Replace AA batteries with AC adapter in parallel?

What are you using for an automatic fish feeder? I'm finally going to be gone to a wedding for a few days and the lid on my reef tank is too heavy for my adult daughter to lift :-(

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
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First of all, this belongs on news:sci.electronics.basic and No, you can't replace batteries with an AC source. How can you be careful about polarity with 3 VAC? If you meant a 3 VDC source, "maybe". You need to know the current required, and if it needs to be regulated.

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Michael A. Terrell
Central Florida

Yes re the adapter; no re hooking a battery pack in parallel.

You can make the "battery backup" work if you use diodes to ensure that the batteries don't try to power the adapter or vice versa, like this:

' BAT + -------->|-------o------ + TO FISH FEEDER ' 1N5818 | ' ADAPTER + ---->|-------' ' ' BAT - -----------------o------ - TO FISH FEEDER ' | ' ADAPTER - -------------'

Use Schottky diodes, like 1N5818, because they have lower voltage drop. Note that you'll only need the diodes on the + side; on the - side you can just connect the adapter and battery pack directly.

This will work as long as the voltage produced by the adapter is a bit more than the voltage produced by the batteries. Otherwise, the batteries will win and they will power the fish feeder until they are drained enough that the adapter voltage is higher again.

To work around that, you might want to use a 4.5V adapter, and two regular diodes (1N4004) rather than a Schottky diode in series with the adapter. Each regular diode has a drop around 0.7V, and the Schottky will drop about

0.3V. So you get 3.1V from the adapter versus 2.7V from the batteries, guaranteeing that no current will flow from the batteries until the AC fails. But that might not be necessary; the 3V adapter probably puts out a bit more than 3V depending on load, so you might just be fine with the circuit I drew above.

Thanks for the help everyone! Thanks Walter for the detailed diagram! Sorry I posted this in the wrong group everyone. Yes, I meant an AC plug to DC adapter output of 3V. As far as Jim's question as to the kind of fish feeder, I am using an eheim feeder for my salt water tank

. It has been reliable in despensing flake or pelet food, and it has moisture control to prevent clogging. My only problem is that it is easy to let the battery run out but still see food in the feeder, and not notice it is not actually dispensing.

Thanks for the help everyone, sorry about the wrong group! Thanks Walter for the detailed diagram, I am going to try that! Yes, I meant an AC to DC converter, giving an output of 3V. As far as Jim's question as to the kind of fish feeder, I am using an Eheim feeder for my salt water tank

. It has been reliable in despensing flake or pelet food, and it has moisture control to prevent clogging. My only problem is that it is easy to let the battery run out but still see food in the feeder, and not notice it is not actually dispensing.