Selecting a power source

Don Leuenberger

I've almost completed a project that uses relays to control the tracking of a telescope. It currently runs off a 9V battery. I would like to add the possibility of plugging in a 12V from an external battery for its power. What would I need for the stamp board (I'm currently using a homework board) to automatically select the power source with the highest voltage? What I want to end up with is a box that will run off of the internal 9V battery if there is no external power source, and to select the external 12V if it it is present. Also, if the 12V source gets lower than the 9V, it should switch to the 9V internal battery.

Don Leuenberger

Chris Savage

Don,

·· I'm not entirely sure a 9V battery is going to be sufficient to run Relays for very long.· I guess it depends on how many, and how much time they are on.· As for your backup source.· I wonder, with the lower cost of UPS systems these days, perhaps that would be the easiest way to go.· You could use a Battery Eliminator (Wall Transformer) to replace the 9 Volt battery, and connect the transformer to an Uninterruptable Power Supply.· When the power goes out, it will keep the system going for awhile.· These are usually made to keep computers running for short periods of time during a power outage, but with as little power as you will be drawing, you could go for much longer.

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Chris Savage
Parallax Tech Support
csavage@parallax.com

Don Leuenberger

Thanks Chris. The power source I have is a 12V gel cell car jump start battery which is a common thing for astronomy equipment. As far as the relays go, they are only 5V reed relays that bring one of the telescope's guiding contacts to ground. The telescope drive system has an internal pull up and drives the motors from there. The relays (4)·are only on one at a time for a few ms at a time. Some of my other equipment has an internal battery and a 12V connector. I'm trying to mimic the same behavior. Everything also has to be portable since it has to be dragged out the the backyard, fields, mountains etc. whenever I want to use it.

Don
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Don Leuenberger

After googling for hours, I think the Max 761 is pretty much what I am looking for. Does anyone have experience with this? Is there something else out there like this?

Don

Tracy Allen

Don Leuenberger said...
After googling for hours, I think the Max 761 is pretty much what I am looking for. Does anyone have experience with this? Is there something else out there like this?

Don

For this systems, it may be that all you need is two diodes. Like this:

12 volts --------->|--------;
                                   |
                                   o--------- circuit
                                   |
9  volts --------->|--------'

Use either rectifier diodes like the 1N4001, or Schottky diodes like the 1N5817 for lower voltage drop. This automatically uses the highest voltage source to power the circuit.

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Tracy Allen
www.emesystems.com

Don Leuenberger

Thanks Tracy. I wondered if something that simple would work when I first asked the question. Sometimes I make things harder than they should be.

Don