I have built a circuit so simple I could not image that anything could go wrong. Ha!
It has a Li-ion cell feeding a boost circuit that raises the voltage to about 28V. This is fed into an LM317 connected as a constant current source with the adjustment resistor variable to make it easy to dial in pretty close to exactly 10 mA. I have not figured out how to move the decimal point on the meter but I have measured the result pretty carefully and it does, indeed, read 100.0X at 10 mA. I was surprised to get four stable digits, expecting to have to put tape over the last two.
The output of the constant current source is connected to one banana jack, the other jack goes to ground which is also the minus terminal of the boost converter.
Across the jacks is a 30 V voltmeter. A shorting switch is across the jacks so it can be closed while I adjust the current to 100.0X.
I had expected the voltmeter to read zero or maybe flutter the fourth digit when the shorting switch is open because there (it seems to me) should be zero current flowing. But actually it is just barely under the voltage at the output of the boost converter; also, the meter reads 000.77 which is .077 mA.
Attached is a copy of the schematic with voltage readings. I have not included in the schematic the power sources which light up the meters but they are completely independent of the circuit.
There is something I don't know or some part of the data sheet I did not read or some other stupid mistake. Help is appreciated.
It has a Li-ion cell feeding a boost circuit that raises the voltage to about 28V. This is fed into an LM317 connected as a constant current source with the adjustment resistor variable to make it easy to dial in pretty close to exactly 10 mA. I have not figured out how to move the decimal point on the meter but I have measured the result pretty carefully and it does, indeed, read 100.0X at 10 mA. I was surprised to get four stable digits, expecting to have to put tape over the last two.
The output of the constant current source is connected to one banana jack, the other jack goes to ground which is also the minus terminal of the boost converter.
Across the jacks is a 30 V voltmeter. A shorting switch is across the jacks so it can be closed while I adjust the current to 100.0X.
I had expected the voltmeter to read zero or maybe flutter the fourth digit when the shorting switch is open because there (it seems to me) should be zero current flowing. But actually it is just barely under the voltage at the output of the boost converter; also, the meter reads 000.77 which is .077 mA.
Attached is a copy of the schematic with voltage readings. I have not included in the schematic the power sources which light up the meters but they are completely independent of the circuit.
There is something I don't know or some part of the data sheet I did not read or some other stupid mistake. Help is appreciated.
Attachments
-
104.6 KB Views: 36