Roger,
I think there's some confusion about the series / parallel question - and the "pulsing" of LED strings.
The main practical difference between a series and parallel light string is the voltage rating of the lamps. A series connected set divides the supply voltage over all the lamps in the string; a parallel connection provides the full supply voltage to each lamp. LED's pretty much need to be connected in series, since they're usually rated at only a few volts each (see the detailed explanation I provide below).
As to the "pulsing", I'm assuming you're talking about the very fast flicker (60 cycles per second) you see with some LED strings. As RJ says, that flicker is due to the nature of LEDs connected to an AC source in a half-wave configuration... The string turns on for less than half the 60 hz AC cycle. And as RJ says, a full-wave rectifier will cause the string to turn on twice during each cycle, dramatically reducing the apparent flicker (making it much more difficult to detect - though the LEDs will still be flashing, they'll be doing so at 120 times per second instead of 60 and will be on for a much larger percentage of the time).
But the LED flicker has nothing to do with whether the lamps are connected in series or parallel - just whether there is a separate full-wave rectifier in the string.
Explanation of a series-connected lamp string:
Just like with "regular" incandescent light strings where the lamps are wired in series, the voltage across each LED (lamp) is only a small part of the supply voltage. Series connections are "voltage dividers". For lamps or LEDs, the 120v source power is divided evenly (more or less) across all of the LEDs. So if there are 40 LEDs (lamps) in a series string, each LED (lamp) will only have 3 volts across it (120/40 = 3), assuming all the other LEDs (lamps) are working properly.
[Technical discussion follows - feel free to ignore]
The nitty-gritty explanation of this is that in series connection, resistances add. Ohm's law (which defines the relationship between voltage, current and resistance) says: Voltage=Current x Resistance (in volts, amps & ohms, respectively). In a series circuit, the resistance (as "seen" by the power supply) is the sum of the resistance of the elements. This sets the current in the circuit. The current from the power supply is the power supply voltage divided by the total series resistance. Note that the same current flows through ALL the series elements - the current from the power supply is the same as the current through lamp(1), which is the same as the current through lamp(n) (current is the rate of electron flow - the same electrons have to flow through each element of the series circuit). Now, because each series element only contributes a part of the total circuit resistance, it only "sees" its part of the total voltage (by Ohm's law, since each element's resistance is only part of the total circuit resistance, it's voltage is also only part ot the total circuit voltage).
So, for example, say we have a light string (or LED string) rated for 120VAC. And let's say that there are 100 lamps connected in series in that string. And, further, let's say that each lamp has a resistance of 1 Ohm. The series resistance is 100 Ohms (100 lamps x 1 Ohm per lamp). The current in the circuit is 1.2 amps (Ohm's law: Current = Voltage / Resistance: 1.2A = 120V / 100ohms). And we can calculate the voltage across each lamp as 1.2 volts (voltage = current x resistance; v = 1.2A x 1 ohm). Note that the values picked for this example were selected for mathematical simplicity - they are not "real world" numbers.
But we can bypass a lot of this analysis. In this example, since there are 100 "identical" lamps attached across the 120volt power source, we know that each lamp will only have 120v/100 lamps or 1.2 volts/lamp across it (though we won't know what the current or resistance is).
Finally, in a parallel configuration, each lamp is connected directly across the power source, so each lamp will be exposed to the full 120v. In this configuration, the current is additive - that is, the current provided by the power supply is the sum of the currents drawn by each of the lamps (each electron flows through only one lamp and back to the power supply - the total number of electrons is the sum of those passing through the individual lamps, so if there are 100 lamps in parallel, the total current is 100x one lamp's current draw).
Hope this helps someone (at least a little).
--Dave
