An ongoing need

One thing that happened during the recent 2 day power outage:

https://wb9kzy.blogspot.com/2025/03/we-are-back.html

was that my portable devices like the tablet and mp3 player (aka cell phone) were getting low on charge.  I have a couple of 12 V lead-acid low self-discharge batteries on hand so I thought: "maybe a simple 5 volt regulator hooked to a USB cable ?"  But all I had on hand was a 1 amp 7805 regulator - with a thirsty USB item like a cell phone or tablet the 7805 regulator will put out 1 amp of charging current which is great but without a good heat sink it will rapidly get screaming hot because it will be dissipating roughly 7.5 watts (12.6 volts - 5 volt regulated output) * 1 amp = 7.6 watts).  In addition, the regulation may not be too good with the high die temperature.  So one way around this would be to add some external series resistance (maybe a car headlamp?) but it wouldn't be efficient.

So that's the need: some kind of step down switching regulator circuit.  One switching chip that I remember is the MC34063.  I looked through my junk box and didn't find one.  So I will have to buy one.  An alternative might be something more modern - I *thought* I had an old National Semi Simple Switcher demo board but I can't find it.  Another way might be some kind of lash up using discrete components.  Finally, since most plug-in chargers are step-down switchers I also thought about just trying to power one of them directly from 12 or 24 volts DC instead of 120 VAC but haven't tried that yet.

Until I get something going I'm keeping the USB charged devices charged !  Never know when the next long power outage will occur here.

Best Regards,
Chuck, WB9KZY
http://wb9kzy.com/ham.htm