What is the deal with 24VAC?
stevels
Posts: 19
So what is the deal with 24VAC?
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I mean, I understand why we have a 9VD, ·6VDV, 12VDC, 1.5VDC.
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I understand 120VAC (60Hz) comes out of US electric outlets, and 220 comes out of European outlets (50Hz).
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However-- why do I constantly see 24 VAC used so much in stuff? ·Is it because it is easy to step down from 120VAC or 220VAC? ·
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Is it safe for a baby to touch? ·If it needs to be a safe, low voltage, like for door bells and thermostats, why isn’t it just 12 VDC? ·That never killed anyone and there is a lot more support for 12DC stuff than 24VAC?
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Why whenever I see 24VAC, I never see a Hz rating to go with it?
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Please someone – what is so cool about 24VAC?
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And as a side not, why to I sometimes see 24VDC? ·Does anyone have a car that uses this voltage? ·I have seen a few UPS devices use it internally, but can’t you just use 12 VDC and have twice the amperage?
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These things keep me up at night.
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Any response would be appreciated.
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Expert in many fields, dumb as a stump with STAMP
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I mean, I understand why we have a 9VD, ·6VDV, 12VDC, 1.5VDC.
·
I understand 120VAC (60Hz) comes out of US electric outlets, and 220 comes out of European outlets (50Hz).
·
However-- why do I constantly see 24 VAC used so much in stuff? ·Is it because it is easy to step down from 120VAC or 220VAC? ·
·
Is it safe for a baby to touch? ·If it needs to be a safe, low voltage, like for door bells and thermostats, why isn’t it just 12 VDC? ·That never killed anyone and there is a lot more support for 12DC stuff than 24VAC?
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Why whenever I see 24VAC, I never see a Hz rating to go with it?
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Please someone – what is so cool about 24VAC?
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And as a side not, why to I sometimes see 24VDC? ·Does anyone have a car that uses this voltage? ·I have seen a few UPS devices use it internally, but can’t you just use 12 VDC and have twice the amperage?
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These things keep me up at night.
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Any response would be appreciated.
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Expert in many fields, dumb as a stump with STAMP
Comments
I mean, I understand why we have a 9VD, 6VDV, 12VDC, 1.5VDC.
I understand 120VAC (60Hz) comes out of US electric outlets, and 220 comes out of European outlets (50Hz).
However-- why do I constantly see 24 VAC used so much in stuff? Is it because it is easy to step down from 120VAC or 220VAC?
Is it safe for a baby to touch? If it needs to be a safe, low voltage, like for door bells and thermostats, why isn’t it just 12 VDC? That never killed anyone and there is a lot more support for 12DC stuff than 24VAC?
Why whenever I see 24VAC, I never see a Hz rating to go with it?
Please someone – what is so cool about 24VAC?
And as a side not, why to I sometimes see 24VDC? Does anyone have a car that uses this voltage? I have seen a few UPS devices use it internally, but can’t you just use 12 VDC and have twice the amperage?
These things keep me up at night.
Any response would be appreciated."
Hehe....how to answer!
24Vac on a doorbell is easy....north american homes have 120Vac (60Hz). So all they need is a transformer to step it down, where they'd need an AC to DC rectifier to convert it if it were 12Vdc.
As far as why isn't more things 12Vdc....it's a legacy thing. (someone please correct me)....but I believe Voltaire had originally tried to sell a DC utility network to a city and it was found that DC loses it's voltage over distance. Whereas AC does not (at least not near as much as DC) so they went with AC as it was cheaper.
I thought I had heard some chatter about some big-wig (google owner or Apple owner) wanting to push DC buss's in homes. You DO see a lot of small appliances that run off of 12Vdc and it'd be nice to just plug in to the required voltage, as opposed to rectifying AC in to DC thereby losing energy in the process!
Is 24Vac any less dangerous? Well, they all say it's the amperage that kills you!! I've worked on 5Vdc power supplies that had a 40Amp output....uhm, sure it's low voltage, but I ain't putting my tongue across those leads....so is 24Vac any different....well, treat it with respect, it'll still make you tingle I'm sure!
As far as a frequency rating on anything other than 120Vac being 60Hz. If you use any type of transformer on it (step up or down) it's still 60Hz on the output of the transformer. You can find frequency douplers within power supplies, but those are for specific applications!
Some manufacturers like to slim down their product by making the power supply external to the unit (xbox 360/laptops). You plug them in to 120Vac and the P.S. generates the required voltages....where, a straight 'corded' device has to have the power supply built in. This means the package is larger and that the unit has to deal with the added heat from the internal power supply.
I assume external modems require 24Vac adaptors so they can interface to the phone line. They also rectify the 24Vac to some flavours of DC (5Vdc for logic and 12Vdc for relays or such).
Lots of things are changing and it really is a waste to plug a 12Vdc device into a 120Vac outlet just to dumb it down to what it wants. They way energy conservation is going, you may see things changing.
Anyhow, hope that gives some info for you!
(hopefully someone will correct me for my heinous mistakes!) [noparse];)[/noparse]
cheers
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Steve
"Inside each and every one of us is our one, true authentic swing. Something we was born with. Something that's ours and ours alone. Something that can't be learned... something that's got to be remembered."
LOL.
Post Edit -- in re. external modems: Hayes "standard" is 9 vac.
TESLA! lol
Couldn't remember his name, but he did have a workshop in Niagara FAlls, NY (I lived in NF, Canada)
Gotta love Google:
en.wikipedia.org/wiki/War_of_Currents
Made a mistake...it was Edison that wanted DC and it was Westinghouse that won with AC
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Steve
"Inside each and every one of us is our one, true authentic swing. Something we was born with. Something that's ours and ours alone. Something that can't be learned... something that's got to be remembered."
I thought it was Socrates… to give you an idea of where I am coming from [noparse];)[/noparse]
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OK, so this makes a bit of sense...
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it is easy to step down 120VAC to 24VAC as it is a factor of 1/5 the voltage. But is true that the intention to step it down was for safety in the first place?· Assuming that is true, it makes sense that you would not want 120VAC doorbells or thermostats.· My next question of course is... why 24V???· Why not make it a nice even factor of 10 and use 12VAC?· That would be even safer, and we all know 1 volts can do a lot of cool things, like keep your beers cold in your car!
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Wouldn’t 12VAC be safer, and (and here is where I know nothing about those “rectum-fires” you speak of) couldn't 12VAC be converted to 12VDC without too much hassle??
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Thanks again!
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SA
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Expert in many fields, dumb as a stump with STAMP
Lets use a common flash light bulb as an example
I am going to use numbers that may not represent an acual flash light bulb
Lets say the light bulb has a resistance of 12 ohms and it is rated at 12VDC
We know using ohms law that I=E/R or Amps=Voltage/Resistance
I=AMPS
E=Voltage
R=Resistance
So we take 12V/12ohms and get 1.0 or 1 AMP
Lets try and put 6V in and see what we get: 6/12 = 0.5 Amps or we say 500 milliamps or 500mA
The bulb will not glow as brightly as 12V because it can only draw 500mA
Now put 24V on the bulb and it would get 24/12 = 2 amps and it would burn up
However, put two of those bulbs in series, and you have now 12ohms +12ohms =24 ohms of resistance
Now 12V/24 is only 500ma and 24v/24ohms is 1Amp so now you need 24V to light the bulbs to their rated brightness.
However, what if you put the bulbs in parallel? equal Resistance in parallel gets cut in half so now two 12ohm bulbs in parallel is only 6 ohms
12V/6 = 2 amps but you have two paths with two bulbs each drawing 1 amp each for a total of 2 amps.
The point is, circuits contain many components each with various current and voltage requirements. The Power source used to operate the device has to have enough voltage and be able to supply enough current to operate the device. Some parts of the circuit would starve for current if there is not enough voltage to push it through.
This is a nice little lab experiment page that uses java applets you can play with and learn about voltage and currents.
http://jersey.uoregon.edu/vlab/Voltage/
This is a circuit simulator that runs in a java window. You can put your own circuits on the page and watch the current move through the wires, using a virtual scope and see voltages and currents change as you change values in the circuits you make. It has many default circuits to look at as well.
http://www.falstad.com/circuit/
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Think Inside the box first and if that doesn't work..
Think outside the BOX!
If you have a transformer that is wound with a 2:1 ratio of wiring....if you put 100V on the side with the most wraps, you'll get 50V on the output.
Now, turn things around and if you put 100V on the other end, you'll get 200V out.
Sounds weird.....however, there is one thing to note....the current available isn't the same.
Follow ohms law. I = V/R Although in this case you'd probably use the power equation P = I * V So, say a Power rating of 100W...at 100V (I'm doing simple math) you have 1Amp (100W/100V)
So, at the same power (100W) if our output is 200V....100W/200V our current is 0.5A. On the opposite end, if we end up with 100V stepped down to 50V...100W/50V is 2Amps.
--hope that made sense!
Consider it like the "Law of Conservation of Energy". You have to get out what you put in. There are losses to consider (heat/copper losses..etc..) as well!
12Vac CAN be converted to 12Vdc fairly easily. For the most part it's just diodes and capacitors (google rectifiers or bridge diode).
A lot of this stuff is legacy stuff!! I had an old email that described why the booster rockets on the space shuttle are the size they are.....and it all boiled down to the wagon wheel spacing back in the old west....if someone has it pass it along. (essentially, the boosters fit on a train that fits on a rail who's spacing is the same as the wagon wheels which probably came from how close you could harness 2 horses...etc..)
So, a lot of times things were chosen for one reason without knowing how things would be in the future!
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Steve
"Inside each and every one of us is our one, true authentic swing. Something we was born with. Something that's ours and ours alone. Something that can't be learned... something that's got to be remembered."
lets take a 100 watt light bulb and put it on a 12 volt, and a 24 volt source.
100W bulb, 12V = 8.3A
100W bulb, 24V = 4.1A
as you can see there is less current with 24 Volts. That means two things, smaller wire, and lower cost.
Now in the automotive world, manufactors are trying to change the current 12 Volt system, and change it to a 48 volt system. Now if I can rember it takes 32 Volts to pirce your skin. I think there will be problems with that.
TC
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We all make mistakes when we are young………That’s why paste is edible!
Not sure what that will do when the cars switch to 48 (really 53.04 (2.21 * 6 * 4)) volts. And the charge voltage will be even hgher!
As TC said reguarding 24 vs. 12 volts,; the higher the voltage -> the smaller the current -> the smaller the wires -> the cheaper the wires.
AC vs DC. AC is easier to switch to a different voltage with low loss/cheaply.
The answers and links you have included are keeping me up late at night and my wife hates you all for it!
I think what I am understanding is broadening my overall understanding about electricity. While I understand the concepts that Steve_b posted about the relationship between current and voltage – and intuitively it all makes sense too, I had a bit of an epiphany with what TC said.
I understand his example about a 100W load drawing proportionally different amperage at different voltages. What I don’t fully digest is this.
I understand that a Watt is a Watt, is a Watt. It is a unit of work, and we don’t care how we get it. It could be from lots of little button batteries or from the 220VAC that the clothes drier is plugged into. What I did not clearly see, is that the guage of a wire is dependant on the amperage and not the wattage.
I thought that the limiting capacity of a wire to conduct without turning into a toaster was based on the bottom line output wattage or total work of the load going through the wire, and that the amperage was fairly irrelevant assuming the load across the wire was inversely compensating for higher amperage.
Does this mean that by increasing the voltage, I can get more work (watts) out of a strand of wire than if I lowered the voltage, but kept the end wattage use constant?
Does this explain how those high voltage wires that carry enough juice across miles of fields seem relatively skinny yet clearly, they are churning out lots of watts?
In that case, is the European system of 220V less wire intensive?
My mind is spinning!
Why don’t we use 100VDC in circuits so we can make our conductive traces even smaller for the same power draw? Smaller chips are good?
Thanks again everyone. I will get this all sorted out, I promise!
SA
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Expert in many fields, dumb as a stump with STAMP
Power = Voltage * Current
Power = Voltage^2 / Resistance
Power = Current * Resistance^2
Now Ohm's Law is:
Voltage = Current * Resistance
I keep erasing my comments....not quite sure how to answer this without confusing things.
Power is directly proportional to Current and Voltage. So, if one doesn't change and the other increases or decreases....your power will change. Having said that, If you double your voltage and halve your current, your power hasn't changed.
Motors require a specific voltage and current. Give them more of one or less of another will either fry them or cause them to chatter (or not run at all). Too much current could fry/weld the motor windings (Tim "the toolman" Taylor didn't do electronics for a reason! haha)
The distribution lines outside your home are at some crazy high voltage (anyone?....10,000V??) They go to a transformer in your neighborhood (either the big green box or overhead pole 'cans) and are dropped down to something more suitable for households. Remember the step-up/down descriptions? If you double your voltage, you half your current!
Not sure about European systems!
100Vdc is a large potential. Remember that electro-static sparks are 10,000's of Volts but very low current and these little 'shocks' will kill an IC quite easily!
Remember the hydro analogy to electricity. Voltage is the force of water and Current is the amount of water. Too much pressure (voltage) and you'll pop your gaskets! hehe
At work we have a radar modulator that runs at 26000Vdc. The 'tube' (similar to a vacuum tube, but different in physical design) has a cathode (negative terminal) and an anode (positive terminal) and the voltage different between the two of them is only 9volts where as the difference between earth ground and either terminal is the 26000V. Scary enough!
Not sure if I helped or hindered!
cheers
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Steve
"Inside each and every one of us is our one, true authentic swing. Something we was born with. Something that's ours and ours alone. Something that can't be learned... something that's got to be remembered."
www.astrodigital.org/space/stshorse.html
Not sure how true it is but it sounds plausible.
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- Rick
24 Volts DC has become preferred in some cases. This is because of things like (back a while in time) diode/relay logic, and more recently things like solid state relays have eliminated the concern for contact life.
The reason there is less arcing is that as the contact is opened, the AC voltage will transit through “zero volts” and the current (and arc) will stop. With DC, the voltage does not reverse, so the arc lasts longer, especially if there is an inductive load.
Also, as a side note, the reason we use AC for "main power" to our homes, industry, etc., isn't that AC electricity looses less voltage over distance than DC. Voltage drop is a function of current and resistance. AC is easier to change voltages up and down via transformers. Power, as mentioned above, is a function of voltage and current. Increase the voltage, and you require less current. Less current through a given resistance (length of cable/wire) means less voltage drop. So you transmit large amounts of power long distances by using a high voltage (i.e. "High Tension Wires"). Then you use a "step down" transformer to lower the voltage to a useable level (120/240/480, etc.).
If you tried to use DC for power transmission, you would not have a practical way of transmitting large amounts of power over long distances.
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John R.
8 + 8 = 10