| Electronics > Beginners |
| DC dummy load mosfet ringing |
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| TechJunkie97:
Hi, I am making a DC dummy load as Dave talked in one of his old videos. However, when I probed at the sense resistor, I got some oscillations. I googled and found similar circuits for dummy load using various resistor and capacitor combinations in the loop but I still got oscillations (but with less ringing). I got the best results with the attached schematic but the oscillations were still there (see attached screenshots). I was wondering what is causing these oscillations and how can we get rid of them. Thanks! |
| MarkF:
I don't see anything off hand. Your sense resistor is quite large. I don't have a R1 in mine and my R2 is much smaller. |
| MarkF:
Consider a +IN voltage of 0-0.5V which yeilds a maximum of 0.5V across the sense resistor. Then, select a Rsense for the desired max load current. I would use a 10:1 voltage divider on the input and a small cap. Your sense resistor would have a 10V drop with a load current of 1A. |
| TechJunkie97:
Thanks MarkF for your reply. I was just trying the circuit on a breadboard and didn't want to deal with high power dissipation right away. That's why I chose a large 10 ohm sense resistor. That way with 2V input, I was getting around 200mA i.e. around 2 watts. I also found the problem with my circuit. I put a 1 uF capacitor across the 12V rail on the breadboard and the signal is much cleaner now. I have seen Dave talk around by-pass caps so many times but never used his advice. Now I have learnt my lesson :) |
| MarkF:
I was wondering if it was noise on the power rails but dismissed it. Glad to hear you found the problem. If you want to build out my dummy load, I can give you my design files (Diptrace). |
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