Need tiny solid state 120vac to 24 vac converter - Page 1
Forum section: electronics / beginners

author: syl20bou
author_rank: Newbie
timestamp: May 23, 2021, 11:43:07 pm
content: Hi,I have a North American standard electric box (2" x 3" x 2 1/2") containing a normal SPST 240V thermostat that I want to replace with a Nest thermostat and link to my computer. The idea is to fit a solid state relay model RA2425-D06T‎ to switch the thermostat and to power the thermostat with residual current only when the heater is off (battery included in the Nest).I need either a 240VAC to 24VAC converter or (if I use ground) a 120 VAC to 24 VAC converter. Tiny! Because there's no room.I am looking for a safe and reliable solid state converter from either high voltages to 24 VAC, Any idea?Thanks.

author: SmallCog
author_rank: Frequent Contributor
timestamp: May 24, 2021, 02:56:21 am
content: AC to AC so just use an itty bitty little transformer with an appropriate number of turns.

author: syl20bou
author_rank: Newbie
timestamp: May 25, 2021, 12:53:11 am
content: Transformer is too big. It needs to fit inside an electric box along with the solid state relay. So I mean T-I-N-Y converter.

author: WattsThat
author_rank: Frequent Contributor
timestamp: May 25, 2021, 03:12:43 am
content: What you’re asking for is a unicorn.I foresee drywall patching in your future. Tons of videos on YouTube. Vancouver Carpenter is quite good.You’ll need one of these, hopefully you have access to all three wires. No way in hell I would control a resistance heater in a home with an SSR and just in case you you’ve overlooked it, those SSR’s need a heatsink.https://www.amazon.com/Aube-RC840T-240-Switching-Electric-Transformer/dp/B0037MXM1C/ref=asc_df_B0037MXM1C

author: BlackICE
author_rank: Regular Contributor
timestamp: May 25, 2021, 03:33:24 am
content: I have a similar situation. However the cable connected to the thermostat has extra wires that are unused. A lazy technician didn't hook up the extra wire for powering the newer type thermostats. However if I'm willing to crawl into the crawl space I believe I can use the extra conductor to connect to the furnace Transformer.

author: Quarlo Klobrigney
author_rank: Super Contributor
timestamp: May 25, 2021, 05:13:16 am
content: Why would there be 240V in a single gang box?Pull the wire back to its origin.Next run the 24V to the box from the furnace, boiler etc. transformer and back again.Even that doesn't explain where the rest of the wiring went to.A picture is worth a 1000 words.BTW, you will never fit a circuit as you intend in a single gang box. Also there is such things as "box fill" Look up "box fill calculator" and the limitations there of.

author: syl20bou
author_rank: Newbie
timestamp: May 26, 2021, 11:22:09 am
content: Thanks WattsThat! The Aube thing will be my best option indeed.

author: Terry Bites
author_rank: Super Contributor
timestamp: May 27, 2021, 02:22:39 pm
content: 0.5VA transformers occupy about a cubic inch.https://docs.rs-online.com/fee4/0900766b80029e0c.pdfI assume you want 50/60Hz, so you are stuck with the iron cross sectional area.A solid state conveter would be more bulky and very hard to make.I thought nest ran from LV DC?Great picture and part number info for world wide standard thermostat.