Survive power off?

alec_t said:

Assuming you use a low-side NMOSFET you could have the gate biased high by a resistor and your bits when active could pull it low as required.

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Never used an NMOSFET before - what value should I be looking at to guarantee that my ATTiny can pull it low? If I'm reading this datasheet right, this one should be more than sufficient for 12-17vdc, 500ma?


EDITED : Hmmm, nmosfet's seem to be unpopular - limited choices! Could I accomplish the same using a PNP transistor to drive a MOSFET? Edited again - knowing what you are searching for helps - "P-Channel MOSFET" gives a few thousand more options!

Power Switch

I'm guessing this is driven from the micro so have a look at this one. It switches ground but it doesn't matter. I think the micro tri-states on power of so this would fail on. The diode is not required if the circuits are all on the same board and there are no inductive loads (like motors).

Hmmm, nmosfet's seem to be unpopular - limited choices!

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Eh? There's loads. Try searching for 'n-channel mosfet'. The one you linked to looks fine.

SystemsGuy said:

Looking at the level of complexity here - and with a good nights sleep and some coffee - maybe I'm wandering down the wrong path trying to "detect" the outage, and would be better served by surviving it? Since anything > 1 minute fails back to "cold start" mode, I really only need to survive for that minute, right?

Some of the ATTiny's are damned power efficient - what are the thoughts about surviving the outage with a small super capacitor? As I said before, I'm much more of a programmer than an electronics guy, so I may be way off base here. Don't know if I could even charge the capacitor fast enough to catch the "one second on, survive one minute off" sort of worse case scenario?

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That seems to be the most appropriate approach.

The ATTINY13A uses 4-6uA in sleep (WDT enabled, 3-5V Vcc) and an appropriate small supervisory IC (ie brown-out-reset circuit) uses <1uA. So if a 470uF cap is used to power the uC, with 10uA power budget and operating within 3-5V, there's 94 seconds of power available. There is not much to it: just sleep and use the WDT interrupt to wake it up (every 64ms) and update the counter and check for power.

If you truly need a unit <0.5cm thick, then an appropriate cap may be a tantalum.

Regulation for the uC can just be a run of the mill 78l05, perhaps with a backflow diode (and one on its GND pin). I don't know what the output you're trying to drive is; apart from the mention of the FET dropping current of a 12V load.

Thanks guys - load of bits and pieces on the way, will let you know what I end up with - or fry!