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Old 08-11-2012, 08:28   #1
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LEDs... What do I not understand

My indicator LED on my propane solenoid circuit failed after 10 years and I decided to replace it. I purchased what I thought was a suitable replacement at Radio Shack. It was a red LED in a plastic lens case that fit the hole in the switch panel. It was rated at 12V nominal, 16V max with a forward current of 12ma typical 20ma max. I installed it and it worked for a short while and was quite bright, but then became intermittent and dimmer. It would sometimes come on dimly and sometimes not at all, then finally would not come on. I got another one and it did the same thing. I measured the voltage at the fixture and it is 13.1 V. At the moment I am on the dock and the battery charger is plugged in and at float voltage. It's a Xantrex three stage charger and it supposedly works on PWM. I therefore assume that it is actually supplying voltage to the battery at something more like 15.5 Volts in pulses that average 13.2 (float Voltage). I took one of the LEDs apart and found that the load resistor was 680 ohms. I calculated that this was about 19.3ma forward current at 13.1V. Even at 15.5 it's only 22.7 ma. It seems to me that since 22.7 is above the 20ma max that in fact the maximum voltage that these LEDs can take is a little less than 13.8v. I other words the rating on the package is BS. Do I understand what's going on here?
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Old 08-11-2012, 09:08   #2
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Re: LEDs... What do I not understand

You are forgetting the the voltage drop of the led.
Vf is typically about 2-2.5v for a red led.

The correct formula is

R=(V-Vf)x1000/I

Some of the cheap LEDs are poor quality. They also tend to get too hot in a tight enclosure (with a resistor producing more heat) when driven at their maximum current.
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Old 08-11-2012, 09:25   #3
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Re: LEDs... What do I not understand

Quote:
Originally Posted by noelex 77 View Post
You are forgetting the the voltage drop of the led.
Vf is typically about 2v.

The correct formula is

R=(V-Vf)x1000/I

Some of the cheap LEDs are poor quality. They also tend to get too hot in a tight enclosure (with a resistor producing more heat) when driven at their maximum current.
So you're saying that in fact the current is substantially lower than my calculation would suggest and that I should really be using around 11.1 and 13.5 volts for my calculations? I'm assuming that the x1000 is to allow I to be stated in milliamps instead of amps.

Given this, it puts me in a safe range vis-a-vis the current ratings. Do you have any idea why the diodes would be blowing up. The last one only lasted 2 cycles before dimming and 3 more before it died completely. I was kind of assuming that the high pulsed voltage might be the issue.
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Old 08-11-2012, 09:42   #4
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Re: LEDs... What do I not understand

We don't know the Vf of the led, but if it was 2.4v the 680 ohm resistor would give a current of 20ma at 16v.

The problem is the crappy emitter (led) or poor design giving too much heat.

You can build you own with better quality components. It's likely a good quality led will be much brighter so it can be undredriven (say at a max of 15mA ) and still give plenty of light, with much less heat. If you can mount the resistor away from the led, or even better use a proper current limiting circuit that would be better again.

The battery will flatten out the pulses so the voltage the led sees will be even, although some protection for very high but very short spikes can be helpful. The better led drivers incorporate this in their circuitry.
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Old 08-11-2012, 09:58   #5
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Re: LEDs... What do I not understand

If the LED started out as quite bright, the current was likely too high. You probably don't really need it that bright, so I would raise the resistance. Radio Shack should have some selection of resistors. I would try a 1K(1000) ohm. With lower current, there will be less heat and the LED will last much longer.
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Old 08-11-2012, 10:35   #6
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Re: LEDs... What do I not understand

It's really hard to find a good quality LED in the market now.

I would probably buy a replacement from a marine panel supplier.

The others are probably repackaged off brand name.

Sorry, but this is the state of electronics in this economy.

James L
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Old 09-11-2012, 17:42   #7
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Quote:
Originally Posted by propellanttech
It's really hard to find a good quality LED in the market now.

I would probably buy a replacement from a marine panel supplier.

The others are probably repackaged off brand name.

James L
This is not true at all. No offense but I work in the industry and parts are fine. In fact boat stores gouge prices up too high. Also a led is typically hermetically sealed so salt should not work it's magic on the little device.

LEDs have specifications that have to be meet in order for proper operation. As someone briefly mentioned above, the led has a voltage drop and a current rating that you'll need to know in order to keep it working for life.

First the equation for selecting a proper resistor (remember resistors limit current if installed in series, so no burning out LEDs)

(V - veld) / currentmax = resistor
Vled is the voltage drop the led will need

So given
12 v battery
2 vled ( specified on data sheet or box that led comes in) most are around 2 volts
20 ma (also specified on box or data sheet)

Now plug this into the equation and you will get the needed resistor to be soldered in series with the led.

If your battery level is not constant then take the highest voltage and use that in your equation. Also this is not rocket science so as long as you are conservative with a resistor selection it will work fine. I say a 1Kohm resistor should be fine for most LEDs for boats


I hope this helps.
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Old 09-11-2012, 19:13   #8
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Re: LEDs... What do I not understand

Radio Shack is convenient ,but I have found the quality of components lacking
There should be little difference in voltage expected on a boat from automotive application (except when equalizing battery's), so it's just a cheaply designed product without quality control
I switched years ago to DigiKey for components when custom building

You would not go wrong with using a quality led from the marine supplier like BlueSeaSystems
They were one of the first companies that provided as standard quality silver tined,rather than common copper in their components
LED Indicator Lights*—*Blue Sea Systems
They are also reasonably priced for a marine item
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Old 09-11-2012, 19:59   #9
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Re: LEDs... What do I not understand

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Originally Posted by Floodhound View Post
This is not true at all. No offense but I work in the industry and parts are fine. In fact boat stores gouge prices up too high. Also a led is typically hermetically sealed so salt should not work it's magic on the little device.
No offense but I build printed circuit boards for a living (self employed). Trust me when I say, you had better buy your components from a reputable supplier.

Just about every electrical device that is bought comes from China. The good ones will have a repeatable process with good quality control. The rest just "get close", and have no quality control.

Now, if your an experimenter, you can go through the different brands to find the ones which will last.

Radio Shack uses the cheaper quality items.

Don't think because two items look the same that they are. I have chips with identical markings. Some are original, and some are copies. I have contacted the original chip manufacturer, and they relayed how to tell the fakes (the finish of the chip casing).

Some times you get what you pay for.

I wish the situation was not this way........

James L
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Old 09-11-2012, 21:22   #10
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Quote:
Originally Posted by propellanttech

No offense but I build printed circuit boards for a living (self employed). Trust me when I say, you had better buy your components from a reputable supplier.

Just about every electrical device that is bought comes from China. The good ones will have a repeatable process with good quality control. The rest just "get close", and have no quality control.

Now, if your an experimenter, you can go through the different brands to find the ones which will last.

Radio Shack uses the cheaper quality items.

Don't think because two items look the same that they are. I have chips with identical markings. Some are original, and some are copies. I have contacted the original chip manufacturer, and they relayed how to tell the fakes (the finish of the chip casing).

Some times you get what you pay for.

I wish the situation was not this way........

James L
You are correct except that the last time I checked Radio shack was somewhat of a reputable supplier. We are still talking about LEDs and passive components not counterfeit ICs from an offshore parts broker. I personally prefer Digikey, Mouser, Allied, and Jameco for many of my parts.

I feel your pain when it comes to counterfeit parts. When I was younger In my career I experienced it when i purchased a reel of 1000 parts that weren't even the correct footprint. I did get my cash back but by the skin of my teeth.

Cheers.
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Old 09-11-2012, 22:09   #11
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Re: LEDs... What do I not understand

Quote:
Originally Posted by Floodhound View Post
You are correct except that the last time I checked Radio shack was somewhat of a reputable supplier. We are still talking about LEDs and passive components not counterfeit ICs from an offshore parts broker. I personally prefer Digikey, Mouser, Allied, and Jameco for many of my parts.

I feel your pain when it comes to counterfeit parts. When I was younger In my career I experienced it when i purchased a reel of 1000 parts that weren't even the correct footprint. I did get my cash back but by the skin of my teeth.

Cheers.
I'm not a big fan of Radio Shack. I only buy from Mouser or Digikey. I prefer Digikey. I have some other places I order reels of passives from. Just better prices for name brands.

Jameco is good, but typically overpriced. Never ordered through Allied.

Even then, I really think the quality of Radio Shack parts has gone down hill majorly since the '80s. They are hurting, and just do not have quality parts like they did back when.

I just don't use them. I've had quite a few parts which I was not impressed with their performance, when they didn't function to design parameters.

I think there are better suppliers out there, and for a LED for a panel indicator for a marine panel, I suggest you try to find one. I can just about bet the panel doesn't have Radio Shack LED's in it. I would think they are of better quality.

James L
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