HP 3478a Standby Power Supply

A while ago I replaced the battery in my HP 3478a multimeter, this battery is in charge of keeping the calibration values inside an SRAM. Since I norally have the multimeter plugged into the wall anyhow I decided it may be a good idea to add a standby power supply such that the battery doesn’t drain that much (other than its internal resistance).

So I did. There is a 78L05M generating a 5V rail, and I also added some super capacitors for good measure that on their own will keep the SRAM supplied for >1 week without the battery. The transformer is short circuit safe.

I have not yet installed it in the multimeter, I will make a new post on how I did that once I do.