Power Supply Advice Please

[Deltatango]

I have made a newer version of my battery powered power supply, my old one was a LM317 with 10x AA batteries with variable output worked well but needed a bit more voltage.

Building around a China made kit on sale with 2A output and 24v AC input, while I would not always need that power for my service work on small items, I will power it also with AA's as they are cheap using mid range brands. My question is :-

1) 24v battery pack direct.
2) 24v battery pack feeding a DC-DC step up covering input voltage drop off as they tire.
3) 12v battery pack (x2) feeding a DC-DC step up which also covers battery fade but has more current for the converter.

Thanks for any insights and thoughts

David

 

[BradtheRad]

In my mind I've dreamed of building step-up converters that let me put one battery in devices that run on 3 or 4 cells. Incandescent flashlights, LED flashlights, a clock, a thermometer... I could deliver the full voltage the device needs, with only the one cell to test and/or replace. However there's the headache of fabricating an enclosure for such circuits. It ought to be a small battery-shaped shell whose electrical contacts match a battery and also the ends of the compartment.

You might manage okay with a few cells and a buck-boost converter. And with less wasted power. As you know the 317 regulator is resistive drop. And doesn't the 317 subtract a volt or two?

 

[Deltatango]

Yea step ups I allow about 2v minimum for LM317 circuits as it can lose the will to live right.

That is why I am building around this China Kit supply, with voltage and current control it is designed to work with a 24v AC input to keep the IC's used happy, while I will have it powered on mains if I need to, for smaller work like radio or similar batteries would do.

I just wondered if anyone had reasoned out if step up converters last longer with 24v of AA battery making say 25v or 12v (2 battery blocks) working to make 25v.

I suppose its not important in the end, but years ago I worked out that 3x a Tandy battery was cheaper than 2x a Duracell for continuous powering before collapse, though that was 20+ years ago.

Thanks so far

David

 

[barry]

I'm afraid I don't understand. You can't just stick 24VDC into a power supply that's expecting 24VAC.

 

[Deltatango]

The AC is fed through its bridge input while the DC is put in after it with a diode acting as a reverse voltage block, that enables me to work on small powered items like transistor radio's which may only want say 50mA. Also under a battery feed it will limit "damage" under short circuits.
When I require higher voltage and current say 1A then the mains side will take over, while in its battery mode makes it portable with no mains lead problems.

After thinking about it I will use 24v DC fed through a step up converter to say 26v (enough to be ahead of input) this will then be mostly battery supply, just partly used for top up, then as its batteries tire the converter will compensate more and more.

Thanks for helping David