DC dummy load mosfet ringing - Page 1
Forum section: electronics / beginners

author: TechJunkie97
author_rank: Contributor
timestamp: April 07, 2019, 12:22:40 am
content: 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!

author: MarkF
author_rank: Super Contributor
timestamp: April 07, 2019, 01:02:33 am
content: 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.

author: MarkF
author_rank: Super Contributor
timestamp: April 07, 2019, 01:14:03 am
content: 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.

author: TechJunkie97
author_rank: Contributor
timestamp: April 07, 2019, 02:06:11 am
content: 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

author: MarkF
author_rank: Super Contributor
timestamp: April 07, 2019, 02:10:53 am
content: 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).

author: Doctorandus_P
author_rank: Super Contributor
timestamp: April 07, 2019, 04:13:50 pm
content: Another thing to consider is the return paths of the currents, especially if there are any high current paths (200mA is high current in this context).Such a current generate a voltage drop over a piece of wire and this can easily be a cause of unwanted feedback if it is picked up by the opamp.Often the best way is to use a star ground connection, and give each electronic compnent a separate wire to the star point.In the circuit MarkF posted that will be 10 ! wires connected together on one end.For breadboarding I have made a star ground connector, which is made of a single row 0.1" header (square pins) and al the pins are simply soldered together. If you make such a connector with 6 pins and put it in a breadboard, then you have 6x4 = 24 holes left to put wires into your star ground connection.

author: TechJunkie97
author_rank: Contributor
timestamp: April 08, 2019, 03:10:29 am
content: Thanks Doctorandus_p for the tip. I'll be more careful with the ground return paths.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: April 09, 2019, 12:09:15 am
content: I just had a closer look at your schematic.TL072 is a bad opamp to use with a grounded power supply. For this opamp there should be at least 3 volt headroom between the inputs and either power supply voltage. It's not a bad opamp, but it is definately not a rail-to-rail opamp.An opamp like the LM358 can work with input voltages very near to the GND level and is a better choice.Had an extra peek at its datasheet, and it looks like the LM358 still works OK with an input voltage of 300mV below the GND pin.You have also connected "everything" to the same 12V rails.Consider this scenario:The MOSfets draws a lot of current from the 12V rails and it sags a bit. which results in a lower voltage on the non-inverting input on the opamp via the potentiometer. A lower voltage on the potentiometer results in the opamp reduicing the gate voltage to the MOSfet, which may result in a rise on the 12V rails and the cycle repeats, which is a cause for oscillations.As MarkF already wrote, it's much better to use a stable reference for the potentiometer, which decouples the current setpoint from the power supply. An easy choise would be 1.25V from an LM317L (in TO92) or even a simple led with resitor and a stabilizing cap. (LED voltage depends mostly on the color of the LED.After a reference voltage has been chosen, then calculate the right current shunt resistor for the wanted maximum current at that reference voltage.