7912 needing minimal load?

[Pierre]

I have added an aux +/-12V output to my switching power supply. I use a single aux. winding, and half-wave rectifying to get +/- outputs.
The +12V is fine with no load, but the -12V doesn't regulate (it outputs up to -20V) until I load it with about 10mA or more.
This doesn't happen with another 7912 model from another manufacturer.

Does anyone know if there are linear regulators like this that need a minimum load? I didn't have notice of this until now.

 

[peranders]

Yes, this is the case. Read the datasheet carefully! Normally it's a couple of mA's.

 

[Eva]

That strange behaviour may be also caused by oscillation, so pay attention to regulator input and output decoupling, and check also that any paralleled decoupling capacitors are not resonating mutually with the help of PCB track inductance.

Consider also that too high amounts of SMPS ripple and switching glitches may disturb regulator operation when applied to its input.

 

[Pierre]

Thanks.
The transformer has a single secondary. One of them is used as Aux GND. The other goes to two MURS120 diodes (ultrafast). For the negative rail, the anode of the diode goes to the regulator input, with a 470uF Panasonic FC capacitor in series. The output goes to another identical capacitor.
I observed the output and saw no signs of oscillation (I experimented that once with a LM1084-T-Adj and it was really oscillating)...
Well, it seems to be cured with 10mA or so...