i didnt quite understand the issue, but if you just need -30V you can get another 30V power supply and join them in series (the ground wire from the old and the positive wire from the new supply), then you can use the joined wire as ground, the ground side of the new supply as -30V and the positive of the old supply as +30
You just need to be careful that the supplies' outputs are completely isolated from mains. Otherwise you'll end up with a lovely spark show.
Measured voltage is completely dependent on what is used as a reference. If you were to measure the voltage of a power supply set to +30V with a voltmeter whose leads were backwards (ie positive on the voltmeter connected to negative on the power supply, and negative connected to positive), then you would measure -30V. All this is to say that if you connect your 30V supply "backwards", you will have -30V. Now, there may be some circuitry in the power supply that only allows current in one direction, and it could be that it's able to source a different amount of current than it's able to sink, so be sure to check its manual.
I've tried -12V from psu i've got lying around but it didn't work.
I'd advise caution against arbitrary decisions with electronics — both for your own safety and the safety of the equipment.
It would be weird for say, a 12V-2A PSU to not be able to source and sink 2A. The current direction didn't change from the PSU perspective, only from the load. If you have multi-rail power supply, then yeah it may have different source-sink capabilities for different rail
The current direction didn’t change from the PSU perspective, only from the load.
You are absolutely right. I don't know why I was thinking the direction changed.
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