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**Hoppy** · 06-28-2009, 07:45 PM

Well, the "adventure" isn't over

Yesterday morning the lights came on as the timer told them to. A few minutes later as I was starting to add the day's ferts, the lights suddenly dimmed. Only 6 LEDs, in one of the 12 LED series groups, remained on, with all 6 being together, the 1st thru 6th from the + lead. The other series group were all off. I disconnected it after checking that the cooling fans were on, and stewed about this the rest of the day.

This morning I tested the LEDs individually, and they all work, so it isn't a failure of LEDs.

I'm stumped, but I do have some ideas about the problem. Here is a question for the electronics skilled members here: If I run 3 LEDs in series, use a constant current device to provide them with 400 mA current, all 3 LEDs will light up as 400 mA current goes through them. Now, suppose my power supply voltage drops below 3X the forward voltage of each LED - for example, if they each run at 3.5 volts at 400 mA (10.5 volts total in series), and my power supply output drops to 10 volts, what happens and why?

I always think in terms of resistors in series or parallel, just from intuition. That thinking says the currrent drops until the voltage available equals the total voltage drop across the LEDs. But, the constant current device won't allow that to happen, so the current goes to zero. (?) With LEDs, each LED will drop the voltage available to the next one in series by 3.5 volts, so the first two will be ok, but the third one only gets 3 volts, which is too little. What does it do, and why? Will a LED supplied with current, but inadequate voltage pass the current along, but not light up?

In my actual circuit, I have about 45 volts available for total forward voltages of the LEDs in series, and 12 in series, each needing 3.5 volts, or 42 volts total. But, suppose a questionable solder joint adds a resistance in the circuit, dropping 20 volts at 400 mA? Would that allow 6 LEDs to light up, but leave the last 6 in the series just passing current without lighting up?

Any other ideas?

EDIT: I just found 3 solder joints at LEDs that were questionable, one of which I could break loose by hand. I resoldered the 3 connections, but that didn't help. Also, I realized that the "-" connection to the two series strings of LEDs is ground, and the heat sink is grounded. So, any current leakage to ground, perhaps at the 6th LED that is lit up, would shut down the last 6 of that string, and that string would then hog the current, leaving the other string underpowered, so it would be off. It may be that my soldering is the total cause of this problem, with the resistance at one or more joints increasing, or bits of solder making a connection to the heat sink, etc. causing the "failure". It is becoming obvious that a 15 watt soldering iron isn't up to doing a good job on mounted LEDs, so I may end up buying a 25 watt one.

06-28-2009, 07:45 PM	#275 (permalink)
Hoppy Planted Tank Guru (View User Tanks) PTrader: (22/100%) Join Date: Dec 2005 Location: Sacramento, CA Posts: 5,686	Well, the "adventure" isn't over Yesterday morning the lights came on as the timer told them to. A few minutes later as I was starting to add the day's ferts, the lights suddenly dimmed. Only 6 LEDs, in one of the 12 LED series groups, remained on, with all 6 being together, the 1st thru 6th from the + lead. The other series group were all off. I disconnected it after checking that the cooling fans were on, and stewed about this the rest of the day. This morning I tested the LEDs individually, and they all work, so it isn't a failure of LEDs. I'm stumped, but I do have some ideas about the problem. Here is a question for the electronics skilled members here: If I run 3 LEDs in series, use a constant current device to provide them with 400 mA current, all 3 LEDs will light up as 400 mA current goes through them. Now, suppose my power supply voltage drops below 3X the forward voltage of each LED - for example, if they each run at 3.5 volts at 400 mA (10.5 volts total in series), and my power supply output drops to 10 volts, what happens and why? I always think in terms of resistors in series or parallel, just from intuition. That thinking says the currrent drops until the voltage available equals the total voltage drop across the LEDs. But, the constant current device won't allow that to happen, so the current goes to zero. (?) With LEDs, each LED will drop the voltage available to the next one in series by 3.5 volts, so the first two will be ok, but the third one only gets 3 volts, which is too little. What does it do, and why? Will a LED supplied with current, but inadequate voltage pass the current along, but not light up? In my actual circuit, I have about 45 volts available for total forward voltages of the LEDs in series, and 12 in series, each needing 3.5 volts, or 42 volts total. But, suppose a questionable solder joint adds a resistance in the circuit, dropping 20 volts at 400 mA? Would that allow 6 LEDs to light up, but leave the last 6 in the series just passing current without lighting up? Any other ideas? EDIT: I just found 3 solder joints at LEDs that were questionable, one of which I could break loose by hand. I resoldered the 3 connections, but that didn't help. Also, I realized that the "-" connection to the two series strings of LEDs is ground, and the heat sink is grounded. So, any current leakage to ground, perhaps at the 6th LED that is lit up, would shut down the last 6 of that string, and that string would then hog the current, leaving the other string underpowered, so it would be off. It may be that my soldering is the total cause of this problem, with the resistance at one or more joints increasing, or bits of solder making a connection to the heat sink, etc. causing the "failure". It is becoming obvious that a 15 watt soldering iron isn't up to doing a good job on mounted LEDs, so I may end up buying a 25 watt one. __________________ Hoppy Last edited by Hoppy; 06-28-2009 at 09:56 PM.