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?
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.
Without knowing the details of the circuit, it is impossible to tell. Specifically we don't know exactly how this cap will be used, and so we do not know how critical the value is or what tolerance is required.
In many applications that substitution is acceptable, but in some it might not be.
But, wait, there is more! 5.0 uF is not a common value of cap. In fact, I have NEVER seen a 5.0 uF cap. I'm sure there is one out there, but it is probably some weird cap intended for some weird application with an equally weird price. Basically, not something that 99.99% of the people will ever use in real life. Even then, 5.1 uF cap would be a more likely value (because it is a more "standard number").
Here is a web page showing standard cap values: http://www.rfcafe.com/references/electrical/capacitor-values.htm
It's likely well with in the tolerance of the speciaiction. and the voltage rating is safe too.
For the oscillator you're referring to because you've exceeded voltage rating required the only effect will be a change in frequency. However a typical aluminum electrolytic capacitor has a tolerance of 20% so a hyphothetical 5uF capacitor could be anywhere from 4-6 uF so your 4.7uF subsitution will be fine, although technically a 5uF would be closer to the original design mark.
For future reference keep in mind if you did need an odd capacitance and were using parts with a low enough tolerance to make sense (some types can be closer to 1%) then you can place standard values in parallel to get a close match. For example a 4.7uF plus a 0.33uF in parallel would yield 5.03uF
