.Running from 12v ac, you can use a voltage doubler rectifier to get that precious "overhead".As in this world, "you get nothing for nothing, & damn-all for sixpence", you need a more complex rectifier circuit, probably with better filtering, but if it takes the project from "impossible" to "do-able", it is worth it.

author: look4awhile
author_rank: Contributor
timestamp: September 23, 2023, 12:17:31 am
content: Quote from: vk6zgo on September 22, 2023, 11:50:35 pmPoint taken. I'll just stick to 18v transformer and not waste any more time on this. Well, almostAttached version works with 12v transformer and said amplifier. No noise to speak of. I don`t think there is any headroom since if I increase the load by another 400mA on electronic load - voltage starts dropping. However it works within the spec, and with a 12v transformer.At the end of the day I replaced active components with straight up 12v zener diode. It does not produce exactly 12v, but it produces stable 'just above 12v' which works for the purpose.Thank you everyone for advice.