[SOLVED] switching from multiple power supplies: battery charger

[eanema]

battery charger

Hi All,

I am designing a small device to operate off 4 NiMH AA batteries (integrated - not removable) The device consists of a PIC and some LEDs plus what ever I come up with for a charging circuit. I was thinking of pulse width modulating a LM317 configured as a constant current source.

The device is going to be small so I would like the part count to be minimum.

I am wondering what is the best/simplest method to connect a PIC to a battery as well as charge the battery from a 10v external source.

Is there a simple controller that I can interface the PIC with that would manage this for me?

I attached an image of what I've been thinking...

All thoughts are much appreciated

 

[eanema]

Re: battery charger

I was thinking that I would like to replace the diode connecting the battery to the PIC with a MOSFET, being switched by the presence of the external supply connected to the gate. The problem is that Vds would probably be to small to allow conductance (especially if you were just topping up the batteries). Any thoughts on this? Any thoughts on what I might use to fix the problem? And is the diode simply enough?

 

[kspalla]

battery charger

Which MOSFET?
why not use PMOS than NMOS.

 

[eanema]

Re: battery charger

Is there sufficient voltage difference between the source and drain to turn fully on and off? I thought that a MOSFET requires a Vds of a couple volts, NMOS or PMOS... is this wrong?

Plus as the batteries charge, the voltage will increase. The FET would need to be able to stay turned off as the battery voltage increased to (and possibly past) the regulated +5v source.

When I have a bit of time I'm going to play around with a circuit... but I would appreciate some input on this.

Thanks

 

[bobcat1]

Hi

Simply use LTC4412 controller to switch between main load and battery using one or two mosfet.

All the best

Bobi