I'm working on a circuit that calls for a couple 5uF caps. I have 50v and 25v 4.7uF at my workbench. No cap voltage is specified in a schematic or parts list. The circuit has 12VDC coming in, one negistor that'll be connected to the path of the caps (though downstream), so I think the 25v will be sufficient.
But is it safe to substitute 4.7uF for 5uF? What's the worst that can happen by having less capacitance in the circuit by 0.3uF?
Answer
Electrolytic caps have a tolerance of minimum 10% (the good ones), so 5 uF may be anything between 4.5 uF and 5.5 uF. For the 4.7 uF that's between 4.23 uF and 5.17 uF, so both ranges overlap for the most part. If the operation in the extremes would be important a capacitor for that value would have been chosen. So it's safe to say that 4.7 uF is a valid alternative for a 5 uF.
For the voltage 25 V may be OK, since you hardly see more than twice the power supply voltage, unless you're actually building a voltage multiplier (step-up converter of Greinacher circuit, for instance). But 25 V on a 12 V supply leaves very little headroom, so I would use 35 V at least.
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