Everything works fine, 1V/A into the comparator and it actually limits the current. But when it limits the current, the voltage goes down and the current as well. Then the current starts to rise and so on. This results in an oscillating current control loop.
I have been searching and reading, but it seems that nobody experiences this problem. What am I doing wrong here?
Second question is the location of the current sensor. What are the pros and cons of placing the current measurement before or after the regulator or even in the negative line.
Hopefully somebody can help me out.
Thanks,
Remon
Thanks void, I will try this tomorrow as it is almost midnight now. I will post my findings here of course.
Dannyf, I have seen the dual diode designs as well. This design requires a negative power supply to work. That is something I try to avoid.
Two issues:
one, for most of the designs, the constant current portion oscillates. That's by design thus unavoidable.
In all my designs (three so far) I do exactly that, 10nF capacitor on the op-amp that controls voltage or current since there is no feedback resistor.
Current limiting sense resistor behind the voltage regulator so that the output voltage is not offset by the voltage drop on the sense resistor. I have had good results with 0.05R (1%) or two 01R 1% in parallel, and an Av of about 10-15 at the diff op-amp stage. Unless you have sense wires on the load, in which case it does not matter where the sense resistor is.
I presume I could have the sense resistor on the ground (negative) terminal, and therefore measure the voltage drop directly on it without a need for the differential amp, I would only need a comparator amp. I am not sure of the implications that the voltage drop might cause eg putting two supplies in series to make a symmetrical one. Maybe it's a good idea.
Uhm... You wil NOT get this stable!
Blackdog
Not the LED test for the current limit again. The current limit is their to protect the power supply not the load, if the load is drawing more then anticipated chances are it’s already fried.
Not the LED test for the current limit again. The current limit is their to protect the power supply not the load, if the load is drawing more then anticipated chances are it’s already fried.
The ADJUSTABLE current limit is precisely there to protect the load, down to a few mA. On my supplies I go down to 0mA, ie the current limit starts from negative values.
Why does a load draw more current then anticipated? Either inrush or something is shorted. If your current limit is hit before your voltage setpoint your current will likely have minimal overshoot if thats what your talking about. Thats because your current limiter has control of the loop early. That assumes the Voltage loop hasn't already pumped up the the voltage to max out.
If the current loop controls the loop before the voltage loop then the current loop dominates and you have a current source, with a voltage limit.
Try your test with your supply outputting max voltage (already powered) and current limit set to 20mA then hook it up to your LED let me know what happens to your LED?
I don't think I would want a PSU that kicks in CL fast enough to save a 20mA LED, that would be similiar to what I'm experienceing with the Analog Discovery. It's a bloody nussiance, you want a supply to be able to deliver short burst of peak current. If I want to test LED's I would have a dedicated CC source output on the regulator.
Here is the schematics v2, so please shoot again!
I agree about the floating supply. All my 4 bench supplies are floating, that allows me to power anything and then attach the Ground crocodile clip of the oscilloscope anywhere I want, pretty much.
However I am not sure why having one more stage (a differential op-amp that references the Vsense to ground) in the current limiting circuitry is considered a problem. I believe we are in a thread where people are discussing placing two LM317s in series... Maybe it introduces some extra latency in the limiting, maybe, I have not checked it on the scope. That would come into play if the DUT is very sensitive to ns spikes, like for example I was reading about how to power/drive laser diodes, where even a 1 ns spike will kill them.
Take a look @ this topic: http://www.circuitsonline.net/forum/view/110029/1
It is in Dutch, but i think you can read it...
No power On or Off Glitch.
Fast acting U en I control.
Low output resistence, DC not measureble (depents on were to connect the sense wires)
Max output current limited outside the "i" control loop.
This is a "real" picture of the load stability, Wire = 2x0.5M to my Active load, over de active a 6,8uF Low ERS capacitor.
That why you see a little ringing, short cable's and no capacitor, no ringing!
http://www.bramcam.nl/NA/NA-01-PSU/NA-PSU-30.png
Its a big topic on that forum, a lot to read, i explane a lot why and how.
Happy reading :-)