Can you use Diac and Triac for dc supply?

The diac conducts in both directions at approximately the same voltage, making it unsuitable in an AC to DC supply. The triac also conducts in both directions, based on a gate voltage which is slightly different in the two usable quadrants. That is why the diac is often used to trigger the triac - it makes the two conduction cycles more symmetric. In both cases, the diac/triac does not make sense for an AC to DC supply, but it works well in an AC voltage regulator, such as a lamp dimmer or motor speed controller.