Voltage drop for LED's question...

[diverjeff]

led voltage drop chart

Hey all:

Am revisiting a previous topic. I am wanting to use LEDs to mark the width boundries of my driveway. Will be using white, 5mm, 3.5 vdc, 20 mA LEDs. My plans are to use some 22 ga. twisted pair conductor I've acquired. I have to run 500ft.

My plans are to use a 120 volt photocell - 120 VAC to 12 VDC transformer - then a voltage/current limiting circuit for the LEDs. The LEDs will be spaced 20ft. apart.

Does anyone have a voltage drop link for smaller wire? I would also really appreciate any ideas. Do you think this will work and if not what are some suggestions?

Respectfully,

DJ

 

[brevor]

led voltage drop

A wire chart I have shows 22 AWG wire can carry .92 amps.
If your driveway is 500 ft. led's are 20 ft. apart that is 25 leds for one side
or 50 led's if you do both sides. If you wire them in parallel (which is what I would do)
20 ma X 50 led's = 1 amp. 22 gauge wire is a bit small. You could run a wire pair for each side of the driveway, That way each side would only carry 500 ma.

 

[nguyennam]

led droop chart

That 22 ga. wire can conduct 1A well, you should not worry about that.

But, as you already mentioned, the problem will come from voltage drop on the wire, not the current conducting capability itself. This wire has 16 ohm per 1000 ft., means a voltage drop would be 16V if you use the above diagram with 1A. It is big compared with 12V power supply.

The more commonly used diagram for your lamp/LED "chain" is connecting LEDs in series, not in parallel as you are talking about.

Say, each side of the road, you will connect 25 (500/20) LEDs in series (with the voltage requirement of 25 x 3.5V = 87.5V) and with a resistor of only 1W dissipation (for the remained 32.5V) and directly to 120V photocell. Using this way, you will use only 1 wire each side (not a pair, so you save 1 wire), the voltage drop on the wire is very small either (8 ohm x 20mA = 0.16V), and you will also save the power, and need no 120V-12V transformer.

On the other hand, connecting LEDs in series make them light equally from LED to LED, as they all light by the same current.

Hope this will help.
nguyennam