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| What is "floated" linear voltage regulator? |
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| Zero999:
--- Quote from: spec on November 21, 2018, 10:33:39 pm --- --- Quote from: Hero999 on November 21, 2018, 04:31:53 pm --- --- Quote from: ArthurDent on November 21, 2018, 03:37:00 pm --- --- Quote from: Hero999 on November 21, 2018, 11:54:28 am --- --- Quote from: ArthurDent on November 21, 2018, 04:26:01 am ---"So how does a floating regulator create/have a low impedance to ground if it never references to it?" There is a difference between impedance and resistance. A supply's output could have an extremely high resistance to ground at D.C. but with a capacitor from the output to ground could basically be a short circuit at R.F.. A lot of supplies I have say the output can be floated +/- XXX volts. --- End quote --- That's true, but with a DC power supply we always need a very low DC resistance. The the linear regulator is too slow, then the output impedance will rise with increasing frequency, which is why a decoupling capacitor is used. --- Quote from: 001 on November 21, 2018, 08:02:16 am --- --- Quote from: spec on November 21, 2018, 07:54:59 am --- The term floating is really a misnomer, because all voltage regulators are connected one way or another to 0V, they must be to function. Having said this, it is possible to have the regulator actually isolated from the 0V line by using an opto isolator for example. Getting back to the 200V supply, you would connect a 100R resistor between the output and sense terminals and a 15k9 resistor from the sense terminal to 0V. --- End quote --- --- End quote --- --- End quote --- That's not what I said at all. I said "high resistance to ground", not impedance. I also said "to ground", not to 0 volts. The output impedance between the output terminals of a power supply should be low but we generally want the resistance to ground of a bench supply to be high or infinite. This gives us the ability to wire one supply in series with another without shorts and this is what is done with dual supplies where they can be switched for either series or parallel operation. Supplies floating with respect to ground allow us to do this or a single output supply to be wired either + or - with respect to ground. If you look at the schematic 001 posted in post #20 to show this is what he meant. You will see this in the lower right hand corner of the schematic as shown below. The circuit has high resistance but low impedance. With a capacitor to ground the 'impedance' decreases with increasing frequency. --- End quote --- You appear to be confused about the context of the word "ground" in the title of the thread. For the purposes of discussing an electrical circuit, the word ground refers to 0V. Whether it is connected to the physical earth or not is immaterial. Ground is purely a reference point in circuit from where all voltages are measured from. A floating linear regulator is a circuit who's common rail is at a different potential to that of the rest of the circuit. The LM317 is a floating linear regulator because it has no ground pin. The adjust pin connects to the centre of a potential divider, thus it floats at a higher voltage than the circuit's ground or reference. The LM7805 is not a floating regulator because its ground pin is normally connected to the same reference as the rest of the circuit. Yes, any bench power supply should be isolated or floating with respect to mains ground, but this has absolutely nothing to do with the definition of a floating linear regulator, such as the LM317 and is completely off-topic. --- End quote --- I do not understand what you are trying to say here. Who is confused about what? What is the relevance of mentioning that a PSU can be isolated from the mains? Please clarify so your view can be understood and discussed. --- End quote --- That was my point: the fact the PSU is isolated from the mains is irrelevant. My reply wasn't aimed at you, but someone else who seemed to have mixed up ground as in the earth, with 0V and floating, as in not being connected to the soil, with a floating regulator, such as the LM317. I've highlighted the part where I believe the confusion occurred. --- Quote from: spec on November 21, 2018, 11:43:06 pm --- --- Quote from: SiliconWizard on November 21, 2018, 10:37:50 pm --- --- Quote from: spec on November 21, 2018, 10:17:57 pm ---Attached is the schematic promised in Answer #41 that shows simplified circuits for a 'conventional' ground referenced PSU and a floating PSU. In both cases the feedback is shown. (...) --- End quote --- I may have missed something, but the OPA192 has limited current output capability so, a PSU yes, but with something like 50 mA max. probably... if it's not further buffered. Just a thought! --- End quote --- No, you have not missed anything. Your observation about the current limitation is correct. There are missing functions too, like decoupling, compensation, and current limiting. But the purpose of the schematics is to illustrate the two feedback techniques which the OP was asking about. If I did a full design you would not be able to see the wood for the trees. :phew: Just for a bit of fun, I did have a go at a 1A version with current limiting and all the twiddly bits. :) --- End quote --- As far as doing a rough simulation of the LM317 is concerned. I prefer to use the generic op-amp model, as its parameters can be edited and is faster to simulate. Here's my basic LM317 simulation. I added in the current limiting but didn't include safe operating area protection. I agree this is only to show the basic concept and not a practical design. |
| spec:
Ahh apologies H99. I did not get your point. |
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