OK, I’m about ready to tear my hair out.
I started this project last year, first because I wanted a variable linear power supply to work on another upcoming project. I have lots of OLD parts from years ago, and wanted to use them as much as possible although after a lot of grief trying to implement this with just 741’s, I did break down and buy some quieter op-amps.
The supply itself wasn’t a problem. It is based on a LM723 with 2N3053/2N3055 pass transistors. Output is 0–30V, 0-2 A. Ground is depressed with a 7805 regulator to achieve the 0V part of the range.
As I said I have lots of old parts and wanted to use one of the analog meters for displaying the current.
This led to a Current sense and limit circuit that runs on separate supplies to avoid Common Mode limits. This system reads a 0.1Ω sense resistor on the board of the main supply.
To allow me to read currents to greater precision, I designed a system to auto scale, with ranges of
0–20mA, 0–200 mA, and 0–2 A.
This measurement system consists of
I started this project last year, first because I wanted a variable linear power supply to work on another upcoming project. I have lots of OLD parts from years ago, and wanted to use them as much as possible although after a lot of grief trying to implement this with just 741’s, I did break down and buy some quieter op-amps.
The supply itself wasn’t a problem. It is based on a LM723 with 2N3053/2N3055 pass transistors. Output is 0–30V, 0-2 A. Ground is depressed with a 7805 regulator to achieve the 0V part of the range.
As I said I have lots of old parts and wanted to use one of the analog meters for displaying the current.
This led to a Current sense and limit circuit that runs on separate supplies to avoid Common Mode limits. This system reads a 0.1Ω sense resistor on the board of the main supply.
To allow me to read currents to greater precision, I designed a system to auto scale, with ranges of
0–20mA, 0–200 mA, and 0–2 A.
This measurement system consists of
- A differential sense amp, X10, Current Sense Out (CSO)
- A low pass filter to remove 60 Hz hum
- Voltage references
- A X10 amplifier
- A LM311 Comparator to detect when current sense is greater than 20 mV.
- Another LM311 Comparator to detect when current sense is greater than 200 mV
- Another X10 amplifier
- TTL logic to generate control signals for switching
- A CD4016 Quad bilateral switch to select the proper signal to the meter control/Full scale adjust. The three input signals are, CSO, CSOX10, CSOX100. The output is 0-2V.
- A 741 buffer followed by a resistive divider to the meter.
- The ranges are not congruent, a sense voltage 10 or 100 times another does not reliably produce the same reading as it should. Seems to be a linearity/accuracy problem.
- Sometimes when I start the supply at or near 0 volts, the current sense system shows a negative current even though independent measurement shows a positive voltage. The voltage is appropriately 0 or positive coming out of the differential sense amp, but negative after the low pass filter, (about -150mV). This self-corrects after raising the Power supply voltage enough.
