richhotrain Mark R. richhotrain gregc Mark R. I just use two AA batteries in series to test my white LEDs. Have never damaged one not using a resistor with it. since white LEDs operate at 3V or above, I can see how this works for white LEDS. But this will probably destroy other non-white LEDs that operate at lower voltages. I agree. There is no good reason not to use a resistor when testing an LED. Rich A white LED is rated at 3.4 volts. Two AA batteries in series usually output 3.0 volts or less (especially when not new). According to the LED resistor calculators, with an LED rated at 3.4 volts using a 3.4 volt supply, the recommended resistor is ONE ohm. So a supply voltage of 3.0 volts (or less) will still light the LED, but the lack of a resistor will not harm the LED because the supply voltage is below the LEDs rated voltage. Mark. I just don't think that is a good reason not to use a resistor, especially on a more expensive SMD LED. How many guys are going to spend the time doing the calculation and getting it right? Testing an LED without an resistor is an excellent way to instantly blow the LED...or, at least, to risk blowing the LED. Rich
Mark R. richhotrain gregc Mark R. I just use two AA batteries in series to test my white LEDs. Have never damaged one not using a resistor with it. since white LEDs operate at 3V or above, I can see how this works for white LEDS. But this will probably destroy other non-white LEDs that operate at lower voltages. I agree. There is no good reason not to use a resistor when testing an LED. Rich A white LED is rated at 3.4 volts. Two AA batteries in series usually output 3.0 volts or less (especially when not new). According to the LED resistor calculators, with an LED rated at 3.4 volts using a 3.4 volt supply, the recommended resistor is ONE ohm. So a supply voltage of 3.0 volts (or less) will still light the LED, but the lack of a resistor will not harm the LED because the supply voltage is below the LEDs rated voltage. Mark.
richhotrain gregc Mark R. I just use two AA batteries in series to test my white LEDs. Have never damaged one not using a resistor with it. since white LEDs operate at 3V or above, I can see how this works for white LEDS. But this will probably destroy other non-white LEDs that operate at lower voltages. I agree. There is no good reason not to use a resistor when testing an LED. Rich
gregc Mark R. I just use two AA batteries in series to test my white LEDs. Have never damaged one not using a resistor with it. since white LEDs operate at 3V or above, I can see how this works for white LEDS. But this will probably destroy other non-white LEDs that operate at lower voltages.
Mark R. I just use two AA batteries in series to test my white LEDs. Have never damaged one not using a resistor with it.
since white LEDs operate at 3V or above, I can see how this works for white LEDS. But this will probably destroy other non-white LEDs that operate at lower voltages.
I agree. There is no good reason not to use a resistor when testing an LED.
Rich
A white LED is rated at 3.4 volts. Two AA batteries in series usually output 3.0 volts or less (especially when not new). According to the LED resistor calculators, with an LED rated at 3.4 volts using a 3.4 volt supply, the recommended resistor is ONE ohm. So a supply voltage of 3.0 volts (or less) will still light the LED, but the lack of a resistor will not harm the LED because the supply voltage is below the LEDs rated voltage.
Mark.
I just don't think that is a good reason not to use a resistor, especially on a more expensive SMD LED.
How many guys are going to spend the time doing the calculation and getting it right?
Testing an LED without an resistor is an excellent way to instantly blow the LED...or, at least, to risk blowing the LED.
I'm TESTING it using the two AA batteries, not permanently installing it. Never damaged one in ten years, and I've tested hundreds this way. I think you guys over-think things sometimes.
¡ uʍop ǝpısdn sı ǝɹnʇɐuƃıs ʎɯ 'dlǝɥ
Alton Junction
One side of the headlight LED is probably already connected to the beacon LED - that would be the equivalent of the blue wire from the decoder which is common to all functions. So you can jumper the other side of the beacon to the headlight, and the headlight should flash with the beacon.
It's very rare for an LED to fail - I'd suspect a bad solder joint first, if the LED is soldered to a little circuit board. Such assemblies are all automated, adn if the machine fails to put a drop of solder paste on the one pad, or not enough, this sort of thing can happen. You need a strong magnifier to look at it, and it's something better fixed with a hot air pencil than a regular soldering iron.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
Having worked in electronics for many years, when LED’s came out, I started putting a potentiometer in series with the unmarked LED and an amp meter on the low amperage scale with a 12 volt power supply on the work bench. Twelve volts seems to be a common voltage for layouts.
All the LED’s I have seen so far are 20 ma types and 1k was fine. Super bright ones now usually require a 2.5 or 5k pot. I would measure the resistance and would grab a close value out of the junk box for the required brightness. Your mileage may vary.
I do remember a few years ago a few were ordering LED testers from ebay, China, quite cheap and happy with them.
If you ever fall over in public, pick yourself up and say “sorry it’s been a while since I inhabited a body.” And just walk away.
I guess I should have mentioned that the LED I want to test is already suspect of being defective.
It is mounted on a small board with a second LED that fits into the bottom of the headlight/beacon enclosure of the loco body shell. One LED for headlight, other for beacon. The beacon works. The headlight has been flaky since day one & now doesn't work at all. I replaced all the wires & soldered connections. Still doesn't work.
Athearn has said they will swap the whole shell under warranty (no LED replacement parts).
I want to double check the LED before I send the shell back just in case the problem is the decorder, not the LED. I don't want to destroy the LED in the process.
Would it be safe to just connect a pair of jumper wires from the beacon LED to the headlight LED & see if the headlight blinks with the beacon?
You know when I don't use a resistor? When I set my bench power supply to limit at 10ma.
Things have settled down a bit and LEDs are much more consistant than they were in the past, as far as voltage requirements. Still, if you use a 9V and a 1K resistor there pretty much no chance of damaging any LED, regardless of color or size. I'd rather take the sure path instead of the "well, it seems to always work" path - especially with SMD LEDs already soldered to fine wire. It's not a money thing, they're all so inexpensive these days, but the tedious process of soldering wires to the SMD ones is not something I want to do more of then necessary. I doubt I can even do the middle and smaller sizes with my eye issues. The bigger size ones I can manage, but they aren't much smaller than the smallest leaded LEDs.
That that everyone has said all this, I kind of think the OP was wondering how you physically test a small SMD LED. If it doesn;t have wires attached, I suppose you can use SMALL probes (some multimeters have really sharp fine tips ont he probes, others are kind of dull) under magnification to touch the contact surfaces of the LED, some meters have the capability to light an LED in the diode test mode. Or connect the other end of the probes to a 9V battery and 1K resistor. If it already had wires soldered on, then you test it like any other LED as has been discussed.
there's certainly no harm using a larger than necessary resistor to test an LED. I've connected LEDs with a 1k resistor across track voltage (~14v).
Mark R.I just use two AA batteries in series to test my white LEDs. Have never damaged one not using a resistor with it.
greg - Philadelphia & Reading / Reading
I suppose that it is possible to test some LEDs, maybe even SMD LEDs, without a resistor using a low voltage source. But, I wouldn't.
Here is what Richmond Controls has to say about their SMD LEDs:
When using all LEDs, the current MUST be limited using a resistor or another means.
Why take a chance and waste an otherwise perfectly good LED? And, SMD LEDs are expensive.
Arto I need to test a small (1.6mm?) surface mount LED. I'm going to use one of those disc 3V batteries for power. Should I still use a 1K resistor in series on one of the test leads? Thnx in advance!
I need to test a small (1.6mm?) surface mount LED.
I'm going to use one of those disc 3V batteries for power. Should I still use a 1K resistor in series on one of the test leads?
Thnx in advance!
What color LED? If it's white (bright white or warm white), 3v is within the normal voltage range and it will light up just fine, albeit really bright (that may or may not be what you want). But it will not burn out and does not need a resistor, unless you want a dimmer glow.Other colors like red require less voltage. With 3v you will not burn it out, but it will light up REALLY bright and would have a shorter lifespan (which would still be months/years depending on how much you turn it on).
I just use two AA batteries in series to test my white LEDs. Have never damaged one not using a resistor with it.
If you know the LED forward voltage & milliamps, you can calculate the resistor.
http://led.linear1.org/1led.wiz
I used a 9v battery with a 1k resistor and 0603 and it lit enough to determine it was ok. IF (I don't know the specifics) your LED needs 2v, at 20mA, the resistor calculates to less than 100 ohms. So, it might not light with a 1k resistor.
Paul
Modeling HO with a transition era UP bent