Battery Backup a Stamp

Guido

Running a Stamp using a Power Supply and 5 volt regulator. Can I use a 9 volt battery on Pin 24 for a back up supply? Or would I have to tie it into the regulator circuit with a diode???
Just running a LCD.
Thank You

Tracy Allen

Yes, it can work to connect the 9v battery directly to pin 24, Vin, for backup, while the 5 volt supply from the external regulator comes in at pin 21, Vdd. However, it is not foolproof. There are a couple of things to keep in mind.

Suppose the external regulator is set for 5.05 volts and the Stamp's internal regulator on its own produces 4.95 volts from the 9v battery, then it will work as you expect, and the 9v battery will back up the external supply. The 9v battery will take over when external regulator outputs drops off below the 4.95 volt mark. the However, if it is the other way around, then the main power will come from the 9v battery. The lesson is, you have to be sure that the external supply is set for a slightly higher voltage than that produced by the Stamp's internal regulator.

Another caveat is that your external regulator may not like to operate with a backup voltage on its output. What regulator are you using? Check its data sheet. Current may flow backwards through the regulator and drain your backup battery. The data sheet will advertise the fact if it can operate with its output at a higher voltage than the input, in a backup situation, and it will state the maximum amount of back current or it will have a graph. If it does not say anything, then you will probably have to add a diode at the output of the external regulator to prevent back current. That will compromise the regulation and lower the output voltage (see the above point), unless the diode can be included in the feedback loop with an external sense terminal.

Power regulators like the LT1529 are designed so that they can operate in backup mode.

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Tracy Allen
www.emesystems.com