Hello,
I am developing an automatic test rig for some electric machines. One part of this rig is a high-side current sensor that I put together. The schematic is attached.
How it is supposed to work is that the (rail to rail) op amp sees the voltage on V- (either 6 or 12V), and adjusts its output into the transistor base to shunt away excess current from V+ to make the inputs equal. The extra shunted current is proportional to the drop across the sense resistor, and is grounded through the emitter resistor. I amplify the voltage across that with a simple NI amp and read that voltage.
I originally built this rig on breadboards, which I still have. The sensor works fine on the breadboard. I recently designed and ordered some PCB prototypes for the rig, and am in the process of finishing up the project.
However, on the PCB the current sensor circuit does not work properly.
The sensor seems to work fine when using the 12V supply; the output of the first opamp is between 0.1V and 2V. However, when using the 6V supply the output of the first op amp is always >11V; the transistor saturates and the emitter voltage is also 11V, and as you can imagine the output of the NI amp is therefore railed.
There is no difference between the components on the breadboard and the pcb, save for part footprints. There is no difference between the routing on the breadboard and the routing on the PCB, save for wires vs. traces.
I have eliminated ground loops, bad traces, shorts, and bad components as possible problems. I am just at a loss now. Any help is appreciated. Thanks!
-SK-
I am developing an automatic test rig for some electric machines. One part of this rig is a high-side current sensor that I put together. The schematic is attached.
How it is supposed to work is that the (rail to rail) op amp sees the voltage on V- (either 6 or 12V), and adjusts its output into the transistor base to shunt away excess current from V+ to make the inputs equal. The extra shunted current is proportional to the drop across the sense resistor, and is grounded through the emitter resistor. I amplify the voltage across that with a simple NI amp and read that voltage.
I originally built this rig on breadboards, which I still have. The sensor works fine on the breadboard. I recently designed and ordered some PCB prototypes for the rig, and am in the process of finishing up the project.
However, on the PCB the current sensor circuit does not work properly.
The sensor seems to work fine when using the 12V supply; the output of the first opamp is between 0.1V and 2V. However, when using the 6V supply the output of the first op amp is always >11V; the transistor saturates and the emitter voltage is also 11V, and as you can imagine the output of the NI amp is therefore railed.
There is no difference between the components on the breadboard and the pcb, save for part footprints. There is no difference between the routing on the breadboard and the routing on the PCB, save for wires vs. traces.
I have eliminated ground loops, bad traces, shorts, and bad components as possible problems. I am just at a loss now. Any help is appreciated. Thanks!
-SK-
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