ways to stabilize Vcc opamp supply in noisy environment.. still not clear - Page 1
Forum section: electronics / projects

author: Houseman
author_rank: Regular Contributor
timestamp: May 25, 2023, 12:05:19 am
content: Hi forum.I need Your valuable knowledge to decipher a schematic around a zener and a BJT I am working on and cannot figure out how it is working.It takes basically a full wave rectified 24Vdc but not filtered as an input and by a zener that fixes the voltage to 13V it powers the opamp (not see in the schema from the emitter of a BJT,Here the LTspice schematic screenshot and output waveforms.Could someone please tell me why has it been used such a method?Was not a zener sufficient?all RM resistors are 1% tolerance ones. Why the hell are there so many?Are diode D1 ad D2 just for reverse protection? but since it is powered by an AC24V trafo and rectified why are there 2 diodes in different parts of the circuit?Sorry for all the questions.Any help appreciated, as always.Steve

author: moffy
author_rank: Super Contributor
timestamp: May 25, 2023, 12:32:20 am
content: A zener is a shunt regulator so it can only supply less current than it is consuming which makes it a power hungry solution compared to the series pass transistor, Q1. Why it was done this way? there is not enough info provided, but it is not a terrible design. R1/DZ1 form the reference for Q1, Q1 acts as the series pass element providing the +12.3V supply. D2 isolates the positive voltage across C13 from the rectified input, RM4 and RM5 could limit current in the event of a fault and R2 acts as a DC discharge resistor for C13. So everything has a reasonable purpose.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: May 25, 2023, 01:26:06 am
content: Quote from: Houseman on May 25, 2023, 12:05:19 amIt does have a filter cap. Your plots show the DC supply not going to 0VQuote from: Houseman on May 25, 2023, 12:05:19 amThere is a zener present - do you mean why not a Zener alone ?A zener alone can be used, but such a shunt regulator causes higher idle current, not a good idea these days.A Zener + NPN is a simple series pass regulator. Nothing fancy in specs, but 'good enough' for many uses.

author: Houseman
author_rank: Regular Contributor
timestamp: May 26, 2023, 01:20:25 am
content: Thanks both from Your valuable answers.I have studied the Series pass transistor, and now every thing is more clear.When you say "but it is not a terrible design" you mean, rather old one? Are there any drawbacks? I mean it broke twice the protection diode D1 and once the Zener regulator without a particular event.. would you suggest a better solution same topology but different approach? or completely change it using maybe dedicated IC's available nowadays?Any suggestion very appreciated,regards and thanks again

author: PCB.Wiz
author_rank: Super Contributor
timestamp: May 26, 2023, 01:52:47 am
content: Quote from: Houseman on May 26, 2023, 01:20:25 amIt is not clear what D1 does, your spice circuit likely lacks other connections.The Zener is not in a particularly stressed position, driven by 10k, so electrical stress seems unlikely. Could it be mechanical stress ?With incomplete information, if you have a small number of these having a few field failures, the normal response is to upgrade the failing parts.

author: moffy
author_rank: Super Contributor
timestamp: May 26, 2023, 03:25:09 am
content: Quote from: Houseman on May 26, 2023, 01:20:25 amI have no idea how it is being used or why it fails, maybe upgrade the 1N4148 to a 1N4004?

author: srb1954
author_rank: Super Contributor
timestamp: May 26, 2023, 10:52:13 am
content: Quote from: Houseman on May 26, 2023, 01:20:25 amUse a 78L12A voltage regulator instead. You will get better performance all round and it will be quite well protected against most abuse. As a bonus it will take a fraction of the PCB space of your current circuit.

author: xavier60
author_rank: Super Contributor
timestamp: May 26, 2023, 11:01:25 am
content: An advantage of a Zener and transistor regulator is that it doesn't need a bypass cap to ground on its input. This avoids coupling hum curent into the opamp circuit's ground.

author: srb1954
author_rank: Super Contributor
timestamp: May 26, 2023, 09:13:38 pm
content: Quote from: xavier60 on May 26, 2023, 11:01:25 amThis is not normally a major problem unless you do something silly with your PCB layout. Even if you do have to put an input capacitor on the regulator because you sited your regulator too far away from the main reservoir capacitor it only requires a relatively small capacitor (330nF). This won't inject much noise current into the ground, unless you have a noisy switching supply as your input source. But that noise should be dealt with at source.

author: David Hess
author_rank: Super Contributor
timestamp: May 27, 2023, 10:12:23 am
content: The transistor is buffering the zener shunt regulator for greater output current capability than the zener shunt regulator would normally provide.The only disadvantage of this circuit is that there is no closed loop control of the transistor output limiting load regulation performance.