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I always err on the side of safety so I used 50 Volt Electrolytic capacitors, but 35 Volts would be acceptable as long as there's always a load on the circuit. A rough guide to the size of the capacitors would be 100uF per watt, so as an example, the capacitor size for a 50 watt LED would be 6800uF. Basic rule: If it flickers, use a bigger cap. The bigger LEDs (>20W) would require a 3 amp diode like the 1N5400.