Electronics > Beginners
LM358 in Consant Current Circuit Problem
<< < (8/9) > >>
orolo:
My own version is quite linear, and fairly accurate, considering the design and components involved. I have tested various currents for several positions of the potentiometer: being a low quality thing, I had to wait some time for the wiper to stabilize. The measurements were:


--- Code: ---  OHMS               CURRENT
    0                0.01uA
   72.7              68.0mA
  109               109.9mA
  114.8             113.5mA
  167.6             176.9mA
  237                0.24A
  440                0.48A
  513                0.56A
  620                0.68A
  745                0.81A
 1018                1.11A

--- End code ---

I tried to use the more precise mode of the multimeter for each measurement, hence the variations in precision. Anyway, it looks pretty linear, and very precise in some of the ranges.

I attach a photograph of the circuit at work in one of the measurements (sorry for the size). I was running out of space in the perfboard, so the mosfet drain is fed via the heatsink  :P . The measurements were made at 40V, as in your description.

TheInfernoMan:
Thanks for your Version  :)

Yeah I think my Potentiometers are not linear, but at the End I think this is not so bad,
because at the End I use the Range between 0-1A and upwards I only need (in most cases) 2,3,4A which I can achieve with the non-linear pots.
orolo:
One thing of note is that the output current of the circuit (my version) follows the formula I =  12·R / (10k + R), so it is clearly nonlinear for high currents. I wanted to see how well the circuit followed this curve, so I plotted it. The matching is pretty good  :) .

Another thing I would test is transient stability under a change of load. What happens if a load resistance is switched on and off, does the circuit ring, and for how long. In simulation there is a bit of ringing, but it dies down rather fast.
Kleinstein:
Ringing and possibly oscillation at the output is expected with a source that is inductive. So for the hard test, add an inductor between the voltage source and the load.

To ensure stability usually an RC element in parallel to the electronic load is used or needed.
orolo:
A final detail: to test the transient behavior, I put a 270 ohm, 10 Watt resistor in parallel with a power mosfet, and used a 555 timer to switch between both in periods of about 0.5s. Then I connected this source to the current sink, after adding a fuse and the RC snubber, just as Kleinstein proposed. I programmed the current to sink 110mA, and made sure that the mosfet and resistor were alternatively conducting the current. Neither the scope nor the multimeter could discern any ringing, and the current stayed stable. If I disconnected the resistor, I could see the square wave as the mosfet switched on and off.

I took a  shot of the mosfet turning on wihtout the resistor, this should give an idea of how the current source starts up. My oscilloscope is not good at all, and the 555 couldn't drive the mosfet to act as a complete switch (the 555 was connected to the 12V supply). The shot is the voltage at the switching mosfet source, with 2V/100us per division, DC coupled. There is a brief peak, and then the voltage stabilizes very quickly.

I think that, with the snubber, the current sink is quite stable. The fuse seems to me also an indispensable idea.

Navigation
Message Index
Next page
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod