: 50mA-500mA constant Dc current source

In summary, Kari's goal is to design and build a 50mA-500mA constant current source. She found a link for a Circuit Center online store that sells a 2N3055 transistor and an LM348 op-amp, but her calculations show that the output voltage will not be independent of the load. She needs help understanding the circuit and how it works, and she asks for someone to help her.
  • #36


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.
 
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  • #37


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?
 
  • #38


ksac said:
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?
 
  • #39


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

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

Thank you for replying!
 
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