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I was reading a linear regulator tutorial and found this.

It is very different from what I read in books.

enter image description here

As compared to LM7805 (Fixed Voltage Regulators)

enter image description here

YouTube link.

Source: Freescale

Both are not same. Maybe I missunderstood.

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  • \$\begingroup\$ What have you read that is so different to what is shown in your picture? \$\endgroup\$
    – Andy aka
    Sep 28, 2022 at 16:39
  • \$\begingroup\$ As in, replace Control element with an equivalent resistance. \$\endgroup\$ Sep 28, 2022 at 17:37
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    \$\begingroup\$ Alan, just look at a typical 7805 regulator and a TL431. Consider the TL431 used to produce 5 V for a load. And then think about all that for a bit. Come to your own conclusions. \$\endgroup\$
    – jonk
    Sep 28, 2022 at 18:01
  • \$\begingroup\$ You'd need to refer to the exact pages of the books you've read, because, as you might imagine, given how many books there are out there on any subject, the books I've read (for example) don't contradict the slide you've shown. We can't know what books you have in mind - you'll have to help us out. And perhaps, re-reading the books in search of the offending citations, you might come to realization that the books were not wrong after all - a very desirable outcome. Or the books you got are junk. \$\endgroup\$ Sep 28, 2022 at 18:53
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    \$\begingroup\$ @jonk Thank you. Yes, LM7805's diagram of "Fixed Output Voltage Regulator" is very different \$\endgroup\$
    – Alan
    Sep 28, 2022 at 19:47

3 Answers 3

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The diagram you have is how the device is used in a typical circuit. Your Youtube screengrab shows a very simplified version of the internal configuration of the devices, enough to illustrate the difference between shunt and series configurations. Here is a slightly more detailed block diagram of a 7805, which is a series regulator:enter image description here

And for a TLVH431, which is a series regulator:

enter image description here

7805 diagram source is: https://www.st.com/resource/en/datasheet/l78.pdf

TLVH431 diagram source is: https://www.ti.com/lit/ds/symlink/tlvh431a.pdf?ts=1664395967436

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  • \$\begingroup\$ Thank you. Sorry, I could not see where the R series, R shunt shown in the both your diagram \$\endgroup\$
    – Alan
    Sep 28, 2022 at 20:31
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    \$\begingroup\$ It's "series pass element" in the series regulator, which is a BJT. The shunt BJT is explicitly shown in the shunt regulator schematic. \$\endgroup\$
    – vir
    Sep 28, 2022 at 21:07
  • \$\begingroup\$ Thank you. I got the meaning now. May I know the second figure (Figure 21. Equivalent Schematic) is related to? I cannot image why this diagram \$\endgroup\$
    – Alan
    Sep 28, 2022 at 21:19
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    \$\begingroup\$ The second figure is a shunt regulator. \$\endgroup\$
    – vir
    Sep 28, 2022 at 21:40
  • \$\begingroup\$ thank you. Very hard to match the shunt regulator (youtube screengrab) and your diagram (Figure 21. Equivalent Schematic). Are they the same? \$\endgroup\$
    – Alan
    Sep 28, 2022 at 21:42
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Both are not same. Maybe I missunderstood.

Yes, you misunderstand - but that's understandable considering the ambiguity in that video.

The 'basics' of a series regulator need not include the 'Ireg' and shunt 'R' shown - only Rseries is essential to its operation.

A practical series regulator will usually have some quiescent current flowing to ground to power its internal circuitry, and a fixed resistance may also be wired between the output and ground to draw the minimum current required to maintain regulation at low output current. But these are implementation details that can be ignored when comparing the 'basics'.

A shunt regulator draws (almost) constant current which is higher than the maximum load current, and diverts the excess current not consumed by the load to ground via Rshunt. Therefore both the fixed 'R' and variable 'Rshunt' resistors are essential to its operation.

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780X series-regulator

Alan, here's a schematic directly from the Signetics uA7800 datasheet:

enter image description here

I've added some arrows and some text. At the bottom, in a kind of brown color, I've identified wires that carry current to the common pin. These currents sum up to some fixed value and could be represented by a single resistor of an appropriate value (if the input voltage stays the same, anyway.) Also, \$Q_{17}\$ is the variable resistor that is being adjusted by the rest of the circuit, located between the input and the output pins.

As you can tell, your block diagram is highly simplified. It's not even a behavioral diagram, it is so simplified. There's no detail to it, at all. But the basic idea is that the series regulator has some kind of "pass transistor" (\$Q_{17}\$ here above) that sits between the input and the output and is controlled/adjusted to regulate the output and that there is also a "resistance" (necessary circuitry) that sits between the input and common/ground to power the rest of the circuitry that does things like "measure" and "protect". This latter resistance can be considered as a fixed value (though, technically, it isn't and is more likely closer to a constant current than a constant resistance) that is as large as possible (to minimize wasted power) consistent with servicing the device's needs.

TL431 shunt-regulator

The TL431 shunt regulator schematic is a little different:

enter image description here

Here all I've done is show you the variable resistor part, with an arrow to it. The rest of the circuit "looks like" a fixed resistor (or fixed current), which is also as large of a resistance (or as small of a current) as is possible, consistent with its operation. Since the variable resistor part is so small by comparison (has such a large current by comparison) with the fixed portions, the fixed portions can essentially be ignored, leaving only the variable resistor part to be worried over.

Here's the example from the datasheet that provides a fixed output voltage:

enter image description here

It's a lot less difficult to see here the fixed resistor, \$R_{\text{SUP}}\$, and the fact that the variable resistor I pointed to in the schematic is arranged where your block diagram has it. In this case, varying the shunt's resistance changes the current (and therefore the voltage drop across) \$R_{\text{SUP}}\$.

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