How can I power a small device long enough so I can switch power sources

[J. Dundee]

I am building a POE powered diagnostic tool which has a wifi access point in it. It works with a smartphone wifi app.

Everything is good except that when you unplug it from one POE switch to put it in another, the wifi will drop (obviously), but worse the phone will possibly acquire a new access point if another is available.

Then to keep using the device you have to disconnect and reconnect wifi which is a pain. But if I could just keep it on for a few seconds while I change cables everything would be fine.

I know it's possible using batteries or capacitors but the problem is that I have very little space in my case left.

So the requirements are
3w at 9v for 60 seconds with components that fit in a space of 38mm X 20mm X 10mm.

Price isn't a major concern.

 

[AnalogKid]

POE is 44 Vdc. Energy storage at 44 V before the 9 V regulator can be much more space-efficient. A sketch of an overall system wiring diagram would help a lot. 3 W for 60 s is 180 W-s, not a large or difficult number.

For example, if you have a 44 V source and the 9 V regulator needs a minimum input of 11 V, then the terminal voltage of the storage capacitor can sag 33 V in 60 s. Very different from storing energy at 9 V for a device that quits at 8 V. Ultracaps might do this in the space you have.

A 9 V lithium battery will do this easily, but it might be too thick. AAA cells might fit the 10 mm space, but you don't have room for 6 of them so using 1 or 2 would need a boost circuit.

ak

 

[J. Dundee]

POE is 44 Vdc. Energy storage at 44 V before the 9 V regulator can be much more space-efficient.
ak

Yes this is an excellent point!
There's not much wiring involved in this part as the Poe module is stand alone and takes two inputs (the PoE +\- @ 48V) and provides choppy DC on the output.
There is a bridge rectifier that I need to place before the module and an output filter cap that comes after. My initial thought was to make the output cap huge to serve both purposes, but as you say the voltage will drop fast.
The output module will output 12vdc but my device only needs nine so there is a little room.
The only problem with putting a cap after the bridge rectifier is that it might screw up the POE signature that the power source uses to determine whether to start full power.
Probably though it will just delay the power until the cap is charged.
What's a good value to start with?

 

[AnalogKid]

The 9800 has a min input of 36 V. Not good. Storage caps would be in farads, not uF. If your space can handle the thickness of a 9 V battery, then this can be done with what is basically a UPS on the 12 V output. Small stupid lithium trickle charger and a couple of Shottkey steering diodes.

ak

 

[J. Dundee]

The 9800 has a min input of 36 V. Not good. Storage caps would be in farads, not uF. If your space can handle the thickness of a 9 V battery, then this can be done with what is basically a UPS on the 12 V output. Small stupid lithium trickle charger and a couple of Shottkey steering diodes.

ak

Yes, but if I don't power the 9800 with the cap, but instead failover and power the unit directly somehow, couldn't that work?
Maybe a relay on the 9800 input with a diode to stop the backflush current? And a resistor to drop the voltage? The device actually can run on 9-24 so not as much to drop.
I am also interested in the trickle charger, but I figured it was tricky... Besides the shottkeys what else is needed?
Thanks, you've already helped quite a bit.

 

[AnalogKid]

As is all over the news these days, lithium batteries have outstanding energy density, and can burst into flames if not handled well. What is the minimum time available for the battery to recharge between discharge events? In other words, how often will this circuit have to leap into action?

ak

 

[Osmium]

Do you have enough room to handle two AG9800M devices with two RJ45 inputs? Connect your second source before disconnecting the first one - then have some switching logic to flip when one of them is disconnected... maybe... just thinking out loud...