I'm currently building a Li-ion UPS project to power my modem when there is is power failure.
The project objective is to power the modem from wall adapter when there is AC power available, and then when AC power fails, the project will switch the power to a Li-ion battery.
The problem is that I don't want the battery to power the modem along with the wall adapter so I'm planning to use a switch which will power the battery off (close battery circuit) when there is AC power available and then switch to the battery when AC fails.
So should I use a small sugar cube Relay switch, or use a NPN transistor as a logic NOT gate switch?.
What are the advantages and disadvantages of both? Does the transistor have any power limitations? Does the slow switching time of relay cause the modem to restart?
The power required for the modem is 9V 0.6A. I'm using a step up converter to boost 18650 Li-ion to 9V.
Yes, you can see them in the transistor's data sheet.
Does the slow switching time of relay cause the modem to restart?
It depends on the Modem.
I would use a dual pole change over relay for this project and have one set of relay contacts switch the power and the other set of contacts apply the battery to the step up converter. That way your step up converter is not always running as this takes some power even though nothing is being drawn from it.
also one of my friend mentioned something about a mosfet. So should i consider using a mosfet?
Another important thing which I forgot to mention is that the boost converter is always running. The wall adapter is a 5v 2a USB power adapter which gets converted to 9v by the boost. The same wall adapter is going to charge the battery too.
So I intend to use the switching between the battery and the Boost converter.
Why not just use two diodes to prevent one supply backfeeding the other? Then all you need to do is make sure that the boost converter output voltage is slightly below the wall adaptor voltage.
russellz:
Why not just use two diodes to prevent one supply backfeeding the other? Then all you need to do is make sure that the boost converter output voltage is slightly below the wall adaptor voltage.
Russell.
I'm attaching a rough circuit sketch of my project.Please make suggestions if it is bad.
I had initially considered the diode option, but doesn't diodes reduce voltage by 0.6v or so?
I don't want the battery to power the booster simultaneously along with the wall adapter,
because the battery charging module won't reset for start charging again when the battery is completely drained until the draw is shut off.( or at least that's what I heard ).
That's why I wanted to shut off the circuit from the battery.
Sorry, there is nowhere near enough detail in that diagram to comment on the circuit. That's more like a system connection diagram. For example, the way you talk about the diode is suspicious. But we can't see where you intend to put it, from that.
