15-11-2012, 07:37 PM
OK, Dr Mark and David, I surrender and will put a digital ammeter on this.
I noticed earlier that set to low output voltages the LED comes on with no load connected. I think I know why - I've even done some sums:-
When R2 is set to zero the only thing across the output is R1. This means that a minimum of 3mA is flowing through Rs and the resulting 3mV across it is enough to set the 741 off. I found a 0.2Ω resistor but this was the same, in fact shorting it didn't effect a cure as the resistance in the wires seemed enough.
I've studied the LM311 data-sheet which Al supplied, and the SGS-Thomson version, but am still a bit unsure as to how to rig one up. What I need to be able to say it it is "If there's only 3mV difference (in fact we'll say 5mV and play safe) between your inputs stay off."
I want to fully re-build this anyway, and fit a meter, but a comparator would be something new to play with and learn about but I've no idea where to start.
Experience and knowledge is never wasted.
- Joe
I noticed earlier that set to low output voltages the LED comes on with no load connected. I think I know why - I've even done some sums:-
When R2 is set to zero the only thing across the output is R1. This means that a minimum of 3mA is flowing through Rs and the resulting 3mV across it is enough to set the 741 off. I found a 0.2Ω resistor but this was the same, in fact shorting it didn't effect a cure as the resistance in the wires seemed enough.
I've studied the LM311 data-sheet which Al supplied, and the SGS-Thomson version, but am still a bit unsure as to how to rig one up. What I need to be able to say it it is "If there's only 3mV difference (in fact we'll say 5mV and play safe) between your inputs stay off."
I want to fully re-build this anyway, and fit a meter, but a comparator would be something new to play with and learn about but I've no idea where to start.
Experience and knowledge is never wasted.
- Joe
