Series pass element

[JJalling]

Hi all,

please excuse my (probably rather stupid) question, but I can't find a clear answer. I have spend quite some time in the weekend, looking at power supply schematics.

What is the advantage of using a LM317 in this schematic (http://www.electro-tech-online.com/attachments/lm317-png.18890/), rather than a NPN transistor (maybe darlington)?

BR Jonas

[digsys]

The LM317 has integrated short circuit limiting and thermal shutdown for a start.

[rob77]

and the most important part is that the LM317 itself is doing the regulation... the opamp is there just to provide a "lift" - the output will be regulated to opamps output + 1,25V by the LM317. so if the opamp will output 4V, the output will be 5,25V

[JJalling]

Yes ok, I see. This was actually the image I wanted to post. Isn't the opamp doing the regulation in this circuit?


BR Jonas

[tautech]

Yes ok, I see. This was actually the image I wanted to post. Isn't the opamp doing the regulation in this circuit?


BR Jonas

No. The 317 is doing the "work", the opamp is just controlling it.
Remember a 317 is an adjustable regulator.
The ADJ bias is set by R5,6,7 and the opamp is adjusting this like a variable pot.

[T3sl4co1l]

No..?

Yes, precisely: the op-amp is doing the control.  The 317 is serving only as a current amplifier, a voltage follower.

The advantage over a BJT is the low input current and negative offset voltage (ADJ = Vout - 1.25V, rather than Vbase = Vout + ~0.7V).  Therefore, any general purpose op-amp can be used.

The circuit will still have much higher dropout than doing it discrete (such as with a R2R op-amp and high-beta follower transistor, or any op-amp followed by an amplifying / level shifting / current gain stage, which would only need a transistor or two), but this way keeps the parts count down, which is likely important to the editors and readers of whatever magazine I presume this is from.

Tim

[JJalling]

Ok, so partcount and complexity is the general answer as I read it.
I don't know if the image is from a magazine or not - i was too lazy to draw my own schematic so found it on google's imagesearch.

Thanks

BR Jonas