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These are protection (to protect chips, not human "safety") diodes often used with older 3 terminal regulators to prevent latch up. Some older regulators could latch if say the - supply comes up before the +. Putting the diode in series with the ground lead, prevents this reverse latch up, but now the regulation voltage is .6v higher, and regulation is slightly deteriorated as diode voltage varies with temp and current.
A second diode is often used, backwards across the regulator to prevent reverse current flow in the scenario where the unregulated supply drops faster than the regulated, and post regulator reservoir caps discharge back through the regulator. This is not that common but can occur if multiple regulators are driven from a common unregulated, with large post regulator caps.
Note: the polarities of protection diodes will be reverse for the positive and negative regulators. Since valid current flow in a diode is from anode to cathode, in the ground leg of positive regulator the diode cathode (banded end) goes to ground and anode end to regulator ground pin. Reverse flow clamp has anode connected to + regulator output and cathode to + unregulated, so no current flows under normal voltages.
Minus regulator diode polarities are reversed to accommodate reversed current flow.
JR
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