Help understanding reason for IC placement on schematic

[seb1982]

Hi guys!

I was looking at the attached simple power supply schematic this afternoon, and noted that the power and ground pins of the LM324 are placed before the 7815 regulator. 

The circuit is designed to be used with a laptop DC adapter, so I wouldn't have thought a stable DC voltage would be an issue, but I'd have thought that the power pin should be placed after the regulator, if for no other reason than the slightly reduced voltage and chip stress and life length.

Can anyone see a reason I've missed as to why it should be placed before the 7815?

*Edited to ensure the schematic fits without scrolling

[David_AVD]

I would imagine the reason is so the opamp I/O are not operating near it's own power V+ limit.

[c4757p]

BUZ22 has a threshold voltage of 4V - if you want it to output 14V, you must feed it at least 18V. The op amp needs a higher supply to reach that.

That said, that means that either the input or output rating of this supply is wrong. LM324 isn't rail-to-rail - it will subtract some of its own voltage. You'll need at least 20-22V on the input to get 14V out.

[seb1982]

Ah - that makes sense.

God, I would literally sell my grandmother (almost) to have your knowledge!  Thanks so much for sharing it. 

[quantumvolt]

It doesn't matter what you feed this circuit - 18 or 24 Volt. The fixed regulator nails pre output-regulation to 15 Volt.

In the datasheet for LM324 it says that Input Differential Range is equal to Power Supply Voltage. Supposing somewhat linear operation - that means that the output can swing very close to the supply. And if not - as long as the op-amp is fed 18 Volts it surely can output anything needed up to 15 Volts (the highest voltage in the actual PSU).  http://www.ti.com/lit/ds/symlink/lm124-n.pdf

So there is nothing wrong with the input and op-amp parts of the schematics.

If the Drain-Source voltage was constantly 4 Volt in this application, the circuit would never output more than 11 Volts whatever you fed it on the left side - 18 or 24 Volt.  Those wanting to know Drain-Source voltage drops in different modes can study the topic  themselves. Imo Vds sat of 1 Volt is not unreasonable under certain circumstances.  Just one of many web links - read and judge for yourself  http://electronics.stackexchange.com/questions/18885/mosfet-as-a-switch-when-is-it-in-saturation

All in all the schematic is sound and is just a single output 2 mode (Constant Voltage, Constant Current) bench power supply. The op-amp is fed unregulated in order to obtain sufficient voltage for the feedback amplifier voltage swing. It is also a kind of 'isolation' of the control power supply from the outputted power, which is good and established practice.

[David_AVD]

If the Drain-Source voltage was constantly 4 Volt in this application, the circuit would never output more than 11 Volts whatever you fed it on the left side - 18 or 24 Volt.

I think it was the gate voltage being discussed, not the D-S voltage.

[c4757p]

quantumvolt - sure, the op amp can put out 15V. But how will you get that out of the MOSFET? If Vgs is less than 4V it will not output anything.

[quantumvolt]

Yep my mistake  . I am so old I forget the source is not a base...

Then the case is solved: Maximum theoretical output less then 15 volt due to the 7815, minimum gate voltage on the regulating series element 19 Volt and minimum 20 Volt supply for the op amp swing  . Hence the op amp must be fed from the higher unregulated DC voltage. Then I also learned something today ...