"Low Intensity": Not just a workout

[tstage]

Can any one of you electronics gurus tell me whether there is a difference between a regular LED and one designated as a "low intensity" LED? I'm guessing from the name that the light emitted through the "low intensity" LED is softer or less intense than a "normal" LED.

The reason for asking is that I wanted to install a "Panel Locator Light" onto the UTP panel that I just bought (an option), to use with my new throttle. I went to Radio Shack to pick up a 3mm T-1 red LED and a 1K 1/4W resistor. RS had the 1K resistor in stock, but they only had the T-1 red "low intensity" LEDs. So, I picked up a two-pack. I soldered both of them onto the UTP board (according to the instructions) but the LED wouldn't light up.

I thought perchance I might have somehow fried the LED, so I removed the first LED and installed another one. Nope, that one didn't work either. The LED IS in the correct orientation. (The cathode is soldered to the "square" solder pad and not the "round" one.) Does a "low intensity" LED take a lower resistor or higher than would a regular LED?

I did watch the second LED, as I powered up the Power Cab, to make sure that it wasn't blowing somehow. I can probably pick up 3mm red LEDs at my LHS but the Radio Shack was much closer...and open.

Anyhow, thanks for the help ahead of time...

Tom

[BRVRR]

tstage,
Depending on your power supply to the LED, the 1K ohm resistor might be too much. Most of the 3mm LEDsI have (Miniatronics Yeloglo White) require a 270 ohm resistor for 4-9 volts and 470 ohm resistor for 10-16 volts.
If your voltage falls into the ranges above, the 1K ohm resistor is too much.
Hope this helps,

[tstage]

Allan,

Thanks. I'm just going by what the manufacturer suggested in the sheet that accompanied the UTP panel:

"Conventional T1 sized red, green or yellow LED can be used...Be sure to solder the shorter lead (cathode) of the LED in to the square hole. You will also need to solder a 1K 1/4 watt resistor in the two holes marked "1K".

...and I followed it to a tee. So, should a "low intensity" LED work fine?

Tom

[tstage]

Hoping to get a few more answers...bump.

[Don Gibson]

TOM:

Since carbon resistors come +/-10% and +/-20%, I USE a cheap V/Ohm meter and small alligator clips to vary the resistance to get 10ma. or the intensity I want.

A +/- 10% 600 ohm resistor can be 540 ohms or 660, a +/-20% 480 - 720 ohms.

[rrinker]

Low intensity LEDs are just that, lesser light intensity. The trade off is usually lower current consumption. The current rating should be on the package. Use abotu haf that when calculating a proper resistor (assuming you know the voltage that will appear at those terminals).

I usually stockpile the resistors that come with the Miniatronics LEDs - 470 ohm is just way too small unless you a certaint he voltage will never rise about 12 volts or so. At 16 volts, a 470 ohm resistor on one of the YeloGlo LEDs would be running at nearly 27ma. I use 1K resistors in all my loco installations for that reason.

For the ordinary color LED, with a 12V source - try about a 680 ohm resistor. That's about 14ma, should be in the safe range for the LED.

Dumb question, I was reading the UTP instructions and I didn;t see where it says to use 2 LEDs, just 1. And it can be either a track status indicator or a locator. For track status they recommend a bicolor LED (since it gets connected across the DCC track power) or a simple colored LED for a locator. You then have to either connect the LED + and LED - holes to the track, or jumper them to nearby holes to source the power from the cab bus. Or connected a wall-wart to supply power.

--Randy

[jim22]

The radio shack packaging should tell you the current requirement and if there might possibly be an internal resister in the LED. I would measure the voltage on the leads to the LED with the circuit on and see if you get any voltage.

To calculate the necessary resistor value, divide the voltage (volts) by the current (in amps, a milliamp, mA, is .001 amps). The voltage is the supply voltage minus the voltage accross the LED when it's on (again, see the documentation for the LED, but it should be a couple volts). For example, if the circuit runs at 5 volts and the LED drops 2 volts, and the LED needs 20 mA, then the resistor value should be (5-2)/.02 = 150 ohms.

I bet you just don't have any voltage on the LED yet.

Jim

[tstage]

QUOTE: Originally posted by rrinker
Dumb question, I was reading the UTP instructions and I didn;t see where it says to use 2 LEDs, just 1.

--Randy

Randy,

As always, thanks for another thorough answer and clearing this all up. [:)] NOW it's making complete sense to me! The part about Powering the Panel locator LED from cab bus power is located right underneath and is a subheading of the INSTALLING AN LED AS A PANEL LOCATOR LIGHT.

At first glance, I assumed it was discussing something else and NOT related to wiring up the LED so I ignored it. The funny thing was that, in the back of my mind, I had this fleeting thought, "Hmmm. Since the UTP panel is daisy chained from the Power panel with the "4-wire" coilcord (which lacks the two extra track power wires found in the "6-wire" cable for use with the Power Cab), where is the LED supposed to draw or receive the power from anyhow?" I guess I thought that it was somehow receiving it from the "4-wire" connection. Guess not. [D)]

And - yes - you are correct. The UTP panel only takes/has room for one (1) LED. The LED packet I bought at Radio Shack came with 2 LEDs in it. I used one, thought I might have fried it, so I installed the other. (FYI: The 3mm "low intensity" LEDs are rated at 3v, 15mA.)

Thanks, Randy! [:)] You've bailed me out of my ignorance once again.

Tom

[tstage]

QUOTE: Originally posted by jim22

The radio shack packaging should tell you the current requirement and if there might possibly be an internal resister in the LED. I would measure the voltage on the leads to the LED with the circuit on and see if you get any voltage.

To calculate the necessary resistor value, divide the voltage (volts) by the current (in amps, a milliamp, mA, is .001 amps). The voltage is the supply voltage minus the voltage accross the LED when it's on (again, see the documentation for the LED, but it should be a couple volts). For example, if the circuit runs at 5 volts and the LED drops 2 volts, and the LED needs 20 mA, then the resistor value should be (5-2)/.02 = 150 ohms.

I bet you just don't have any voltage on the LED yet.

Jim

Thanks, Jim! [:)] You and Randy's expertise in this area are invaluable.

Tom

[rrinker]

Of the 4 wires in the normal NCE cab bus wiring, 2 are used for power to each cab, and the other 2 carry the signal. Like others, when you get beyond a certain number of cabs (not possible with the PowerCab), you have to add some extra form of power through the UTP panels so there is enough power to run all the connected cabs. Normally the other 2 wires are unused, except in the PowerCab where it carries the track power back to the panel.

--Randy

[tstage]

Thanks, Randy. I successfully soldered 22ga jumpers from the LED- pad (on the UTP board) to the "-" cab bus power pad; and the LED+ pad to the "+" cab bus power pad. The red panel locator LED lights up just fine now. [:)][tup]

Tom