Can you change 680 microfarad capacitor with 1000 microfarad at 35 volts?

Answer 1

Yes you can - in principle, and in many cases. As long as the form factor of the new capacitor is the same as the form factor of the former capacitor, and as long as the maximum voltage of the new capacitor is higher than the max voltage of the former capacitor, you can often replace a lower-rated with a higher rated capacitor.

However...

Apart from the nominal capacity and maximum voltage rating, you should also consider the capacitor's design. There are many ways to make a capacitor, from tiny ceramics over various plastics and metal foil combinations to electrolytic capacitors. Apart from the capacity and voltage rating, the different types of capacitors also vary in a lot of other aspects: impedance, precision, drift with time and drift with temperature, are just some.

Electronics designers typically choose the capacitors carefully, and considering all those aspects. Only when those aspects don't matter much, designers tend to chose the cheapest technology. In such a case, you're safe to replace.

In most cases of capacitors rated at 1 micro Farad and above, electrolytic capacitors are used (typically small cylinders). Those are typically selected because they offer excellent value (capacitance) for money, and you're typically safe to replace them, subject to above rules.

CAUTION: some capacitors have a distinct polarity. You must make sure to observe the polatiry when replacing polarized capazitors. Failure to do so can result in serious harm (basically, the part might explode).

Answer 2

You risk having an unexpected explosion or a circuit fire if you use the wrong type of capacitor just as much as if you use one of too low a voltage rating for the particular end-application. You can replace a low-voltage capacitor for one of the same microfarads size only if the "type of construction" of the replacement capacitor is the same as the original AND the rated voltage is equal to or higher than that of the the original. In other words if the original is a "non-electrolytic" type - meaning that the insulation between the plates is plain paper or plastic and not a chemical compound - the replacement capacitor must also be a "non-electrolytic" type, even if the rated voltage is higher.

If the insulation between the plates is "electrolytic" the replacement capacitor must also be "electrolytic", even if the rated voltage is higher.

The technical reason for having to use exactly the same type when a replacement has to be made is to make sure that only a non-electrolytic will be used if the circuit carries high-voltage ac (alternating current), because an electrolytic type will not be suitable, or safe, to use in that situation.

Electrolytic types are much smaller than equivalent-sized non-electrolytics - which is very handy for miniature products. But electrolytics can only be used on dc (direct current) circuits which carry ac "signal" currents - and hence voltages - which are tiny in comparison with the standing dc voltage.

Answer 3

This is likely to produce problems, but the exact nature of the problem and if it might be acceptable anyway will depend on the purpose of the capacitor in the circuit. Value of a component is NOT the only criterion for successful substitution when the original is not available.

Answer 4

If it's use is an input, no you can not, if it is an output yes you can.