Question on resistor label.

[davelectronic]

My eyes glazed over when I saw this was Yet Another Thread on the infernal '723...

But, I did noticed that no one correctly answer the OP's actual question: the "sc" in Rsc stands for "short circuit". This resistor is sized to produce around 0.6V of drop when the desired maximum current flows through the pass transistor, at which point it will provide enough forward bias to turn on another transistor that starts shunting away current from the pass transistor, resulting in automatic overcurrent protection. For example, to trip at 5A you would select a 0R12 resistor for Rsc.

The trip level is roughly defined and responds somewhat linearly to overcurrent (that is, acts like a CC regulator during moderate overloads) so it is best to set the trip level well above the max operating current for the regulator but, of course, well within the SOA of the pass transistor(s).

Also note that the presence of Rsc automatically increases the output impedance of the regulator so voltage feedback sampling should be done after it.

So not sense current ? Seems an odd label to give it, although I understand your explanation.

[Ian.M]

Either - However, expanding  R_SC as R_{Sense_Current} makes more sense from a practical viewpoint as one of the pins it connects to is called 'CURRENT SENSE', and from a semantic viewpoint as it will cause the regulator to limit on all current overloads (if implemented properly) not just on short circuits

[MagicSmoker]

This is more of a semantic argument, than a technical one, but historically speaking, Rcs refers to a current sense resistor, while Rsc refers to the resistor which sets the short circuit trip current in the classic two-transistor current limiting circuit, which is precisely what is used in the infernal '723.

To put it another way, when the resistor supplies a signal that an error amplifier uses to regulate current you might call it Rcs, but when it is sized to drop ~0.6V at a certain current to then turn on a transistor which shunts base drive away from the pass transistor, you call it Rsc, because then its purpose is to protect the pass transistor from shorts.

But, like I said, this is more of a semantic argument. 

[davelectronic]

This is more of a semantic argument, than a technical one, but historically speaking, Rcs refers to a current sense resistor, while Rsc refers to the resistor which sets the short circuit trip current in the classic two-transistor current limiting circuit, which is precisely what is used in the infernal '723.

To put it another way, when the resistor supplies a signal that an error amplifier uses to regulate current you might call it Rcs, but when it is sized to drop ~0.6V at a certain current to then turn on a transistor which shunts base drive away from the pass transistor, you call it Rsc, because then its purpose is to protect the pass transistor from shorts.

But, like I said, this is more of a semantic argument. 

Quite a bit to get my head round, but very interesting, I'm picking up vibes, your not an LM723 fan ? I know its been extensively used over the years, even in retail units. Do you think its problematic  ? Or is it just "it gets everywhere" curious on your thoughts of the chip.

[MagicSmoker]

Quite a bit to get my head round, but very interesting, I'm picking up vibes, your not an LM723 fan ? I know its been extensively used over the years, even in retail units. Do you think its problematic  ? Or is it just "it gets everywhere" curious on your thoughts of the chip.

I don't have any real problem with the '723 (where ' is replaced with LM, or UA, or about a dozen other manufacturer prefixes) it's just that it is - literally - a 50 year old IC that isn't all that impressive by today's standards. All it does is combine a mediocre reference with a slow and drifty error amplifier, a paltry 150mA pass transistor, and another transistor for short circuit protection. For a 13.8V/10A PSU it will get the job done, I just cringe a little when I see people try to make a "lab" power supply with it.

[davelectronic]

Quite a bit to get my head round, but very interesting, I'm picking up vibes, your not an LM723 fan ? I know its been extensively used over the years, even in retail units. Do you think its problematic  ? Or is it just "it gets everywhere" curious on your thoughts of the chip.

I don't have any real problem with the '723 (where ' is replaced with LM, or UA, or about a dozen other manufacturer prefixes) it's just that it is - literally - a 50 year old IC that isn't all that impressive by today's standards. All it does is combine a mediocre reference with a slow and drifty error amplifier, a paltry 150mA pass transistor, and another transistor for short circuit protection. For a 13.8V/10A PSU it will get the job done, I just cringe a little when I see people try to make a "lab" 3power supply with it.

Yes I can see what you mean, it's output to drive other transistors is quite low. And I can remember seeing it in bench cb psu's back in the 80's. Every thing is smps now days, so I guess not much call to design another high tech linear regulator really.