Feb 19, 2014

Why “wait 30 seconds” when powering off and resetting a modem?

Whenever I have an issue with my home network, my broadband provider’s tech support always tells me to power cycle my modem, and “wait 30 seconds” after I power down before restarting. Why wait that long before restarting the modem?

This thread has some helpful answers:



You want all capacitors to discharge. A poorly designed/constructed device could be damaged. But the more likely issue is that since you're power-cycling it to reset an unexpected/unhandled failure, a capacitor not being discharged could leave the system/circuit/device not fully reset.

On computers I tell people to wait for all fans to stop spinning. It's a fair compromise. This 30-second advice is much more relevant to a non-computer (simpler, bigger capacitors) device. We know that the complex parts of a computer will be reset upon power cycling them, regardless of any random capacitors.

I've certainly power-cycled things quickly, had it not work, then waited a significant time with it off, and had it work. No evidence if this mattered of course."

There actually is a reason behind the “wait 30 seconds” thing, and that reason is capacitors. Capacitors hold an electric charge, and it takes a bit for them to discharge. Ever notice LEDs on your computer staying on for a few seconds after you power it down? That’s the residual charge from capacitors. The other reason tech support says 30 seconds is that they actually want you to wait about 10 seconds for the capacitors to discharge, but if they say 10 seconds, many people will only wait for 5. It might also be to give the CMTS enough time to purge the association with your modem and start afresh. 

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