capacitorfailurepower
I need a cheap way of powering my internet modem (rated at 0.8A, 12V) for approximately 1 minute in case of power failure (which happens every hour or two) till my secondary power takes over.
I am thinking capacitors but i am not sure if they will be able to power for 1 minute.
Edit: If i connect a 12 v battery in parallel and update the power adopter to 14V to charge the battery, would this kind of setup damage my modem?
I don't have an answer but do have a suggestion: Are you absolutely positive that your +12V rail is always live? Many vehicles disable what would be considered to be the main 12V rail anywhere from 10 to 60 seconds after the vehicle is shut off.
Second suggestion: a series resistor on the FET gate with a 12V zener diode clamp connected right between Source & Gate. The value of the resistor isn't critical because it's after the timing capacitor. Anywhere from 1k through 22k is good - I'd be using 4k7.
I regularly see significant transients on vehicle power rails. Some of that comes from what is called "Load Dump", where a heavy load is suddenly removed while the engine is running and the alternator voltage regulator can't respond quickly enough. That's only one source of high-voltage transients, there are others.
A simple battery backup IC will do what you want.
Essentially, a comparator samples two voltage inputs, the main power, and a backup power supply (Batteries). Based on that, two cascading inverters paired with two p-channel fets power the circuit at Vo, while two n-channel fets are used as open-drain outputs. Pull them up with a resistor, and they are the perfect interrupt source for your arduino.
The specific one shown is the ICL7673, but it's maximum current is 38mA. It's designed for rtc and ram with a coincell, but an extra pair of P-channels can be used to increase the current.
There are other battery backup or power switches which will do higher current builtin.
Best Answer
One problem with capacitors for power backup applications is not so much if they can be used, it's also a matter of cost. Just using an on-line supercap discharge calculator as an example and plugging in a few numbers assuming a 12 -> 9V drop would still keep it running OK I came up with a 20F capacitor for a bit over a minute.
Although a bit larger than required on Digikey at the moment the cheapest part that I found that met or exceeded those requirements was a 58F 16V Capacitor at a bit over $US140. That's not too over the top but if you were trying to connect across a normal modem power supply you'd also need to consider the inrush current as the capacitor charges so it might not be as simple as just placing one across the supply.
I'd expect that for that voltage / current assuming that the modem is normally powered from mains that a small UPS using an SLA battery would be a cheaper and better solution. While SLA batteries do degrade over time I'd expect a small UPS to be able to provide that small amount of backup time for quite a long period, maybe say five years until the batteries needed replacing.
As an addendum to your updated question when connecting an SLA battery in parallel you may experience similar problems with an inrush current. While I wouldn't expect the battery to draw much current after a few minutes of discharge (if fully charged at the start) for safety you should also consider what would happen if power was out for hours and the battery discharged completely.