: 50mA-500mA constant Dc current source

[Bob S]

Maybe I misread the specs, but if the output range is 500 mA max into a 1 to 100 ohm resistor, then you need to use a power supply capable of 60 volts at 500 mA, and output transistor ratings of Vceo of 60 volts and 50 watts. I suggest a PNP darlington TIP125, which has an hfe of 1000. Also I suggest that the output load be ground referenced, so with the suggested PNP darlington, the sense resistor (about 10 ohms) is between the +60 volts and the emitter, with the load connected between the collector and ground.

 

[ksac]

Hey, I know this thread is about a year old, but I was working with the same thing, so thought I'd post here itself..
Even I derived the output current as I= (+Vs-Vin)/R
but I found a book that says I=Vin/R
and what's more! I tried the circuit out in lab and found the latter to be the current.
I am confused..
Any thoughts anyone?

 

[berkeman]

Hey, I know this thread is about a year old, but I was working with the same thing, so thought I'd post here itself..
Even I derived the output current as I= (+Vs-Vin)/R
but I found a book that says I=Vin/R
and what's more! I tried the circuit out in lab and found the latter to be the current.
I am confused..
Any thoughts anyone?

If you mean for the circuit shown in post #16, your first equation is correct. Your 2nd equation can apply to other topology current source/sink circuits. For example, if you are sensing the current on a low-side resistor, with the load on top of the resistor, and the bottom of the resistor grounded. Do you see how that equation would then apply?

 

[ksac]

Well my circuits exactly the same, except there's no voltage divider at V_in (I've used a zener for reference voltage) and a resistor between the op amp output and pnp base. My sensing resistance is between V_s and emitter (so no low-side current sensing) and the load between collector and ground.

How would low-side sensing work here? Emitter to V_s, load to collector and the other end to R_sense, which is grounded, and the inverting terminal of op-amp connected to the junction of R_load and R_sense? Is that how?

Thank you for replying!