Safety Question

[jghfslk]

Can a capacitor be potentially dangerous if the circuit never gets more than 9 volts and the capacitor is rated for much more than 9V?  Even when using very large values?

[EBK]

The voltage rating is independent from how much energy is actually stored.  Instead, it represents the maximum voltage you can apply across the capacitor before damaging it.
The amount of energy that gets stored depends on the capacitance value and the actual voltage applied.  Although the energy will somewhat dissipate through leakage over time, the formula is:

E=C•V²/2.

This only partially answers your question, I know, but a cap with a higher voltage rating is no more dangerous than a lower rated cap under the same circumstances, i.e., with the same voltage applied across it, unless you apply a voltage higher than the rating, in which case, the cap could potentially explode.

[jghfslk]

Re-reading my post, I realized I should have worded things differently.  Could using a capacitor with a very high capacitance be dangerous even though the voltage would be low? For example, is a 10000uf capacitor any more dangerous than a 10uf capacitor if they are both in a circuit that never exceeds 9 volts?

[EBK]

Short answer first:  No, not dangerous at 9V.

Longer answer:

I'll tell you that the cap is not dangerous if you promise not to touch it to your tongue. 

Think of it almost like a 9V battery that can only deliver current for a short period of time.  The "danger" always depends on how much current can flow through your body and for how long.  That will depend on additional things like skin resistance.  I don't want to complicate this too much by mentioning that there are circumstances where a circuit powered by 9V can charge a capacitor to more than 9V (e.g., charge pumps), but I guess I just did.   

So, the answer is:
No, it is not generally dangerous at that voltage. 

If you want an easy to remember rule, here is one:  if the capacitor is larger than your fingertip, then discharge it with something other than your fingertip before working near it.

[Zigcat]

Keep them away from goldfish, though...

[matmosphere]

Short answer first:  No, not dangerous at 9V.

Longer answer:

I'll tell you that the cap is not dangerous if you promise not to touch it to your tongue. 

Think of it almost like a 9V battery that can only deliver current for a short period of time.  The "danger" always depends on how much current can flow through your body and for how long.  That will depend on additional things like skin resistance.  I don't want to complicate this too much by mentioning that there are circumstances where a circuit powered by 9V can charge a capacitor to more than 9V (e.g., charge pumps), but I guess I just did.   

So, the answer is:
No, it is not generally dangerous at that voltage. 

If you want an easy to remember rule, here is one:  if the capacitor is larger than your fingertip, then discharge it with something other than your fingertip before working near it.

So, like my tongue right?

[EBK]

So, like my tongue right?

I'm glad I wasn't sipping my coffee when I read that.  Well played.