Daves Dummy load, Question about the sense resistor in the circuit.

[rwgast_lowlevellogicdesin]

Ok so I built a digital version of daves dummy load and so far everything is working the way it should but I am confused about the current sense resistor. Originally I just thought it was there to sense current, but it has to be there in order for the circuit to work. I.e if you take it out and just through a DMM before the test load to measure the current nothing happens. So exactly does this resistor do besides sense current? Is there anyway to take it out and use a hi side sensor or even a hall sensor?

[rwgast_lowlevellogicdesin]

Maybe im not really wording my question right, I guess what im asking is why does the circuit require the low side sense resistor to operate? Is it becuase the resistor is what is acually drawing the current from the attached load? I was under the impression the mosfet was drawing current based on its gate voltage?

[lapm]

Sense resistor is there to provide means to measure current flowing throw.

Its dummy load - variable resistor with some smart to controll how much current it draws. This requires feedback mentioned above...

Just becouse your resistor is able to consume 25 wats, docent mean you want to consume all that power - hence need to control current drawn...

Take out sense resistor and you brake the feedback loop. Your multimeter probably will not have same value of shunt resistor inside it...

[Nermash]

Try searching among Mjlorton videos on Youtube, he did quite a lot experimentation and thinkering regarding the sense resistor in his dummy load build.

[idpromnut]

I have been having issues in my reproduction of Dave's DC load as well, but mine are of the flavour that the pass-mosfet bangs on or off with a very very small window of adjustment (2.5V into the feedback op-amp turns it on, and the mosfet saturates at 3-4V into the feedback op-amp). I thought that this might be a function of the type of mosfet I am using, and specifically the VGS threshold (I was initially using a logic-level mosfet). I swapped this out for a higher VGS mosfet and gt the same thing. I am using a LM358 in the same basic voltage follower -> feedback/control op-amp configuration as Dave.

But to answer the question, the pass 'fet/transistor are there to absorb the power (voltage drop X current) that the sense resistor is not.

Example: if you have a 1Ohm sense, and feed 10V into the load and want to set the load to 1A, the sense resistor should be dropping 1V across it (Ohms Law), and there for the remaining power of the 9V drop @1A must be dissipated somewhere, and this is in the pass 'fet/transistor.