| Electronics > Beginners |
| Dummy load circuit mods - why? |
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| Lee697:
G'day.... I put this together 5 years ago, based on Daves dummy load circuit (EEVBlog #102). I'm thinking of rebuilding it, so have been drawing out the circuit. I can't recall why I have the ceramics across the output/inverting input, or why the resistor is in series with the inverting input or output is there. Searching doesn't reveal the source of the mods (and I'm pretty sure I didn't come up with them myself!). Can anyone help out? Why would they be there? The circuit works well between near zero and 2A, although the lone MOSFET on a small heatsink gets a bit 'warm' at high current..... Cheers! |
| schmitt trigger:
Loop compensation? |
| Jwillis:
The mosfet is acting like a precision variable resistor controlled by the opamp . So it a sense its not so much passing current to a load but sinking current to ground. So the Mosfet becomes the load . So its going to get hot and will require a very large heat sink .The resistors as schmitt trigger are there to compensate any change in the load voltage back into the opamp.The caps are to prevent oscillations. The resistor between the ouput and the gate I think also dampens any oscillation but I'm not sure about that one. |
| ledtester:
The capacitors are there to prevent the circuit from oscillating. The amount of capacitance you need will depend on the transistor you are using for the load. If you have a scope you should observe the voltage at the sense resistor as you increase the current. With no capacitance you probably will see oscillations beginning at 500 mA to 1 A of load current when using a MOSFET. |
| Jwillis:
--- Quote from: ledtester on September 28, 2019, 03:01:24 am ---The capacitors are there to prevent the circuit from oscillating. The amount of capacitance you need will depend on the transistor you are using for the load. If you have a scope you should observe the voltage at the sense resistor as you increase the current. With no capacitance you probably will see oscillations beginning at 500 mA to 1 A of load current when using a MOSFET. --- End quote --- But what is the resistor between the output and gate for? |
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