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| Lab Power Supply - The Lost Current |
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| xavier60:
In your old design, The output Darlington functioned as an Emitter follower which buffered the output of the CV op-amp which was able to swing by the whole range required for 0-30 volts output. In the Elektor design and many others, everything is referenced to the positive output. This includes the Darlington Emitter, 0 volt reference for the low voltage split rails(+/- 12 volts) and the voltage reference source, a TL431 possibly. A good way to properly drive the Darlington's Base is to source current from a voltage source that is always a fixed amount higher than the Emitter so that there is enough available Base drive current even when rising Emitter voltage approaches Collector voltage. This can only be from the +12 volt rail. This makes it possible to fully turn on the Darlington. This is desirable when the PSU is operating at high output voltage and current so that as much as possible of the unregulated voltage on the Collector can be transfered to the output. The regulator will be said to have a low dropout voltage, a good thing. In the Elektor design, the Darlington can no longer be thought of as an Emitter Follower. It is a series pass element conducting current from the unregulated rail to the output under control of either the CV op-amp or CC op-amp. Google "Darlington pair" Do you understand what op-amps do? --- Quote from: radoczi94 on February 07, 2018, 09:31:44 am ---One thing I do not really understand about this old schematic. 723 makes the reference. As far as I know emitter followers have a voltage gain of 1. How is 0-30 volts on the output, if the Vref is 7,15V? I'm shure, that some resistor makes this magic. Maybe R17 is talking back? Why don't just pull it up to the collectors? I've seen in some HP and Tektronix supplies, the base were just pulled up. What I do not like about this, is the 723. Maybe a little bit too crusty. I'm shure there are much better solutions for a good reference. --- Quote from: xavier60 on February 07, 2018, 12:12:18 am ---Because of the way the voltage adjust potentiometer is configured as a VR in the Elektor design, it is not fail safe. If the potentiometer becomes scratchy, the PSU output could spike to full voltage. I think it is better to use the potentiometer as a potentiometer to directly vary the reference voltage. Including a wiper bleed resistor would make it fail safe. The reference voltage will need to be negative. I don't like the remote sensing. It should be omitted. --- End quote --- You mean, like the current set pot? Dividing the vref with the pot itself? --- End quote --- Yes |
| radoczi94:
So the floating supply's ground is referenced to the output.This means, that the floating positive rail and the Vref is on higher potential than the output, and the floating negative rail is lower. This is because the floating supply'ss ground tied up to the output by R23 47R. Right? This way the Vref is able to source current to the Darlington bases, because the Vref's potential is higher than the emitter(s). As I understood, the opamps are sucking away current from the Darlington base, or the CV oamp tries to push it, if the output voltage is not high enough, but it can not be happen, because the diode is reverse biased,so the Vref opens the Darlington more. If I'm right, the Vref needs to be capable of providing enough current to drive the transistors, and the voltage dividers at the opamps.In the same time, it needs to be stable with low drift. Maybe a bare TL431 is not enough for this, a lot depends on the beta of the Darlington. Edit: The total beta of the darlingtons in worst case 400, so 8-10mA is enough for 3-4A output. --- Quote from: xavier60 on February 07, 2018, 12:10:49 pm ---A good way to properly drive the Darlington's Base is to source current from a voltage source that is always a fixed amount higher than the Emitter so that there is enough available Base drive current even when rising Emitter voltage approaches Collector voltage. This can only be from the +12 volt rail. --- End quote --- There are some violations to this, because there is some voltage drop across the shunt. But we want this, so the voltage drop is compensated by the control circuit. Right? --- Quote from: xavier60 on February 07, 2018, 12:12:18 am ---Because of the way the voltage adjust potentiometer is configured as a VR in the Elektor design, it is not fail safe. If the potentiometer becomes scratchy, the PSU output could spike to full voltage. I think it is better to use the potentiometer as a potentiometer to directly vary the reference voltage. Including a wiper bleed resistor would make it fail safe. The reference voltage will need to be negative. I don't like the remote sensing. It should be omitted. --- End quote --- Why would be practical to have a negative reference voltage? Just in case of pots getting scratchy? |
| xavier60:
--- Quote from: radoczi94 on February 07, 2018, 12:53:19 pm ---So the floating supply's ground is referenced to the output.This means, that the floating positive rail and the Vref is on higher potential than the output, and the floating negative rail is lower. This is because the floating supply'ss ground tied up to the output by R23 47R. Right? --- End quote --- That's right. That area of the circuit can be left the same but with the sense wire tied directly to the + output terminal. --- Quote from: radoczi94 on February 07, 2018, 12:53:19 pm ---This way the Vref is able to source current to the Darlington bases, because the Vref's potential is higher than the emitter(s). As I understood, the opamps are sucking away current from the Darlington base, or the CV oamp tries to push it, if the output voltage is not high enough, but it can not be happen, because the diode is reverse biased,so the Vref opens the Darlington more. If I'm right, the Vref needs to be capable of providing enough current to drive the transistors, and the voltage dividers at the opamps.In the same time, it needs to be stable with low drift. Maybe a bare TL431 is not enough for this, a lot depends on the beta of the Darlington. Edit: The total beta of the darlingtons in worst case 400, so 8-10mA is enough for 3-4A output. --- End quote --- Both low voltage rails should be regulated for best performance and the drive current for the Darlington should come from the +12v rail. You won't have a positive reference if you decide to go for a negative reference anyway. The low voltage rails don't strictly have to be +/-12v. A bit lower would be ok if your small transformer doesn't output enough to regulate 12v. --- Quote from: xavier60 on February 07, 2018, 12:10:49 pm ---A good way to properly drive the Darlington's Base is to source current from a voltage source that is always a fixed amount higher than the Emitter so that there is enough available Base drive current even when rising Emitter voltage approaches Collector voltage. This can only be from the +12 volt rail. --- Quote from: radoczi94 on February 07, 2018, 12:53:19 pm ---There are some violations to this, because there is some voltage drop across the shunt. But we want this, so the voltage drop is compensated by the control circuit. Right? --- End quote --- That won't be a problem. --- Quote from: xavier60 on February 07, 2018, 12:12:18 am ---Because of the way the voltage adjust potentiometer is configured as a VR in the Elektor design, it is not fail safe. If the potentiometer becomes scratchy, the PSU output could spike to full voltage. I think it is better to use the potentiometer as a potentiometer to directly vary the reference voltage. Including a wiper bleed resistor would make it fail safe. The reference voltage will need to be negative. I don't like the remote sensing. It should be omitted. --- End quote --- Why would be practical to have a negative reference voltage? Just in case of pots getting scratchy? --- End quote --- Pots going scratchy is all too common. It is actually the wiper losing contact with the track. Have a think about what happens next if the pot is configured as a VR as in the Elektor design? As far as I can see, the reference needs to be negative if the control is to be wired as a potentiometer rather than a VR. Time to start sketching some circuits. |
| C:
Simple question The Elektor link How much trust should you give to an article in a mag where the operating principles does not match the final design? The article describes how it works in figure 2. A classic design where CC mode is hacked on to a VC mode regulator. Some of this hack shows in final circuit. The final circuit figure 3 does not match the theory of operation in figure 2. Here you have the 723 suppling power to R9 & have the analog OR for CC & CV ( D4, D5) So Author could not describe a simple analog OR to make sense of circuit. Then you have if it was truly a VC & CC regulator, why only one indicator of mode? How much effect does driving that indicator have on mode switch? Note that it is possible that a load is CC & VC at same time. With load noise a good supply can be changing fast between the three states. With out a scope, trying to update this very old design I would say is a fail. It is what it is an attempt at a cheap precision power supply. some thing to keep in mind, If you are going to be building analog circuits a Positive & Negative supply is nice to have during design. |
| radoczi94:
--- Quote ---Both low voltage rails should be regulated for best performance and the drive current for the Darlington should come from the +12v rail. You won't have a positive reference if you decide to go for a negative reference anyway. The low voltage rails don't strictly have to be +/-12v. A bit lower would be ok if your small transformer doesn't output enough to regulate 12v. --- End quote --- I think I will go with +-15V. I have a few salvaged 2*18V*0,15A PCB mains transformers, I want to use them. Then put around some 78-7915 regulators some filtercaps, and a preload resistor or whatnot. --- Quote ---Pots going scratchy is all too common. It is actually the wiper losing contact with the track. Have a think about what happens next if the pot is configured as a VR as in the Elektor design? As far as I can see, the reference needs to be negative if the control is to be wired as a potentiometer rather than a VR. --- End quote --- If the pot works as just a voltage and the contact is failing in the wiper, the output goes low, because the voltage difference between the inputs is positive. If the VR fails, the goes to full chooch.Oops. --- Quote from: C on February 07, 2018, 04:43:01 pm ---Simple question The Elektor link How much trust should you give to an article in a mag where the operating principles does not match the final design? The article describes how it works in figure 2. A classic design where CC mode is hacked on to a VC mode regulator. Some of this hack shows in final circuit. The final circuit figure 3 does not match the theory of operation in figure 2. Here you have the 723 suppling power to R9 & have the analog OR for CC & CV ( D4, D5) So Author could not describe a simple analog OR to make sense of circuit. Then you have if it was truly a VC & CC regulator, why only one indicator of mode? How much effect does driving that indicator have on mode switch? Note that it is possible that a load is CC & VC at same time. With load noise a good supply can be changing fast between the three states. With out a scope, trying to update this very old design I would say is a fail. It is what it is an attempt at a cheap precision power supply. some thing to keep in mind, If you are going to be building analog circuits a Positive & Negative supply is nice to have during design. --- End quote --- Had a feeling about that, but only concentrated on that voltage setting pot. Which was basically the 90% of the theoretical part.As far as i know, this thing is able to work, my buddy made two of these and both worked first-second try. Spent half a day googling around to see in search of a usable schematic, nothing. I will probably end up poking parts into breadboard. With only a multimeter, for now. I wanted to buy a scope since ages, so I guess, the time has come. |
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