This is a subproject designed for my SONATA tube tester and analyzer. This circuit can measure a wide range of current utilizing two ranges and is powered directly from input voltage that can span from 60 V to 280 V. This makes it a suitable block for building tube testers and other test equipment that deals with 0.15 to 150 mA direct current and 60-280 V supplies.
The heart of the circuit is the op-amp (U2) circuit with shunt resistors (R4, R5). Shunt resistors create a voltage difference when the current flows through them. This potential is then used by the differential input of the operational amplifier to control the FET (Q3) that creates a voltage divider, converting the current to voltage. This circuit was taken from the Art of Electronics book; however, it has one major problem when a high dynamic range of input voltages is expected. Namely, the circuit is supplied from the same rail that the measured current passes through using a Zener diode (D2) to drop the voltage and create an artificial ground for the op-amp. This circuit is fine for one specific supply voltage because the proper value of the resistor from the anode of D2 to ground can be chosen for small power dissipation. However, if the input voltage increases, the power dissipation can become very large. On the other hand, for a small voltage, the circuit might stop working. This problem was solved using a voltage-controlled current sink. No matter the input voltage, the op-amp U1 will control the Q4 in a way that the current through the Zener diode is roughly constant. Such implementation still causes significant power dissipation in the artificial ground network, but it is much better than using a small value resistor. In addition, the range of measurement can be changed by providing 3.3 V or 5 V at J1. For switching ranges, MOSFET Q2 is used. When a small value of current is measured, both shunts should be used in series. When a larger current is being measured (>15 mA in the proposed version), the MOSFET is open, and only R4 is used.
Different wire wound resistors were used as a load. I have used my homemade HV-DC PSU that can be adjusted from 50 V to 300 V to sweep the input voltage.
Offset voltage was also measured, this is a value that appears on the output even withouth load with input voltage present. Some offset is expected since output voltage divider used for measuring the HV line potential also draws small amount of current. Due to imperfections of the circuit this is most likely not the only reason but the good news is that this offset is linear and can be very eaisly removed in software.
Ratios of mV/V also known as sensitivity were also measured for tested cases.
Current sink was also measured, using potentiometer is was set to maintain 10 mA of current draw.
Below is the schematic; it also includes a second method of artificial ground optimization that I have tested only in simulation. I think the current sink is much better since it provides a constant value of current instead of rapidly switching it between two values.
PCB includes screw terminals that can be used to attach cables. In addition, PCB has also fuse holder and footprints for protection diodes at the sensing outputs.
Remember that this is a high voltage project! You are working on your own risk, watch out where you put your hands.
In the future I will upload the gerbers and KiCad project on my GitHub. If you need it now, please email it.
sp6gk 'at' protonmail.com
Here is demonstration of a restored radio from 1938.