I have an EE background at school, but am truly humbled by what I don't know as I start doing practical projects. So talk to me like I'm a noob that knows some theory, with little power experience.

I'd like to create a circuit, driven by one 24V AC. I'd like to have a fence lighted up by having single LEDs placed every 20 feet, on a 300 ft fence. The wire would be covered in PVC and waterproofed within the fence, but we want to expose the LED bulb. We want this circuit to be as cheap as possible but safe. All I know is that we'd need to connect these LEDs in parallel, and each are connected with a current-limiting resistor.

>What type of wire would I use that would work best?
>Would I have to convert the 24V AC to DC, and using what component?
>Is there anything I need to modify in this long circuit?
>What LED parameters would I need to have to make it a bright-enough fence so that people a block away might be able to see the fence light up? Would these LEDs http://www.radioshack.com/product/index.jsp?productId=2062548 work?
>Is exposing the LED to the outside dangerous, e.g. would rain be dangerous to a kid who tries to play around with it, since it's powered by 24V?
>What exactly is considered "high voltage?" (I'm worried about safety)

 

>What type of wire would I use that would work best?
Just about anything would be OK. Your biggest stress is the sun warming the PVC conduit. You can find low voltage lighting wire at a home center rated to work without the PVC.

>Would I have to convert the 24V AC to DC, and using what component?
While each LED acts as a diode they make poor diodes, so you would need another series diode to protect the LED from reverse voltage, plus a series resistor (all 3 parts in series). I'd try something like a 2K ohm 1/2 watt resistor to start and experiment to see what value makes the intensity you like.

Also, to balance the system I'd wire alternating LEDs in opposite polarity. Ignore the last Odd led, it can go either way.


>Is there anything I need to modify in this long circuit?
Probably not, unless you use very very thin wire where the resistance of 600 feet of wire (the current makes a round trip) becomes significant.

>What LED parameters would I need to have to make it a bright-enough fence so that >people a block away might be able to see the fence light up? Would these LEDs >http://www.radioshack.com/product/in...ductId=2062548 work?
Probably. It takes a ridiculous small amount of power to see the LED itself.

>Is exposing the LED to the outside dangerous, e.g. would rain be dangerous to a kid >who tries to play around with it, since it's powered by 24V?
Such lighting circuits are so intrinsically safe that the US National Electric code (NEC) has a special category for them: if you use an isolation transformer in a power limited circuit you don't even have to ground the silly thing.

>What exactly is considered "high voltage?" (I'm worried about safety)
Generally a conservative number is over 40 volts is high.

 

Thanks very much ErnieM. I feel more confident in the circuit. A couple more questions just came up:

>I thought resistance was only found in ohms, so how do you find the wattage of the resistor?

>I have a 120VAC to 24VAC transformer powering this circuit, how do I determine if it's an isolation transformer or not?

 

...>I thought resistance was only found in ohms, so how do you find the wattage of the resistor?

Resistance is measured in Ohms. Power is measured in Watts. The size of the resistor can give you a fair idea as to the amount of power it can handle. This can give you a relative visual reference.

>I have a 120VAC to 24VAC transformer powering this circuit, how do I determine if it's an isolation transformer or not?

It is most likely an isolation transformer. If the secondary winding of the transformer is not physically tied to the primary, it is isolated. Use an ohmmeter to check the secondary output to the primary.

 

You don't have an EE background if you don't know about watts.
Or you went to MIT
Hoax or not I love that youtube video with the graduating class from MIT being given a simple wire/battery/lightbulb and being asked to make the bulb light up and they were stumped..

 

>I thought resistance was only found in ohms, so how do you find the wattage of the resistor?

Power for a resistor is current thru times volts across. Assuming the entire 24V is across the resistor the current is 24V / 2,000 ohms = 12 mA. 12 mA times 24V is .288 watts. I guestimated the power on the high side but to be safe I (mostly) doubled it anyway to .5 or 1/2 watt, a standard value you may well find at the Shack with the other parts.

Note since the LED only lights every half cycle I'm at least a factor of 2 high anyway, so a 1/4 watt resistor may well work too, but here more wattage is better for a long life (he says glibly to cover his previous over estimation).

>I have a 120VAC to 24VAC transformer powering this circuit, how do I determine if it's an isolation transformer or not?

It probably already is. There exist things called auto transformers, but more transformers have separate primary and secondary windings and thus intrinsically isolate. To be 100% sure just try an ohmmeter on it.

Now to get the 24V power limited I would add a fuse into the works too. Your draw is under .2 amps so anything from 1/2 to 1 amp fuse should be good. The point here is to limit the current to some safe value way above what you are drawing so you don't have to change the fuse ever except for a catastrophic failure.

 

What type of fence? Do you want to illuminate the fence or just have LEDs visable. The R-S LED looks like an indicator, not verry bright. I would full wave rectify the 24 V AC to 22V pulsating DC & put two groups of 8 LEDs in series with 1 resistor for each group. LED color is red?? So: 22V - [ Vf about 2.2V X 8 = 17.6 V ] = 4.4V @ 20 mA = 220Ω. The 2 groups connected in parallel.

 

You don't have an EE background if you don't know about watts.
Or you went to MIT
Hoax or not I love that youtube video with the graduating class from MIT being given a simple wire/battery/lightbulb and being asked to make the bulb light up and they were stumped..

I do know that power dissipates through resistors, I should've reworded it. "I thought resistors was only labeled* in ohms, so how do you find the wattage?" and thanks to Ernie, now I know which V and I to use.

Bernard: Block fence; just want the LEDs to be visible, but have them bright enough so people a block or more away can see it. What's the reason for using 2 parallel groups?

 

I'd be lazy and not include an extra diode for every LED, but just use 4 to make a single full wave bridge and power them all off of that.

You need to choose your LEDs wisely. Many you can find are VERY bright (much brighter than what you'll find at the shack, even at the same power). But they may also be focused into a narrow beam, and that is probably NOT what you want unless you want them to be seen at a particular spot and you aim them at that area. If you want to point them all up or all down and have them seen from 360° around, you'll need LEDs meant for that and/or specialized reflectors to diffuse the light. The commercial reflectors, for instance in LED gaslights, look like a mirrored hyperbolic cone, pointing at the LED which is pointed back at the cone.

If each post's LED just points horizontally to eye level, I think you could get away with the focused LEDs. You wouldn't see all the posts as bright as others, but it would be hard to miss.

Just to elaborate on what Bernard is suggesting, if you drop over 20V of the supply 24V across a resistor and just 3-4 across a single LED, most of your power is dissipated in the resistor. You could add a bunch more LEDs in series and get all that much more light for free (with respect to the power, not the cost of the LEDs, but they're cheap). You could point them in different directions, make a strip, go wild.

 

You efficiencies go way up if the LEDs are in chains (series), as the same current powers more than one LED, however wire is expensive, so it is a trade off. The resistors will stay much cooler as they drop less voltage (and therefore dissipate less power).

LEDs, 555s, Flashers, and Light Chasers

Bill's Index (blog)

 

The LEDs in my solar garden lights are enclosed and protected from rain. But they still rust away in a year or two from the humidity when it is raining.
Your RadioShack LEDs might rust away in a couple of months.

The RadioShack LED in the holder that you posted has a maximum voltage rating of 12V so it must have a resistor inside the holder to limit its current. Two can be connected in series and be powered by rectified 24VAC.