Power supply for Blink doorbell

[j.a.mcguire]

Hi there,

I'm installing a Blink video doorbell, which requires a 24V AC supply to the two terminals on the back.  But it also requires 2 x AA batteries internally, in addition to the permanent 24v AC supply.

I have a pair of wires running through my door frame to my existing door bell push button, which I intend to use to supply the 24v AC.  I was hoping to be able to put some sort of convertor inside the battery bay, that can accept the 24v AC and output 3v DC to replace the batteries, but I'm struggling to find a suitable unit.

* It has to be fairly small to fit in the dimensions of the 2 x AA battery bay.
* Ideally it will be contained in a small enclosure, but this is not necessary as the body of the doorbell is plastic.
* I'm not certain on the current, but I would imagine it's <500mA.
* It might be nice if it had a large capacitor to compensate for any surge in current demand (such as video recording), but this is more of a nice to have rather than essential requirement.

Issues:

* On amazon I can find a number of small boards which can handle the voltages, but they are DC-DC converters, which means I then have to squeeze in a separate bridge rectifier which I'd rather not do.
* I also found this but I'm not familiar with the chip, not sure if anyone can comment on efficiency: https://www.amazon.co.uk/DollaTek-MP1584EN-Step-Down-Adjustable-Converter/dp/B07DJ5HZ7G
* The majority seem to an LM2596 which I've read aren't considered to be particularly efficient these days? Such as these: https://www.amazon.co.uk/Yizhet-Efficiency-Regulator-Converter-Adjustable/dp/B0823P6PW6

If anyone has some recommendations I'd appreciate it.

[tunk]

Remember that rectified and smoothed 24Vac is ~33Vdc.

[themadhippy]

then have to squeeze in a separate bridge rectifier

4 diodes aint exactly massive,4 1n4001 will squeeze into a 6mm cube

[SeanB]

Put the 2 AA cells in, and set a calendar in your phone for 12 months to replace them. They are there to provide battery back up in case of mains failure, and hold the configuration and the stored video before it is uploaded to the server.

[j.a.mcguire]

Remember that rectified and smoothed 24Vac is ~33Vdc.

Thanks I hadn't realised this, if anything I expected a slight voltage drop due to the diodes.

then have to squeeze in a separate bridge rectifier

4 diodes aint exactly massive,4 1n4001 will squeeze into a 6mm cube

True it's just I'd hoped I could get everything self-contained onto a little board, no big deal though I suppose.

Put the 2 AA cells in, and set a calendar in your phone for 12 months to replace them. They are there to provide battery back up in case of mains failure, and hold the configuration and the stored video before it is uploaded to the server.

From the reviews, and my own witness of the power draw I will be lucky to get a month out of a set of batteries.  The doorbell seems to oscillate between approximately 110ma and 240ma so will swiftly drain the 5Ah available from the batteries.

Because the unit seems to have been principally designed for the US market it operates on 24v.  However in the UK, you can't seem to find a domestic bell which operates on anything more than 8v.  As a result I've had to wire the 24v circuit via a relay which triggers my 8v mechanical bell.  The blink doorbell pulls a trickle current over the 24v line.  I want to also draw the additional current to feed the battery supply from this 24v rail, which if at 3.3v it pulls max 240mA it should only need < 40mA on the 24v rail to power it I believe?  My relay switches at around 200mA so hopefully this won't cause an issue.

I think this might do the job: https://www.sparkfun.com/products/18357

Does anyone have any opinions on suitability, efficiency or alternatives?  Thank you.

[Faringdon]

It sounds a very small space you have for this converter for 24vac to 3vdc.
I assume your 24vac is isolated.
So what about a full wave diode rectifier with tiny little diodes...just big enough to manage the volts and current.
Then you need the step down from approx 33vdc to 3v.....so you could do that with RT6204

https://www.richtek.com/Products/Switching%20Regulators/DC_DC%20StepDown%20Convertor/RT6204

Woudl need a PCB...can you do that?

There are free pcb layout packages  on the web

[j.a.mcguire]

Thank you for that, I could make a PCB on Vero board but that's all I have the capability to produce.

How about the board which I linked above, do you think it would be ok?

[Faringdon]

The FET on that is only rated 32V...thats a bit tight for your app.

What about this
https://www.digikey.co.uk/en/products/detail/recom-power/R-78CK3-3-0-5/21284863

Or search on digikey under "DCDC converters"

https://www.digikey.co.uk/en/products/filter/dc-dc-converters/922