Foldback current protection for sink circuit

[seyyah]

Say I have a circuit like in the schematic. Is it possible to add foldback current limiting to this circuit for situations like the load is shorted in order to protect the transistor?

 

[KlausST]

Hi,

internet search for "current limit open collector circuit" should give some ideas.

Klaus

 

[srizbf]

your load current is approx. 6mA.
TIP122 required for this current?

 

[seyyah]

your load current is approx. 6mA.
TIP122 required for this current?

Its just for example. The values and components are yet to be determined.

 

[seyyah]

Hi,

internet search for "current limit open collector circuit" should give some ideas.

Klaus

I could not find, generally they are for constant current limit or the load is at the emitter side.

 

[dick_freebird]

"Foldback" implies that the current limit will be further
reduced if the output is driven higher against the limited
current. This is more of a "power limit". You may not
want to get into the multiplication that a true power
limit scheme would entail, adding in an output voltage
term by summing could be "good enough".

If this is discretes based, can you accept adding an
emitter sense resistor (~0.7V to turn on a base shunt
NPN)?

If integrated such that you can depend on matching
and one more transistor costs roughly nothing, you
could go to schemes that impose a lower voltage "tax"
(like using an emitter coupled amplifier to sense a
lesser emitter resistor's voltage, maybe 100mV or so)
and find a place where summing in VOUT/Rxxx
current gives you the output-reduces-further-with-
IOUT profile.

If you find old timey linear regulators which advertise
foldback limiting specifically, their datasheet / app note
schematics might offer details of specific implementations.

 

[KlausST]

Hi,

I could not find

--> there are hundreds, maybe thousands

generally they are for constant current limit

--> how else do you want it?

the load is at the emitter side.

--> No, just a resistor for current measurement is at the emitter, load is at collector

Klaus

 

[seyyah]

Hi,

--> there are hundreds, maybe thousands
--> how else do you want it?
--> No, just a resistor for current measurement is at the emitter, load is at collector

Klaus

The right one(cl.png) from wikipedia has no foldback action it just stops at a certain limit while the other one(fb.png) folds back.

 

[KlausST]

Hi,

if the current limit is not sufficient...

How much foldback do you want?
A complete down to zero or just partly reduce? To what level? What timing? How to go back to normal operation?
A drawing could be helpful.

Klaus

 

[seyyah]

Hi,

if the current limit is not sufficient...

How much foldback do you want?
A complete down to zero or just partly reduce? To what level? What timing? How to go back to normal operation?
A drawing could be helpful.

Klaus

No, partly reducing is enough just to prevent damage from overheating. I estimate 150mA@24V will be drawn continously at max. But I want to add a 100-150mA extra room just in case to handle transients etc. It should folds back as soon as it exceeds 250mA to ~150mA or below and returns to normal once it falls below 250mA.