I'm working on an ESP8266 thermostat that should be powered by the 24V AC from the furnace. I'm looking for a converter to 5V DC that would be small enough to fit in a smaller enclosure and not dissipate too much heat. Not fully sure about the current, but I'm planning for up to 500 mA for short periods of time. I found on ebay this, but it accepts input from 85V upwards. What would happen if I would feed 24V AC to it? Would it still work. Or alternatively doesn anyone have a link for something similar that would work with 24V?
1) No. Those converters will not work on 24 VAC.
2) This is not a site which permits product recommendation.
3) If you don't mind some extra size and weight, you can get what are called bell transformers (used in doorbells, naturally) which take 120 VAC in and put 24 VAC out. If you take one of these and feed your 24 VAC into the secondary, you'll get 120 VAC on the primary, and you can use this to feed your converter.
5 volts time .5 amps is 2.5 watts. Ignoring converter losses, this implies that 24 Volts will only need 0.1 amps, and this is a very small transformer.
Get a 120-240VAC to 5VDC Power supply like this one:http://www.hlktech.net/product_detail.php?ProId=60
Connect your 24VAC to a 240/120VAC - 24VAC transformer on the secondary side(the 24V side)(You can get this transformer from any old appliance or hobby electronics store). Connect the other side of the transformer(the 120/240VAC) to the input of the Hi-link power supply.
There will be very less heat dissipation and the package will be small depending on your transformer.
One alternative to this is:
Make a rectifier and filter circuit like the one below and connect it to a buck converter like this:https://www.amazon.com/Lysignal-LM2596-Supply-Adjustable-Converter/dp/B074J4NXCM/ref=pd_day0_hl_23_14?_encoding=UTF8&pd_rd_i=B074J4NXCM&pd_rd_r=3516ad17-2cfc-11e9-a095-811ee4313434&pd_rd_w=6lXwy&pd_rd_wg=8G2xP&pf_rd_p=ad07871c-e646-4161-82c7-5ed0d4c85b07&pf_rd_r=VHE2RA0J3G6ST9KPK3MJ&psc=1&refRID=VHE2RA0J3G6ST9KPK3MJ
Also, tune the potentiometer for fine 5V output.
Good luck. If you have a doubt, drop it down in the comments.