Electronic – ny pitfall to using resistors in series AND parallel for an LED array

• Question: What happens when we set the resistor so that it sets the current to be within safe levels for a single diode and then connect several diodes in parallel to it?

No harm will be done, but this result will seldom be useful.

One possible use is for eg a backlight where you only need as much light as one LED would make but you want it more widely distributed physically than you can achieve with only one LED. I have actually done this BUT it is an unusual application.

• Question: The LEDs themselves should be safe from overload, becasue the current is limited. I expect that in the LED case, the distribution of current among them will not be equal since those with lower initial forward voltage will get more current and will be brighter. After a while the forward voltages will match and the current distribution will stay uneven.

The forward voltages will match instantaneously "by definition" and will stay matched as they are connected together electrically. What WILL vary is common Vf and the current that each LED takes. The LED that takes the most current will heat the most (subject to absence of differing ventillation and heatsinking etc). With 10 LEDs the effect of variable individual cooling could have a significant effect on results.

For practical purposes the LEDs will settle down with currents about equal to the currents that they initially drew at the group Vf. See "Oscillations?:" below for comment on why the result is somewhat more complex in reality - BUT that you will not notice this.

• Question: Is there anything else that will happen? I've read the answers in this question and some of them mention oscillations, but don't elaborate on the topic.

The above are the main things that will happen.

• LEDs run at below rated current may change colour - especially ones that use a phosphor based emission technology. The exact effect varies with manufacturer and product. Some white LEDs run at low currents assume ugly green or purple tinges. Some get more blue or more yellow depending on the blue LED / Phosphor reradiation mix that is used to usually make white.

Luminous efficiency increases slightly with decreasing current. ie if you run 1 LED at x mA or 2 LEDs at x/2 mA each you will get slightly more light from the two LEDs.

Lifetime to say 70% of initial output will increase significantly if you share X mA among 2 LEDs and very very very much so if you share it amongst 10 LEDs.
This is based both on the current density per LED and the likely reduced die temperature. These are largely independent effects.

• OSCILLATIONS? : As shown in the typical graph below, the effect of increasing temperature is to slightly decrease the forward voltage for a given current, As Vf is common to all LEDs this will instead have the effect of slightly lowering the overall group VF and of increasing the share that this LED takes.

  As an LEDs VF/current curve is not linear this will not "run away" indefinitely but will assume a new higher current value. Other LEDS may then have very slightly less current so cool somewhat so draw less current so decrease group Vf so decrease net current so ... . This is the "oscillation that is referred to in the post you mentioned by Rocket Surgeon and it is a real effect but it will essentially always be such a small effect that you will not notice formal oscillation - just a very small continual shift of overall operating point.

I think the way to think about this is to think about load lines.

(Public domain image from Wikimedia)

What the load line graph shows is two equations that need to be solved to get the operating point of the circuit. I is the current going around the circuit in the clockwise direction. V_D is the potential as indicated in the schematic. The diode response curve shows all the possible combinations of I and V_D that are consistent with the diode's characteristic equation:

\$I(V_D) = I_s(\exp{(nV_D/V_T)}-1)\$.

And the load line curve shows all the possible combinations of I and V_D that are consistent with the characteristics of the Thevenin source formed by the battery and the resistor:

\$I(V_D) = (V_{DD} - V_D)/R\$

Since there's only one combination of I and V_D that satisfies both equations, shown by the intersection of the two curves, that will be the operating point of the circuit.

So, how does this help answer your question?

When you put two components in parallel, their voltage is the same and their currents add, so the response curve of the parallel combination stretches in the "I" direction. When you put two components in series, their current is the same and their voltages add, so the response curve of the series combination stretches in the "V" direction.

if we had an LED connected in between a 5v Battery such as +----LED----(Ground) We'd experience a voltage drop of 5v over the LED,

This isn't a realistic scenario, because of the steep shape of a diode's response curve. If you put 5 V across a diode you would be more likely to blow up the diode than have a working circuit.

That said, real voltage sources like batteries have some parasitic series resistance (and real diodes also have some parasitic series resistance), so if you had a beefy enough diode, you could find its operating point when powered by a 5 V battery using a load-line analysis like the one shown in the picture above.

however 2 leds would just be 2.5 voltage drop a piece.....I don't understand WHY that is? Shouldn't all the "Pressure" from the battery's voltage be used after the first LED?

If you have two devices in series, their current is the same. If they're identical components (with identical response curves), that means the voltage across each one has to be the same as the other one. So if you put 5 V across a series combination of identical parts, you know you'll get 2.5 V across each of them.

Furthermore how can equal pressure/voltage get distributed in a parallel circuit

In a real circuit it's not perfectly distributed because there's some resistance in the wires connecting the parts. But in a model with ideal wires, it's the definition of the wire that the voltage is the same at all points on the wire. And this approximation is good enough for analyzing the vast majority of circuits.