I have seen designs of 6 and 12 V power sources where 1N4004 diodes are indicated. Would not it be better to use the 1N4002 which is for lower voltage?
In fact, in a 6 or 12 V source, both use 1N4002 and 1N4004. The two will work in the same way as they are for currents up to 1 A. The 1N4004 that supports a voltage of 200 V is oversized and there is even the possibility of using the 1N4007 to 400 V. The price difference between the types is minimal , hence a small project is not important what does not occur in an industrialized type. Using a diode with higher voltage is just giving a safety margin to operation, but in a 6 V or 12 V supply we will never have the 200 V that the diode supports. If you have 1N4002 available, do not waste your 1N4004 or 1N4007. Store them for assembly that requires a larger voltage diode.