"A common mistake that people make when trying to design something completely fool-proof is to underestimate the ingenuity of complete fools" : Mostly Harmless Douglas Adams
I was reading through the past posts on ETO yesterday, and very interesting they are too. A lot are beyond my field, but when I came across throbscottle's post, below, about power supplies blowing up, it stuck a chord. It got me thinking about all the lengths you need to go to in order to protect certain circuits, and also comply with the many specifications and standards: temperature, shock/ vibration, humidity, EMC, safety, through life costs...
In terms of protection, bench power supplies are some of the most difficult: quite simply, you never know what will be connected across the output terminals. Because of this, they are also difficult to stabilise in the frequency domain and at the same time give good fast regulation. This is not so difficult with the low gain Darlington output of this simple voltage regulator, but even emitter followers can make good oscillators.
So, out of interest, I took the most basic voltage regulator, the kind that was popular before the three terminal regulators, like the LM78xx/ LM79xx and LM317/LM337, swept the board. Then, I added most of the components needed to make the basic regulator reasonably robust, and also to function ok with worst-case components.
In the attached schematic below, PSU 1 is the original circuit and PSU2 illustrates what it turns into with protection. As is often the case, the additional components completely overwhelm the basic circuit. In voltage regulation terms, both PSUs perform the same. In fact, the original simple circuit performs better.
These circuits should work OK, but I haven't spent any time optimizing, so don't worry about the design details, just the principles.
ERRATA
(1) As AnalogKid noted in post 11, 2D-4 should be replaced by a short circuit and D2-5 should show the correct symbol for a TrasZorb (Transit Voltage Suppressor) 2015_12_04
I was reading through the past posts on ETO yesterday, and very interesting they are too. A lot are beyond my field, but when I came across throbscottle's post, below, about power supplies blowing up, it stuck a chord. It got me thinking about all the lengths you need to go to in order to protect certain circuits, and also comply with the many specifications and standards: temperature, shock/ vibration, humidity, EMC, safety, through life costs...
In terms of protection, bench power supplies are some of the most difficult: quite simply, you never know what will be connected across the output terminals. Because of this, they are also difficult to stabilise in the frequency domain and at the same time give good fast regulation. This is not so difficult with the low gain Darlington output of this simple voltage regulator, but even emitter followers can make good oscillators.
So, out of interest, I took the most basic voltage regulator, the kind that was popular before the three terminal regulators, like the LM78xx/ LM79xx and LM317/LM337, swept the board. Then, I added most of the components needed to make the basic regulator reasonably robust, and also to function ok with worst-case components.
In the attached schematic below, PSU 1 is the original circuit and PSU2 illustrates what it turns into with protection. As is often the case, the additional components completely overwhelm the basic circuit. In voltage regulation terms, both PSUs perform the same. In fact, the original simple circuit performs better.
These circuits should work OK, but I haven't spent any time optimizing, so don't worry about the design details, just the principles.
https://www.electro-tech-online.com/threads/variable-psu-output-vulnerability.145895/
I'm in the last 1% of my hybrid psu design, and think I have found out a problem that must affect many variable bench psu's.
So, take an adjustable psu with an emitter follower output, connect a capacitor across the output, turn up the voltage, quickly turn down the voltage. Suddenly the emitter of the output transistor is now at a much higher voltage than it's base, possibly exceeding its vbe and killing it.
So I think that's why I keep blowing output transistors anyway. Does the theory hold water? Does it affect all variable PSU's? Or is it a nasty trick I have uncovered because the pre-regulator lowers the collector voltage when the output is turned down?
ERRATA
(1) As AnalogKid noted in post 11, 2D-4 should be replaced by a short circuit and D2-5 should show the correct symbol for a TrasZorb (Transit Voltage Suppressor) 2015_12_04
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