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24VAC to DC

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mariush:
Here's a suggestion

You could use a switching regulator that runs at higher frequency, allowing you to save money on the inductor and other components.

See for example :

https://www.digikey.com/product-detail/en/monolithic-power-systems-inc/MP2456GJ-Z/1589-1894-1-ND/5292060
https://www.digikey.com/product-detail/en/monolithic-power-systems-inc/MP2459GJ-P/1589-1121-1-ND/5298208


If the relays are so expensive, what's stopping you from using cheaper 5v relays? Change to a 5v switching regulator and you can add a 10-20 cent 5v to 3.3v LDO for your microcontroller, but you can save money on the relays. You have 4 relays... you'd save around 1-2$ on relays alone if you go with 5v relays.

example of 5v relay: https://www.digikey.com/product-detail/en/kemet/EE2-5NU-L/399-11017-1-ND/4506469

Also, if you need let's say more than 50, consider other stores that have better prices than Digikey.
For example, TME.EU in Europe stocks some brands less known in US, here's an example: https://www.tme.eu/ro/en/details/rs-5-l/miniature-electromagnetic-relays/rayex-electronics/
But pay attention at the secondary, the AC and DC current rating... for ex. the one above is only rated for 1A and 120Vac or 24v DC.

If you have multiple relays, you could use transistor relays like ULN2003A (7 transistors with resistors built in) : https://www.digikey.com/products/en/discrete-semiconductor-products/transistors-bipolar-bjt-arrays/277?FV=-8%7C277&quantity=0&ColumnSort=1000011&page=1&k=uln2003a&pageSize=25

joeyjoejoe:
I'll check out those regulators, hopefully they are simple and the part count is low.

That 5v relay costs more then the 3v one I am using. https://www.digikey.ca/products/en?keywords=ec2-3nu

Zero999:

--- Quote from: joeyjoejoe on January 26, 2020, 05:52:16 pm ---It would only shave a few cents off, but I do have two BC817's already for programming. And it means a bunch more questions

Sorted by price and still active components, I have

Currently,
DMN63D8LDW- N-channel mosfet (45 cents)

Options (sorted by price)
MBT3904DW1T3G - dual NPN (31 cents)
MUN5213DW1T1G- dual NPN, prebiased (33 cents)

I need 50mA for the coil. Will the MUN5213DW1T1G save me a pull-down resistor?

Resistors/diodes/led's are basically free, but the labor to place them isn't. I'd take fewer parts over a 10cent BOM increase anyday - the relays themselves from a legitimate supplier are just over 2$ a pop in small quantities, ESP32 5$ and coil for the switching regulator is 2$.

--- End quote ---
The dual BJTs seem sensible, but a base resistor of 47k is too high, so it won't saturate. You need around 1k, as per my schematic and there is no need for pull-down resistors, because the BJT will turn off, if there's no base current, so if the MCU's I/O ports are set to inputs, the BJTs will be off.

joeyjoejoe:
I need to brush up on transistors/current, but I did find MUN5230DW1T1G with a 1k base resistor and 1k base-emitter.

thinkfat:

--- Quote from: blueskull on January 19, 2020, 06:18:01 am ---
--- Quote from: james_s on January 19, 2020, 04:19:47 am ---For a single quantity from a retail supplier?

--- End quote ---

We don't know if the OP wants to make a few or just a one off.

There is no place for LM2676 anymore. It runs at low frequency (meaning needs large LC), it is expensive from big distributors, and from elsewhere you get 100% fake.

The chip itself is a huge package, combined with a stupidly large 100uH choke, it has no power density.

--- End quote ---

The LM2676 is actually not half bad a choice. It is a step up from the LM2596, switching at 250khz instead of 150khz, works reasonably well with a 33µH coil which you can get in a 10mm square package. Of course it is not a synchronous buck converter and the diode you need is also relatively big, but it should be possible to cram it all into a 15mm by 30mm area.

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