LED Power Concern

I always like some sort of current limiting on LED lighting.  Did you ever try measuring the current to the LED's?  If they're 20ma or less, you're fine.

This has been mentioned many times but it seams there will always be someone who seems to know better. The reason your LED buss reads 3.3 volts is that appears to be the forward voltage drop of the LED's you are using. All those LED's are regulating the voltage to 3.3 volts and the power supply you are using just seems to be working with it. Common practice would call for a current limiting resistor in series with each LED to limit the current to the value the LED will withstand for long life. This is why most devices that use LED's run on at least 4.5 volts, 3 batteries. It gives them that extra 1.5 volts for the limiting resistor and a few other things.

Are you shortening the life of your LED's by running them the way you are? Probably yes but it may still be long enough that you will not notice it. Shouuld you have a series resistor? Again yes but with a 3.3 volt supply you may not have enough voltage overhead to make it work properly. Would I change anything? Not at this time. It is not correct but it has worked all these years so why change.

One word of warning though. If you add an LED and strange things start to happen it may be that the LED you added will not work properly with the 3.3 volt buss. This could affect that LED or some or all of the other LED's.

Al

That's why I recommend at least measuring the current.

I did not think to measure the current but will do so.  The wall wart being used was provided by the supplier of the LEDs, so I assumed it was properly matched.  Should its listed ratings indicate 20mA?

In regard to a current limiting resistor, what value of resistance would be appropriate?

The value of the current limiting resistor is dependent on the voltage being supplied.  If the wall-wart was supplied specifically to drive those LED's, it may well be properly matched.  What does the wall-wart measure running open circuit with nothing attached?

The ratings of the power supply are normally more than a single LED, without specific product knowledge of this specific unit, I can't evaluate what you have.

As to limiting resistors, I'll give you an example.

If you have red, yellow, or green LED's, they operate normally at around 1.5 volts, white or blue normally operate at around 3.0 volts.  Assume a white LED and a 12 VDC (measured) power supply.  You have 9 volts at 20ma to drop, that would require a 450 ohm resistor, probably 1/2 watt as the actual power dissipation is .18 watts. 

Each volt of drop at 20ma requires 50 ohms, simple rule of thumb for calculating current limiting resistors.

The wall wart is rated for 2A.  I tried to measure amperage with LEDs on and got a range of .01 to .o6 amps.  I question the accuracy because the digital reading was constantly varing.  To obtain the measurement, I placed the amp meter in line with one of the wires from the wall wart.

Any suggestions?

Well, the .06 reading is three times the maximum current of most common LED's unless you have special high-power models. 

I hope this is the output wire you put it in series with, right?  Is this for more than one LED?

The reading was at the source.  However, as I mentioned, the number on the meter was moving constantly.  Is that to be expected?  Is my method correct? 

The number of LED lights supported is about 50.

Am I correct to conclude that I need to be concerned with limiting the current versus attaching the (-) to the layout common (assuming no current increase)?

Sorry for the remedial questions, but I would rather be safe than sorry.

Thanks for your help!

I'm sorry to be so vague, but I'd have to know a bit more about the environment to make an intelligent answer.

If you have 50 LED's, you could logically have an amp of power going to the whole group if they're all wired in parallel. 

I'm not sure what attaching the - to the layout accomplishes, I don't think that enters into the current consumption or applied voltage questions.

I would be concerned with the Phasing of the wall wart.

You are effectively paralleling the 2 transformers if you connect the commons.

And if it's a switch mode supply all sorts of nasty things can happen.

I would NOT tie the commons together for these reasons.

Tying one side of a transformer to another isn't a phasing issue.  It's only an issue if you are ever going to tie the other side of each together.  Since the wall-wart is DC, that's not likely to happen in this case.