27vac to dc
BobVal
Posts: 48
Hello:
I am replacing a thermostat with a Javelin and other devices and was wondering if there was a CHEAP and easy way to take the 27 volts AC that is coming in and convert it to 6 - 10 volts DC for the Javelin and other devices?· I would still want access to the 27vac because I need it for the zone values.
I have looked around and everything that converts ac to dc costs hundreds.
There must be a nice cheap and easy circuit that I could buld?
Anyone have this type of background?
Thanks
Bob
I am replacing a thermostat with a Javelin and other devices and was wondering if there was a CHEAP and easy way to take the 27 volts AC that is coming in and convert it to 6 - 10 volts DC for the Javelin and other devices?· I would still want access to the 27vac because I need it for the zone values.
I have looked around and everything that converts ac to dc costs hundreds.
There must be a nice cheap and easy circuit that I could buld?
Anyone have this type of background?
Thanks
Bob
Comments
A full-wave bridge rectifier converts ac to pulsating DC and·the addition of a smoothing capacitor takes out the ripple; $1 for the former and another $1 for the latter.
I thought that pulsating dc was created by using a Half bridge rectifier.. That would have an equal amount of pause between the hyperbola shape sine wave on an ocope. A full bridge rectifier due to using 4 diodes gives you a full dc sine wave because the second set of diodes gives you the second hyperbola on the oscope when the ac alternation goes negative. But due to some sort of lag in the diodes causes the sine wave to be imperfect thus the ripple effect. By using a capacitor you can smooth out the sine wave and produce a steady dc current .
am i correct here or am i not learning this correctly. I am just trying to see if what I think is correct is correct ok
Badger the confused LOL
thanks for answering my question. I did not take the time to look up the ripple freqs. in my text book. I should have done that before posting. For that I apologize but the half and full wave question you cleared up for me. I watch these forums like a hawk and read all posts of interest. I capture all posts from the Basic Stamp, Hydra, Prop, Robotic, and the Sand Box. I know about who knows what and such. I never question but just ask to clarify some things as i did here. Thanks for your time to answer.
Badger
How do I get a parts list and hookup drawing.
Would be nice if you guys could give some digikey numbers for parts, etc
Thanks
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Chris Savage
Parallax Engineering
Post Edit -- With 27vac in, the output will peak charge to appx. 38V, which is a little high for most regulators.· It's a lot to take on if you have no experience in this area.
Post Edited (PJ Allen) : 11/10/2008 4:24:29 PM GMT
It can be made into a pretty decent bench top supply if you put it in a case, change the pot on the board to a panel mount type and add add a meter.
If you want to build one yourself, just Google "LM317 power supply circuit".
edit: PJ is right, the input is a little high for most of the common regulators. This would preclude the use of most available kits, doing it yourself is still possible. Some options would be an input transformer on the AC, or some diodes to drop the DC prior to the regulator, or building the circuit with a higher voltage tolerant regulator such as the LM317HV.
www.national.com/mpf/LM/LM317HV.html
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- Rick
Post Edited (RDL2004) : 11/10/2008 4:47:29 PM GMT
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- Stephen
·If you use what PJ Allen·in his post earlier and you use the switching power supply that i have the link below that all you will·need
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The full-wave bridge is the heart of a power supply
Take a look at this kit this might be·what looking·
http://store.nutsvolts.com/product.php?productid=16719&cat=379&page=1
I used one in this project and it works very well now to this day
····························· >>>>>Click on this<<<<<<<<<
Elapsed Timer For Testing Running Time on A/C Units or other Equipment·
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··Thanks for any·
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Sam
Post Edited (sam_sam_sam) : 11/16/2008 3:12:52 AM GMT
Thanks but I already ordered a few of the ones that one in the post by RDL2004, they are a little less money and seem a little smaller.
Amazing when I posted this the first couple of replies made me feel like I could do this for 3 bucks.
Thanks
Bob
I want to point out that the description of the ebay item notes a maximum input of 24vac (vs. your nominal 27vac supply/transformer.)
Why isn't this easier.
Since I already ordered one, I guess I will have to find a use for it or return it (the return shipping plus the shipping I already paid to have it sent to me probably makes it worth keeping).
But before I do anything else I better put a meter on my zone tranformer to see if it is 24vac or 27vac (never actually tested the line, just assumed it was 27vac - but it could be 24vac).
Now the big question is what happens if I put 27vac into this item? Is it toast?
Bob
The simplest and cheapest thing to do that I can think of is to string enough diodes together to drop the voltage to a safe level. Just use 1N400X types and figure about 0.7v drop per diode.
Another possibility would be to rectify the incoming AC then use a zener pre-regulator.
A transformer on the AC is a third possibility, but will be bulkier and probably the most expensive way to do it.
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- Rick
they can be as big as a car or as small as the tip of your pinky
i have seen real small ones that pack a punch
if you have anything that had a power adapter you could probably get away with using that
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Parallax posesses power beyond belief.
Believe in it.
You should take a careful look at the circuitry used by the furnace/air conditioner you are going to control with your circuit. In most basic systems the thermostat is a switch that closes the circuit where the 27 volts goes to the coil of a relay whose contacts turn on the furnace/ac when closed. If you use the 27v to run the Javelin you will be turning off power to it the moment you turn on your furnace/ac. There are a couple of ways around this but they require a bit of circuitry.
You are making this harder than you need to
Do·you have two power wire·from the transformer to the·thermostat
Most of the time
One wire goes to the thermostat from the transformer· unless you have a digital thermostat that dose not have battery's then both wire are there at thermostat some time this dose not hold true
Look at all of the wires at the transformer·do both power·wires go to the thermostat
If they do then you have it made
If they do not then you will have to add one more wire to it ·to power your· Javelin Board
one of those wire run the fan most of the time that color wire is·green
These are your color wire you should have
RED is power from the transformer
GREEN controls the fan
YELLOW controls the compressor
Orange controls the reversing value you may or may not have this wire hook up
WHITE is for heat
PURPLE or BLUE is for power from the transformer·most of the time you may or may not have this wire hook up
I hope this help you
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Sam