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Post by oldmuley on Apr 25, 2015 18:32:07 GMT -8
This morning I purchase two BLMA modern signals for my layout. Specifically I picked up the single head unit with the green/yellow/red LEDs. When I went to test one out I found the red and yellow LEDs worked fine, but the green one didn't. Figuring this was just a fluke, I unboxed the second one and test it out as well. It had the same issue as the first- red and yellow worked fine, but no green. I'm using a lab-grade power supply, so I know I had the voltage and amperage dialed in perfectly. It is correct that these are all common anode and each color led has its own cathode lead and all three LED's use the same voltage (2.0 to 2.2)? Seems odd that I'd have two that failed in exactly the same way.
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Post by downunder on Apr 25, 2015 21:24:19 GMT -8
I'm a retired electronics tech and I managed to blow two up also. Your bench supply is not the issue, the concern is the current through those micro diodes.
First off there are no specs supplied with these signals, ie they don't tell you what the maximum voltage and current ratings are.
I had assumed the old adage that most silicon diodes run happily on 20mA...WRONG.
While I don't have the specs for the coloured micro surface mount diodes used by BLMA, I looked in a sales catalogue where they showed standard 3mm diodes of various colours and realised that the max current varies by colour. The particular ones I checked on varied from 14-17 mA max for the three colours of interest.
So back to the BLMA diodes, I have since run them at about 15 mA without issues, but that may still be too high.
John
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Post by bdhicks on Apr 25, 2015 21:26:40 GMT -8
I'm not sure what LEDs are actually in there, but I think as a general rule LEDs nearer the red end of the spectrum take lower voltage and LEDs near the blue end take a higher voltage. I checked the specs for some 0603 LEDs and the red and yellow are in the 1.8-2.6v range and the green is in the 3.0-3.2v range. The operating current should be the same across a family of LEDs, though.
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Post by edwardsutorik on Apr 25, 2015 22:17:15 GMT -8
I believe BLMA says they test everything before it goes out.
That said, either
1.) the green LED was delivered defective
2.) the green LED was blown out by the user
3.) the green LED is fine and is not being fed properly
Am I missing something?
I've got a couple of their signals. And the stress level was at about 3 when I lit them up. But they worked. They're supplied by the appropriate Circuitron supply through the (again) appropriate resistor.
I think that BLMA could have done a better job of support. I found what I needed, but it wasn't as user friendly as I would have wished. Is anyone familiar with the modern computer age surprised? But they did supply a formula for determining the resistor, and I followed it.
I REALLY like these signals. See 1, 2, 3 above.
Ed
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Post by downunder on Apr 25, 2015 22:31:51 GMT -8
BDHicks, current is not the same across a range, sorry.
Looking at spec sheet for a brand of general purpose 3 mm LEDs, red has typical IF of 15 mA, while other colours are 20 mA, but white is 25 mA
VF is typically 1.8 V for RED narrow view angle, through to 3.5 V for aqua.
IF max continuous for this brand is 15 mA for RED Diffused, 50 mA for RED Waterclear, etc.
The issue really is they all vary so much that we need BLMA to tell us what the safe operating condition is.
Regards
JOHN
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Post by edwardsutorik on Apr 25, 2015 22:54:37 GMT -8
The issue really is they all vary so much that we need BLMA to tell us what the safe operating condition is. Regards JOHN Here is what BLMA posts on their site: "All four wires will connect to the circuit of your choice to operate the micro-LED's (each of which takes no more than 2.0-2.2 Volts; 25 to 30 Mili-Amps)." And, with that, we in certain sections of the northern hemisphere of this planet will retire. Bon appetite! Ed Edward Sutorik |
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Post by oldmuley on Apr 26, 2015 7:20:42 GMT -8
Well I figured it out. The BLMA instructions and website clearly state 2.0 to 2.2 volts, 25-30mA just like Ed said. The thing is, I've never seen or heard of a true green LED that operates within that range. All the true green 603 LEDs I have used in the past require 3.0 to 3.2v. Since I figured these were going back one way or the other, I thought I'd try an experiment. My power supply was set to 2.0 volts and I rechecked everything; red and yellow were both OK, but nothing on the green. I then isolated the green and started to increase the voltage slowly. Around 2.4v I could see the very faintest glow from the green LED. As I increase the voltage to 2.8v it was becoming bright enough that it looked pretty good. I stopped at that point since I didn't want to drive it any harder.
So problem solved. Surprising that BLMA hasn't updated their information.
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Post by canrailfan on Apr 26, 2015 9:07:49 GMT -8
Vf for an LED is the voltage drop across the device when it is powered, not the maximum voltage that can be applied to the LED. The reason for the differing Vf values of colour LEDs is the different semiconductor materials used to output each colour. Vf is indicating the minimum voltage that must be applied to the device before it will emit light. In most applications the applied voltage should be slightly higher to account for variations in individual devices of a given type.
What is critical for LEDs is the current allowed to pass through them. In the forward direction LEDs have effectively no resistance and will therefore pass all current available from the supply. Typically we need to limit the LED current to 15-20ma using a resistor in series with the LED, because most DC supplies are fixed voltage and do not limit the current to a connected circuit. (LEDs can be powered directly by the constant-current sources provided on some DCC decoders, by design these limit the current to 15-20ma.)
I use a 12v DC power supply with 680-750 ohm resistors to power BLMA signals and have had no failures. There are charts available on several websites that provide suggested resistance values for connecting LEDs to higher voltages.
David
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Post by canrailfan on Apr 26, 2015 9:44:06 GMT -8
Here's a helpful website for calculating resistor values for LEDs.
ledcalc.com
David
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Post by canrailfan on Apr 27, 2015 19:32:51 GMT -8
I measured the Vf of the LEDs in BLMAs #4001 Searchlight signal head as follows
Green = 3.00V
Yellow = 2.01V
Red = 1.98V
These values will vary slightly from signal to signal but will always be close to these values.
The supply voltage was 11.65V, with 750 ohm resistors on each LED.
David
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