There has been a lot of question about the fact that the slug on Luxeon emitters is not electrically neutral.
It is well known that the slug for Red/Orange/Amber luxeons is tied directly to positive. Those that were not aware of this fact, or were not able to sufficiently isolate the slug of these luxeons from the often ground-bodied flashlights serving as hosts were given a grim reminder of this fact when their luxeon lived a very short life. Allowing the slug of one of those types of luxeons to contact ground while in operation causes a short directly across the bond wire of the positive lead, ultimatley fusing the bond wire open. The luxeon could still be used in this case, since the bond wire merely connected the positive lead to the slug - but to the unknowing user, the LED appears to be toast.
It was long thought that the slug of white/green/blue Luxeons was tied to negative. This, is in fact, not true. The slug of these models of luxeon are tied to the positive and negative leads through zener diodes that serve as ESD protection. You can verify this yourself: using the diode test function on your DMM, connect the positive probe to the slug, and the negative probe to either of the power leads. Your DMM should forward bias these ESD diodes, and read something like 0.6-0.7V as the diode's Vf.
Thus, the circuit diagram for a white/blue/green Luxeon (heretofore refered to simply as Luxeon) looks like this:
Now, based on my testing today, and the above circuit configuration, I'd say that it is "safe" to tie the slug of a luxeon to ground in the single luxeon case. In addition, I'd deem it "safe" to tie multiple luxeon slugs together when in a parallel configuration, provided that all of the slugs and all of the grounds are tied to a common point.
Now, I use the term "safe" loosely, because, if it were myself, I'd still prefer to keep the luxeon slugs isolated from each other, and the heat sink, just in case.
However, it is absolutely imperative that all slugs be completely isolated both from each other, and from ground, when running multiple luxeons in series.
Below, is a diagram, based on the above schematic including ESD diodes, of 3 Luxeons in series:
I did some measurements on a set of 3 bare Luxeon emitters in series, at about 28mA test current. I then simulated the slugs being electrically tied together by pushing them against a strip of aluminium bar stock. This was driven with an LM317 in current mode, with a 47 ohm resistor, yielding 27.6mA of test current. The emitters could be run bare at this test current level, without any heatsinking.
Here are the test voltage results of running the 3 emitters in series with the slugs completely isolated:
The voltages across the ESD diodes should be fairly accurate - bypassing the ESD diode on the - terminal with the 200uA current scale on my DMM, I measured several microamps flowing - so in normal operation there is several uA of current flowing from the '+' terminal, through the first ESD diode, through the slug, and through the second ESD diode, to the '-' terminal. This forward biases the '-' terminal ESD diode, so there is usually ~0.6V between the slug and '-' terminal when the slug is isolated.
Now, I first caught wind that these were zener ESD diodes from some Lumileds documentation - though not from the datasheet its self. IMO, the first diagram I showed is really needed in the Luxeon datasheets - it allows an engineer to truly understand the electrical relationship of the slug to the two power terminals. Anyways, being that these are zener diodes, that means there is a zener voltage where the diode is reverse biased, and current begins to flow rapidly. The zener volage of the positive terminal ESD diode is at the heart of why luxeons in series must have their slugs electrically isolated from each other and ground.
In my testing, based on the above diagram, I experienced issues when the slugs of Lux #1 and Lux #3 were in electrical contact. Connecting the slugs of Lux #2 and #3 together had no effect, as was the case with connecting Lux #1 to #2.
When the "failure" mode was introduced, all 3 luxeons went fairly dim. The connection of the Lux #2 slug had no effect on the behavior when #1 and #3 were connected.
Below, is the diagram that illustrates the failure mode:
The only difference that connecting Lux #2's slug makes is on the voltages of the ESD diodes for Lux #2, so the rest of the information in above diagram is correct regardless of the connection of Lux #2's slug.
From this diagram, we can see that, in the failure mode, the majority of the current (path in red) is being dumped through the now reverse-biased (zener-biased) '+' ESD diode of Lux #1, into the slug, to the slug of Lux #3, and through the forward biased '-' ESD diode of Lux #3. A small amount of current still flows through the 3 series luxeons, but at 1.2mA, they are quite dim.
Another important thing to note: the total volage of the series string has dropped from 8.49V to 7.58V - almost 1V drop. My luxeons survived without damage because I was using a current source - but those that would be using a resistored setup, or even direct drive for 3 series luxeons, would see the current increase dramatically.
I do not know how much continuous current the ESD diodes on luxeons are designed to handle. I would imagine that they aren't designed to be carrying continuous current of more than a few 10s of mA for very long. I have no idea if carrying hundreds of mA to an amp of current would destroy the ESD diodes or not. If they failed to an open circuit, then the luxeon would operate as normal, but without ESD protection. But if they failed as a short, then they would either pass all the current through the slug into the other ESD diode, or (without a current limited power source) possibly pass so much current that the bond wires fuse. In the former case, the luxeon still passes current, but with a Vf of approx 0.7V, no current flows through the LED portion. In the latter, the luxeon passes no current. In either case, the luxeon is rendered essentially useless.
Now, from what I observed, it would appear that you only need to ensure slug isolation if running 3 or more luxeons in series - but that's not so. I was only running these at ~30mA. If running them at higher currents, the combined Vf of just two in series would easily exceed the ~7.5V voltage required to reverse bias the '+' ESD diode of one Luxeon, and forward bias the '-' ESD diode of the second. At this point, some current would be passing through the ESD diodes and slugs of the luxeons, and not through the LED junctions them selves - proving the extra current to be completely wasteful. Thus, even two luxeons in series can experience the same failure.
So that covers the situation where the slugs of series luxeons are tied together, but not connected to ground. As a last test, I connected the aluminium bar to ground, and repeated the test.
With the slugs connected and tied to ground, things are a little simpler. Once the slug of Lux #1 comes in contact with ground, then all 3 luxeons in the string go out. There is no effect if either or both of the other two slugs contact ground.
Below illustrates what's happening when the slug of Lux #1 contacts ground:
Again, the '+' ESD diode of Lux #1 is reverse (zener) biased, and passes pretty much all of the current (in red) directly to ground, bypassing the LEDs. I've labled the current passing through the Luxeons themselves as ~0mA (approximately zero) - because, while the luxeons did not light up at all, I did measure distinct voltage drops across the terminals of each luxeon - telling me that a tiny amount of current is actually flowing, but not enought to produce any light. I just ran out and measured it - it's about 18uA of current.
So in this case, there isn't any indication of what's going on - the string simply doesn't light up. And for those not using a current regulator, it's worse - the voltage has dropped down to 6.75V - almost 2V lower than if the string were functional - a resistored or DD setup will dump a lot of current into the poor ESD diode. But not all current regulators will handle this scenario. Current regulators that rely on a sense resistor being between the '-' terminal of the LED and ground will also have issues. Since that sense resistor is being bypassed, the regulator will run wide open, dumping as much current into the LEDs as possible - and all of it will go through the single ESD diode - likely leading to its destruction, and /or other damage to the Luxeon.
Again, as with the non-grounded example, I did not experience issues with connecting the slug of Lux #2 to ground, but given a setup that will deliver higher current, a series string of 2 will also experience this failure if the slug of the "first" (connected closest to "+" of the power source) luxeon touches ground.
So - after all of that - what does all of this amount to? Simple - when running luxeons in series, you absolutely must isolate all slugs from each other and ground. Failure to do so could result in one of the two failure modes described above where a) the luxeons are dim (possibly flickering), or b) the luxeons fail to light up at all. Contunuous high current operation in one of these failure modes could lead to damage of one or more of the luxeons in the series string.
When connecting luxeons in parallel, it is still a very good idea to isolate the slugs of all emitters from each other and ground - in fact, I would even stronly suggest it just for a single luxeon, as it is the only way to be absolutely safe.
One more note - it is best to treat Luxeon III and V stars as bare emitters with respect to isolation. The star construction and thermal bonding that is applied for these stars is such that it cannot be guaranteed that the slug will be isolated from the metal heat sink star. While it may appear that your stars are isolated, best practices would dictate that you isolate III and V stars from each other as well.
It is well known that the slug for Red/Orange/Amber luxeons is tied directly to positive. Those that were not aware of this fact, or were not able to sufficiently isolate the slug of these luxeons from the often ground-bodied flashlights serving as hosts were given a grim reminder of this fact when their luxeon lived a very short life. Allowing the slug of one of those types of luxeons to contact ground while in operation causes a short directly across the bond wire of the positive lead, ultimatley fusing the bond wire open. The luxeon could still be used in this case, since the bond wire merely connected the positive lead to the slug - but to the unknowing user, the LED appears to be toast.
It was long thought that the slug of white/green/blue Luxeons was tied to negative. This, is in fact, not true. The slug of these models of luxeon are tied to the positive and negative leads through zener diodes that serve as ESD protection. You can verify this yourself: using the diode test function on your DMM, connect the positive probe to the slug, and the negative probe to either of the power leads. Your DMM should forward bias these ESD diodes, and read something like 0.6-0.7V as the diode's Vf.
Thus, the circuit diagram for a white/blue/green Luxeon (heretofore refered to simply as Luxeon) looks like this:
Now, based on my testing today, and the above circuit configuration, I'd say that it is "safe" to tie the slug of a luxeon to ground in the single luxeon case. In addition, I'd deem it "safe" to tie multiple luxeon slugs together when in a parallel configuration, provided that all of the slugs and all of the grounds are tied to a common point.
Now, I use the term "safe" loosely, because, if it were myself, I'd still prefer to keep the luxeon slugs isolated from each other, and the heat sink, just in case.
However, it is absolutely imperative that all slugs be completely isolated both from each other, and from ground, when running multiple luxeons in series.
Below, is a diagram, based on the above schematic including ESD diodes, of 3 Luxeons in series:
I did some measurements on a set of 3 bare Luxeon emitters in series, at about 28mA test current. I then simulated the slugs being electrically tied together by pushing them against a strip of aluminium bar stock. This was driven with an LM317 in current mode, with a 47 ohm resistor, yielding 27.6mA of test current. The emitters could be run bare at this test current level, without any heatsinking.
Here are the test voltage results of running the 3 emitters in series with the slugs completely isolated:
The voltages across the ESD diodes should be fairly accurate - bypassing the ESD diode on the - terminal with the 200uA current scale on my DMM, I measured several microamps flowing - so in normal operation there is several uA of current flowing from the '+' terminal, through the first ESD diode, through the slug, and through the second ESD diode, to the '-' terminal. This forward biases the '-' terminal ESD diode, so there is usually ~0.6V between the slug and '-' terminal when the slug is isolated.
Now, I first caught wind that these were zener ESD diodes from some Lumileds documentation - though not from the datasheet its self. IMO, the first diagram I showed is really needed in the Luxeon datasheets - it allows an engineer to truly understand the electrical relationship of the slug to the two power terminals. Anyways, being that these are zener diodes, that means there is a zener voltage where the diode is reverse biased, and current begins to flow rapidly. The zener volage of the positive terminal ESD diode is at the heart of why luxeons in series must have their slugs electrically isolated from each other and ground.
In my testing, based on the above diagram, I experienced issues when the slugs of Lux #1 and Lux #3 were in electrical contact. Connecting the slugs of Lux #2 and #3 together had no effect, as was the case with connecting Lux #1 to #2.
When the "failure" mode was introduced, all 3 luxeons went fairly dim. The connection of the Lux #2 slug had no effect on the behavior when #1 and #3 were connected.
Below, is the diagram that illustrates the failure mode:
The only difference that connecting Lux #2's slug makes is on the voltages of the ESD diodes for Lux #2, so the rest of the information in above diagram is correct regardless of the connection of Lux #2's slug.
From this diagram, we can see that, in the failure mode, the majority of the current (path in red) is being dumped through the now reverse-biased (zener-biased) '+' ESD diode of Lux #1, into the slug, to the slug of Lux #3, and through the forward biased '-' ESD diode of Lux #3. A small amount of current still flows through the 3 series luxeons, but at 1.2mA, they are quite dim.
Another important thing to note: the total volage of the series string has dropped from 8.49V to 7.58V - almost 1V drop. My luxeons survived without damage because I was using a current source - but those that would be using a resistored setup, or even direct drive for 3 series luxeons, would see the current increase dramatically.
I do not know how much continuous current the ESD diodes on luxeons are designed to handle. I would imagine that they aren't designed to be carrying continuous current of more than a few 10s of mA for very long. I have no idea if carrying hundreds of mA to an amp of current would destroy the ESD diodes or not. If they failed to an open circuit, then the luxeon would operate as normal, but without ESD protection. But if they failed as a short, then they would either pass all the current through the slug into the other ESD diode, or (without a current limited power source) possibly pass so much current that the bond wires fuse. In the former case, the luxeon still passes current, but with a Vf of approx 0.7V, no current flows through the LED portion. In the latter, the luxeon passes no current. In either case, the luxeon is rendered essentially useless.
Now, from what I observed, it would appear that you only need to ensure slug isolation if running 3 or more luxeons in series - but that's not so. I was only running these at ~30mA. If running them at higher currents, the combined Vf of just two in series would easily exceed the ~7.5V voltage required to reverse bias the '+' ESD diode of one Luxeon, and forward bias the '-' ESD diode of the second. At this point, some current would be passing through the ESD diodes and slugs of the luxeons, and not through the LED junctions them selves - proving the extra current to be completely wasteful. Thus, even two luxeons in series can experience the same failure.
So that covers the situation where the slugs of series luxeons are tied together, but not connected to ground. As a last test, I connected the aluminium bar to ground, and repeated the test.
With the slugs connected and tied to ground, things are a little simpler. Once the slug of Lux #1 comes in contact with ground, then all 3 luxeons in the string go out. There is no effect if either or both of the other two slugs contact ground.
Below illustrates what's happening when the slug of Lux #1 contacts ground:
Again, the '+' ESD diode of Lux #1 is reverse (zener) biased, and passes pretty much all of the current (in red) directly to ground, bypassing the LEDs. I've labled the current passing through the Luxeons themselves as ~0mA (approximately zero) - because, while the luxeons did not light up at all, I did measure distinct voltage drops across the terminals of each luxeon - telling me that a tiny amount of current is actually flowing, but not enought to produce any light. I just ran out and measured it - it's about 18uA of current.
So in this case, there isn't any indication of what's going on - the string simply doesn't light up. And for those not using a current regulator, it's worse - the voltage has dropped down to 6.75V - almost 2V lower than if the string were functional - a resistored or DD setup will dump a lot of current into the poor ESD diode. But not all current regulators will handle this scenario. Current regulators that rely on a sense resistor being between the '-' terminal of the LED and ground will also have issues. Since that sense resistor is being bypassed, the regulator will run wide open, dumping as much current into the LEDs as possible - and all of it will go through the single ESD diode - likely leading to its destruction, and /or other damage to the Luxeon.
Again, as with the non-grounded example, I did not experience issues with connecting the slug of Lux #2 to ground, but given a setup that will deliver higher current, a series string of 2 will also experience this failure if the slug of the "first" (connected closest to "+" of the power source) luxeon touches ground.
So - after all of that - what does all of this amount to? Simple - when running luxeons in series, you absolutely must isolate all slugs from each other and ground. Failure to do so could result in one of the two failure modes described above where a) the luxeons are dim (possibly flickering), or b) the luxeons fail to light up at all. Contunuous high current operation in one of these failure modes could lead to damage of one or more of the luxeons in the series string.
When connecting luxeons in parallel, it is still a very good idea to isolate the slugs of all emitters from each other and ground - in fact, I would even stronly suggest it just for a single luxeon, as it is the only way to be absolutely safe.
One more note - it is best to treat Luxeon III and V stars as bare emitters with respect to isolation. The star construction and thermal bonding that is applied for these stars is such that it cannot be guaranteed that the slug will be isolated from the metal heat sink star. While it may appear that your stars are isolated, best practices would dictate that you isolate III and V stars from each other as well.
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