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| LM358 in Consant Current Circuit Problem |
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| orolo:
I have built the circuit and tested it, and it works (tested the load with an ammeter), at least up to 1 amp @ 40V. I have used your parameters, 40V for the source, an MC7812 regulator for the opamps, one LM358, etc. The schematic is attached below. If you are interested, I can post some images and probe the circuit with an oscilloscope to see how it behaves. The load current increases in an apparently linear way with the pot, but I haven't checked the precision. Some obvious points: -> If you connect the current sink directly to 40V, the mosfet will dissipate a lot of power (about 40W at 1 amp). If you are planning to go into the multiamp range, you'll need a massive heatsink and probably a fan. Study carefully the thermal aspect of the project. A quick look at the IRFZ44 datasheet tells it manages up to 50 Watts, so without source resistance that's 1.25 amps per transistor. Without a good heatsink, your mosfets will die very quickly. -> I designed the voltage divider thinking in very high currents, and used a low quality potentiometer. Design the divider carefully for the desired current range, and use a quality pot. The current is controlled by a voltage divider, which is buffered by one half of the 358, then divided by 5, and sent to the regulator. -> In my circuit I used an alleged IRF1405 bought from ebay, who knows what's really inside. It works, though. The 358 is also an ebay cheapy, bought in a lot of 50. -> Don't forget to load the 7812 with at least 5mA for it to regulate reliably. I used an LED for that. To increase stability, I thought of using the opamp to drive a 2N7000 (from the 12V supply!), which in turn drives the bigger mosfet. The 2N7000 has much lower capacitance. The idea worked in simulation, but seemed to give no real advantage. Simulating some more, I got oscillations for very light load currents with no gate resistor: with a gate resistor, the oscillations went away. |
| TheInfernoMan:
Thanks for your work on your schematic, it is near to my current schematic I think :) I will solder my PCB tomorrow and test it out, I think i will post my results as an answer. Temperature is not the problem in my case. The MOSFET is not directly on the PCB it is mounted to a CPU Cooler which is capable of dissipating to 100W heat. The limitation comes from the MOSFET (IRFZ44N) which can only dissipate around ~90W maximum (Infineon Datasheet). So the CC Load would be rated to 60W at the End (to go safe). I used the L7805 for a voltage reference and I included a minimum load in my schematic, so it would be not the problem. Precision is not the most important point, important is that the current is constant, so that I can record a nice decharging graph of a battery for example. With kind regards, TheInfernoMan :) |
| Kleinstein:
To ensure stability with an inductive source, one might need a snubber (R and C in series) at the output. Something like 1-10 µF and 10 Ohms should be about the right size. Also a fuse in series is a good idea - just in case the FET fails with a short, which more like the normal mode of failure. The permissible power at the MOSFETs is not only limited by there Ptot rating and thus the chip temperature. There usually also is a SOA limit, that can be significantly lower if the voltage is relatively high. This especially true for relatively modern MOSFETs made for lower voltage (e.g. < 200 V). So if you need 40 V operation, look for FETs specified for 200 V switching. Something like a IRFP250 is such an older type. Different from switching applications you want types with a low trans-conductance compared to the rated power, and not the lowest R_on. For the IRFZ44 mentioned here 1.5 A at 40 V could already be too much - the DS I found has no curve specifying a DC SOA, so no guarantee it works. With MOSFETs of a dubious source, like Ebay, be prepared that there might be a smaller FET inside and thus sudden failure is possible. You would be lucky if you get an old sucker instead - but this is possible too. |
| TheInfernoMan:
So I've built the schematic up on a pice of perfboard and it works very well. One improvement possibility is that it is hard to set a good Current with the help of the potentiometers (also because I used a 100k and a 25k (for fine adjustment), but it looks like its not a linear adjustment (can that be?)). So at the End, I am really thankful for the help of all here and from my side the topic is closed :) ;D Regards, TheInfernoMan |
| Seekonk:
Did you by the pots as linear or are they out of some audio equipment. Those are not linear. Set pot midway and measure resistance. |
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