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opamp output not 'calibrated'

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MrAl, I'll try a LM358 and also what MikeMl suggested. With the large time constant you are talking about, would that introduce some delay?


Hello again,


Ok good with the LM358. The time constant i was talking about was to be as large as possible but without interfering with the requirements of the circuit. This means you make the resistor as large as possible but without bothering the required speed of the circuit. You also dont need a 1uf cap but can use a 0.1uf cap that way too. It would probably be better to get the main problem fixed first though before adding to the complexity.

Note that in my notes i am assuming that you are testing the op amp circuit alone, without the current resistor, by inputting a voltage of say 0.1 volts, but if you do use the current resistor then you measure the voltage that gets to the op amp circuit which should still be in the range 0.1 to maybe 0.2 volts. With 0.1v input you should see about 8v output. If not, try the LM358 and let us know what happens.
 
current sense

Seems like it has to be one of the 2 problems mentioned above. Maybe you could give a little more info. How are u measuring the output - scope, plc trip or??? If it is the actual circuit shown can you scope the .2 volts across r sense to see if there is a capacitive spike due to the stray capacitance of the big inductor? Or as the guys have pointed out both inputs to the op amp should go directly to Rsense to eliminate any trace resistance. If you are measuring with the actual circuit you might short out the inductor so you only measure the resistive load.
 
Regarding crutschow suggestion about the inputs of the opamp - the non-inverting input is fed straight from Rsense but for the inverting input, they are from 'separate' ground points. I have fix this, in that Rsense and Rn are now connected at the same node.

I have also added a 10uF cap across the non-inverting terminal. I am measuring the voltage across Rsense (V-Rsense) using a DMM. Added a scope across and no spikes can be seen. However I note that there is quite a large difference in V-Rsense between what's measured from the scope and the DMM. e.g. on the DMM V-Rsense is 95mV and the scope reads 109.4mV.

Based on the above discussion I have made the changes described above. There seems to be an improvement - mainly when I connected the grounds 'together'. The gain has now dropped from 120 to about 84.

Could not get an LM358...
 
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Hi,


Sounds like you may have found the problem...
 
MrAl, thanks to you guys part of the problem has been identified. I still can't understand why the gain fluctuates so much? ideally it should be 81 but it varies between 80 and 85 for different inputs.
 
What different inputs?
 
Your op amp input offset voltage can make it appear that the gain varies with input. Measure the op amp output with the MOSFET turned off (gate at zero volts).
 
Hello again, It sounds like something else very basic is wrong. I would suggest testing the amp all by itself first and get that working without question. To do this all you have to do is input 0.2v right to the terminals (after disconnecting the sense resistor of course). This will tell you if something is wrong beyond the sense part of the circuit. If it works ok, then it must be in the sense part. In other words, we break the circuit into smaller parts and test each part separately and carefully.
 
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