I've been looking to build a linear power supply with current limiting and output adjustable down to 0V and I found this circuit in a LM317 datasheet.:
So I've built it on a breadboard with minor changes. I used J111 JFETs as current sources, with 1k resistors between source and gate to limit the current to about 6.5mA. Negative voltage rail is provided by a 555-based charge pump, outputting about -13V at this load level. And while this works and regulates down to 0V, it can be adjusted even to negative voltages. When using in CV mode, it can go down to negative few-hundred millivolts, consistent with 1.25V minus the voltage drop across D3, D4. When the current limit is set to minimum and voltage to maximum, I can get the output to go as low as -4V!
I think this should not be the case, since the design above uses a polarized tantalum cap on the output. Even if it's expected, I would like to eliminate it because I don't want my circuits damaged from negative voltage when I turn the pot too far. So far I thought about using a more precise shunt reference below 1.25V in place of D3, D4 and/or adding fixed resistors in series with the pots to limit their regulation range.
So to sum up my questions are:
- Is this supposed to happen or am I doing something terribly wrong?
- What is the best way to limit regulation to positive voltages/currents only?
Thanks in advance.