Way to switch between two power supplies
Way to switch between two power supplies
(OP)
Dear all,
what is the most power efficient way to choose between two power sources?
To be more precise, I have a solar powered circuit with a dc regulated output and a rechargable battery, and want to be able to automatically select the best of those two upon voltage level conditions (i.e.always keep a 3V output).
Any ideas/suggestions welcome!
Thanks a lot!
what is the most power efficient way to choose between two power sources?
To be more precise, I have a solar powered circuit with a dc regulated output and a rechargable battery, and want to be able to automatically select the best of those two upon voltage level conditions (i.e.always keep a 3V output).
Any ideas/suggestions welcome!
Thanks a lot!





RE: Way to switch between two power supplies
Are you going to build the circuir your self?.if so you could use a voltage comparator and a couple of transistors,out put to switch a relay.
RE: Way to switch between two power supplies
could you please elaborate on your suggestion?
Thanks!
RE: Way to switch between two power supplies
xnuke
"Do you think you used enough dynamite there, Butch?"
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Way to switch between two power supplies
RE: Way to switch between two power supplies
However, voltage levels may not be the best indicator of which source to use. You may be trying to makesure the two solar sources do not feed each other. It should be OK and even desireable for both to supply the load. Xnukes diodes would optimally share as a function of the source impedance of the two supplies under the rare condition of nearly matched voltage source. But, the highe voltage supply might sag down until there is benefi from the lower volatage supply. If you can not live with the diode drop you would have to get more elaborate.
RE: Way to switch between two power supplies
...................solarCell
..................|..........|
....dcConverter.......rechargableBattery
..................|.........|
...................\......./
..............powerSourceSelectCircuit
......................|
...................LOAD
A collegue suggested to you use p-channel mosfets driven from a low-power comparator through npn transistors, but i do not really know how to design this circuit. Can you please help?
RE: Way to switch between two power supplies
RE: Way to switch between two power supplies
RE: Way to switch between two power supplies
RE: Way to switch between two power supplies
Then u stop charging the battery and still run the load?
Normally you break this up differently....
RE: Way to switch between two power supplies
Solar panels > 'Battery Block' > VR (if req'd) > Load.
Why bother routing around the 'battery block' if it will automatically regulate its own charge level?
The only concern might be to make sure that your VR (dcConverter) is highly efficient or switched-off when not in use. You don't want it to drain the battery flat on a cloudy day.
RE: Way to switch between two power supplies
If I use the serial line architecture you propose, do you think adding a dc converter after the solar cell will enhance the effieciency any any way? I am asking this first of all because the solar cell is rated at 6Voc and 400ma Isc, hence need to step this voltage down to 5V (for IC to work) plus I read that this will in a way increase the ability of the circuit to extract the max power out of the solar cell (track its maximum point of operation).
Thanks in advance for any comments
RE: Way to switch between two power supplies
What exactly is the dc converter for ?
Do you need 5 volts or 3 volts ?
RE: Way to switch between two power supplies
The 3V I need for my load.
I think that plugging the cell to the battery is not the the most efficient way to charge the battery.
RE: Way to switch between two power supplies
<clunk> <clunk> [the sound of two unneeded circuits hitting the junk box...]
Then you'd be back to simply paralleling the 6v solar panel across the 6v battery (like everyone else in the world), and then use a suitable low drop-out, efficient three terminal regulator to provide the 3v for the load.
You'll need to confirm that this will work with your gear.
Watch out for the quiescient current draw of the 3v regulator and load. You should be able to calculate the expected battery life.
RE: Way to switch between two power supplies
Don't these batteries require a constant current charging algorithms to charge properly?
RE: Way to switch between two power supplies
6 volt solar panel,
5 volt chip in the charging circuit,
2.4 volt battery pack, and finally
3 volt load.
That's a lot of different voltages for such a simple application. You're going to burn up all your solar energy as heat in all the various circuits (6.0 to 5.0, 5.0 to 2.4, 2.4 back up to 3.0)...
I'd use a 6 volt lead acid battery and a voltage regulator for the 3 volts. That'll get you maximum efficiency up to the 3 volt regulator.
RE: Way to switch between two power supplies
any other suggestions
RE: Way to switch between two power supplies
Parts list: Panel, zener diode, 3 cells, diode, wire.
* the efficiency of the shunt regulator doesn't matter because it tosses away energy that you don't need (once the battery pack is fully charged).
**higher voltage than 2.4 volts to avoid having to step the voltage back up to 3.0 volts again. The batteries should be selected so that they are not fussy about how they are charged (it's a solar panel after all - not really reliable). Fussy batteries are the wrong batteries.
All this assumes that your 3 volt load can handle some minor variation in the supply voltage (3.6 to 2.7). If not, then you'll need to bump up the voltages (another cell to 4.8 volts perhaps) and use a carefully selected 3 volt regulator.
RE: Way to switch between two power supplies
all sounds good on your approach, but the battery is what makes this circuit tricky.
What I mean is that the battery type to use, imposes various restrictions and requires extra circuitry in order to avoid situations like overcharge, overdischarge etc. Since SLA batteries are ruled out due to size and weight isuues, we are left with recheargable NiCd, NiMH and Lion.
NiCd are also ruled out due to the memory effect they present, hence I have to use NiMH or Lion cells, both of which are not easy to charge (at least not as easy it is with lead acid cells)....
While NiMH need constant current to (fast)charge Lion require a constant voltage/constant current algorithm..which brings me back to the begining, wondering what to do....
RE: Way to switch between two power supplies
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RE: Way to switch between two power supplies
RE: Way to switch between two power supplies
The above link provides some interesting details on the care and feeding of LiIon batteries. It doesn't seem reasonable that you would be able to accomplish all that, while being powered by a solar panel (with the added amusement of clouds occasionally blocking out the sun).