DC Ornament on an AC Light String

This page details the original problem I was trying to solve and the current status of solutions. As I learn interesting things about the situation and circuits, I'll update this page. My hope is that it will become sort of a tutorial for others interested in various aspects of this problem.

[Last updated 11-30-2005, 12:15 pm]

Goal

Put a device that wants to run at 3 V DC into a string of Christmas tree lights that runs from 120 V AC.

The Light String

The string runs from 120 V AC (regular wall socket).
There are 50 bulbs in series in the string.
With all 50 bulbs in the string, voltage drop across 1 bulb varied between 2.3 V and 2.4 V (AC).
With all 50 bulbs in the string, current measurement was 125.7 mA.

The "Ornament"

Preferred voltage is 3 V (DC required).
Voltage range of 2.7 to 5.5 tolerated.
There are 16 LED/R pairs; LEDs will be turned on and off by microcontroller (uC), causing current use to fluctuate.
LEDs see a voltage drop of ~1.8V; resistors are all 270 Ohm.
Current use (at 3V) will vary from ~2 mA (no LEDs on) to ~74 mA (all LEDs on).

Circuits

Thanks to everyone who replied (especially Doug!) to my query. Here's an explanation of the situation (as I understand it).

General useful formulas / information:

V = I * R
P = I * V
I = C dV/dt

V: voltage, in Volts (V)
I: current, in Amps (A)
R: resistance, in Ohms (Ω)
P: power, in Watts (W)

C: capacitance, in Farads (F)
dV: change in voltage
dt: change in time, in seconds (s)

When a diode only has a small current through it, it drops less voltage.
AC voltage, as Doug explained:
```
	Vs = instantaneous supply voltage
	   = 120 * sqrt(2) * sin(2 * pi * 60 * t)
```
This means that there are portions of the cycle where there's not enough voltage for the diodes in the bridge.
The current will vary as the AC voltage oscillates!

Interesting information about the situation and the circuits below:

As various people explained, the startup condition is interesting... The bulbs have a much lower resistance when they are "cold," causing a larger current through the string until they're "warm." This means that various components need fairly high current and power ratings.
The diode stack or zener diode will control the voltage level, shunting excess current.
The capacitor will supply current during the portions of the cycle where the bridge isn't getting enough voltage from the (AC) supply. This will supply the necessary current to the ornament. The capacitor needs to be large enough that its voltage won't drop too much during this time.

"Circuit 1" -- The Diode Stack

Here's the first circuit Doug suggested:

Explanations

The diode stack will maintain a voltage drop between 3.5 V (5 * 0.7 V) and 5.0 V (5 * 1.0 V). The voltage drop is reduced when current is low.
When the current through the string is greater than the current needed by the ornament, the extra is shunted through the diode stack.
When the current through the string is less than the current needed by the ornament, the capacitor supplies the difference. That will reduce the capacitor's voltage by:
```
	limit  Vc1(t+1)  =  Vc1(t) + 1
	dt->0               ---------- * (Io - Is) * dt
	                        C1
```
(where Vc1 is the voltage across C1, Io is current through the ornament, and Is is current through the string).

"Circuit 2" -- Zener Variation (1)

Here's a variation (zener diode instead of diode stack):

Explanations

The zener diode will maintain a consistent voltage drop based on its rating (3 V, 3.3 V, etc.).
When the current through the string is greater than the current needed by the ornament, the extra is shunted through the zener diode.
When the current through the string is less than the current needed by the ornament, the capacitor supplies the difference.
The value for C1 needs to be larger here than in circuit 1 (the schematic doesn't accurately reflect this, yet). C1 needs to maintain enough voltage for the ornament circuit. The capacitor in the diode stack circuit (1) starts at a higher voltage, so it can discharge further and still remain in the acceptable range.

"Circuit 3" -- LM350 Regulator

This was from a suggestion from Danny (off-line) to use LM350, which can regulate current as well as voltage. I put together what was apparently a poor circuit using this. I've learned more about how to solve this sort of thing since then. Maybe I'll try again with the LM350, but for now we'll just skip this one...

"Circuit 4" -- Zener Variation (2)

Doug's most recent suggestion:

Explanations

This circuit should provide the smoothest DC to the ornament, but requires more voltage.
Excess current is absorbed by C1 and shunted through the zener diode.

Measurements / Experiments

For the following 2 graphs, I used a simple DC circuit: battery (~4-5 V), diode, and resistor (for controlling current). I measured the voltage drop across the diode and the current for various resistances (using a multimeter).

The graph below compares the options for the rectifier bridge. (The "bridge" is Digi-Key part PB66DI-ND.)

The graph below compares the options for controlling the voltage drop across the ornament.

Based on the above, I decided to use the bridge (not individual diodes) for the AC to DC conversion, and a diode stack to control the voltage (as in Circuit 1).

Interestingly, when I tried experiments with a diode stack and zener diode, the zener did a better job of ensuring the higher voltage. As expected, the zener also did a better job keeping a consistent voltage level. So, I've decided to use Circuit 2.

I've finally gotten around to taking lots of measurements for various configurations. Here's a page showing oscilloscope measurements. (Note that there are lots of oscilloscope images, each of which is 50k-60k, so this page may take a while to load, depending on your connection speed.)