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Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
I assume this has a simple solution, since the circuit works fine in theory. The problem is the op-amps aren't working correctly. Every op-amp is a TL071CP, powered by 5 volts, single supply. The one op-amp in comparator mode swings it's output from 4 to 1.2 and back to 4 volts as the voltage on the positive input is increased from 0 to 5 volts. The output sinks to 1.2 when the positive input is around 1.2V. Instead it should stay close to ground and jump to 4 volts when the voltage passes 2.5. The other two amplifying the current sense voltage have 4 volts on their output without any current flowing through the cs resistor. The voltage here should be close to ground until current flows through cs.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141
The single supply is the problem I think.
TL071 input range does not include the negative supply (0V in this case)
You need either a negative supply voltage for the op-amps as well as a positive supply. OR change to op-amps that have input common-mode to the negative supply.
Registered Member #505
Joined: Sun Nov 19 2006, 06:42PM
Location: Yorkshire!
Posts: 329
This caught me out with a circuit I was designing (photodiode zero bias amplifier). I swapped from TL082 to LM358 when a colleague pointed out that the input range did not include anything from 0V to about 1.5V.
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Op-amps behaving strangely
