Sizing 24V Transformer for HAVC Thermostat and Cool Valve - Page 1
Forum section: electronics / projects

author: suryaputhra
author_rank: Regular Contributor
timestamp: October 29, 2016, 03:38:07 pm
content: I have designed a power supply for a thermostat and cool valve, both works on 24VAC. Cool valve's inrush current is 280 mA and normal working current is 80 mA. Thermostat power consumption is not available. Our unit is halfway between thermostat and AC unit, from our power supply four one square mm wires will go to thermostat which is 20 feet away, from thermostat three wires come back to our power supply with 24V signal to activate 24V relays. Each wire runs 40 feet so seven wire into 40 feet 280 feet is the electrical cable running length. My questions are below1. What could be the sizing of 230V,24V step down transformer by considering voltage drop across the seven core one square mm cables subsequent current loss. How much voltage is the drop. Best voltage regulation 10% or 5%. What could be the best amp capacity of the Transformer.2. In the below functional block diagram where can I connect oscilloscope to observe step down transformer's sine wave, output frequency and other parameters.3. Suggest me one simple DIY 24VAC load bank to test this unit.3. The power supposed to work 24/7 365 days non stop let me know any other important inputs.-suryaputhra

author: suryaputhra
author_rank: Regular Contributor
timestamp: October 30, 2016, 03:06:33 am
content: Can someone put on light on my topic.-suryaputhra

author: suryaputhra
author_rank: Regular Contributor
timestamp: October 30, 2016, 05:31:28 am
content: "Right after step down xfmr", is xfmr stands for transformer, where to connect ground lead of oscilloscope.-suryaputhra

author: suryaputhra
author_rank: Regular Contributor
timestamp: October 30, 2016, 06:39:08 am
content: I do not have isolation transformer, can I remove ground wire from the havc power supply (so that no ground loop happens) and connecting oscilloscope leads to transformer output wires phase red wire to probe and neutral wire to ground wire of oscilloscope.-suryaputhra

author: suryaputhra
author_rank: Regular Contributor
timestamp: October 30, 2016, 07:14:41 am
content: Quote from: blueskull on October 30, 2016, 06:59:35 amI read that small difference in the guage of transformer wire will change in frequency, pls explain me really transformer will sync to grid.Total of seven wires a total length of 280 feet going this is powered by our transformer. I believe there will be 1.5V drop am I am wrong. Fan motor is of 230V/3amp relay will bypass the line based on output from thermostat.-suryaputhra

author: suryaputhra
author_rank: Regular Contributor
timestamp: October 30, 2016, 07:27:51 am
content: What could be maximum voltage these 24 VAC thermostat and 24VAC cool valve can take longtime because our transformer sometimes gives 25.5 VAC.-suryaputhra

author: suryaputhra
author_rank: Regular Contributor
timestamp: October 30, 2016, 08:01:53 am
content: Cool valve says 10%, but unfortunately thermostat spec sheet or data sheet not mentioned about the regulation.-suryaputhra

author: SeanB
author_rank: Super Contributor
timestamp: October 30, 2016, 09:21:23 am
content: Your normal off the shelf thermostat transformer is almost always a 24VAC 1A unit, which will run all the above without any issues for decades. The longer wire will drop the voltage slightly, but in all cases the designers of the thermostats, relays and valving take the voltage drop into account, and almost allways they will work from 22VAC to 26VAC with no issues, so the slightly higher voltage on the secondary is not going to cause any issues, and in any case this will vary with the incoming mains voltage in any case, as this is also not a constant voltage, but varies within a band of 210VAC to 250VAC in most parts of the world, and in India this will most likely be on the low side due to high loading on the power grid in most cities.The one that comes with the airconditioner or the boiler control will work here, and will likely already be there in the system as supplied from the manufacturer. This transformer will be in the unit already, and the 4 wire control is standard to supply the 24VAC, a common wire and then 2 control wires to the unit from the thermostat. One is for the fan control, and the other is for cooling control. The thermostat will be supplied from this 24VAC supply, and on some systems they use a 3 wire connection and the thermostat has a 3VAC drop across the control outputs when on to power the electronics, and a low current draw when off, so it can emulate a mechanical thermostat without any rewire needed. 4 wire only allows a brighter display backlight or for those units that need more than 100mA of power off the 24VAC line for them to function, as the relays in the indoor unit might not release at 100mA of current through them.

author: suryaputhra
author_rank: Regular Contributor
timestamp: October 30, 2016, 12:06:49 pm
content: @SeanB, the power supply unit did not come from the manufracturer of HAVC or thermostat company. We have designed the power supply from the scratch and need to supply 3000 units for the client. So are the questions and doubts. Thanks for info.-suryaputhra

author: SeanB
author_rank: Super Contributor
timestamp: October 30, 2016, 01:44:57 pm
content: As you are buying them best to use an off the shelf part from a supplier then, and this will be cheaper than a custom part, and with a 3k order, that will be a quantity they can supply off the shelf. The biggest issue is to get a part with low losses off load, as it will spend all of it's life running with no load, so either get the no load power consumption, or get a sample and measure the no load temperature rise in a small plastic electronics project box ( so there is no convective cooling) and see which one has the lowest temperature rise after being run for 6 hours. lower loss will have a larger core, more laminations and more turns of copper so the core losses and copper losses are lower.

author: suryaputhra
author_rank: Regular Contributor
timestamp: October 30, 2016, 02:00:36 pm
content: @SeanB, yes, as you suggested we are going to test it with the same principle you mentioned. We are using IP40 Hager box for power supply because client asked for IP40, no proper cooling using this box, anyway we are doing thermal profiling.-suryaputhra