Superpositioned

Electrical losses in the flashlight

In this flashlight, each LED/resistor combination consumes 4.5volts at 30mA or about 135mWatts. The ballast resistor alone consumes 1.1volts at 30mA or about 33mWatts. Therefore, 25% of the power being drained from the batteries is lost in the resistor of each LED chain!!

Battery life

The ‘C’ size batteries in the PVC flashlight have a capacity of around 4 500mAh. If you build your flashlight with seven LEDs, there is a constant current drain of 7 * 30mA or 210mA. Dividing this into our battery capacity, it becomes obvious this flashlight will only stay lit for about a day. This is a far cry from the 50-100 hours claimed by commercial flashlights running on smaller ‘AA’ batteries.

Cutting your losses

As mentioned above, the ballast resistors are wasting 25% of our battery power. Electronic Design proposed a simple circuit to resolve this in a recent article. The front end of their circuit draws less than a milliamp of extra current.

The circuit is best described in two parts: one, the boost circuit function of Q1 and Q2, and two, the control circuit of Q3 and JFET1. Assume Q1 is off. With the battery voltage slightly above Q2’s VVB, a positive Q2 base current [iB = (battery voltage VBE)/RJET1] would flow. Q2 turns on, which switches inductor L1 to ground.

The end result is a 23volt pulse (as shown in Figure 2) across the series of ultra-bright LEDs. At 278kHz, the human eye cannot distinguish the difference between these pulses and a constantly lit LED. This saves even more battery power.

As the battery voltage decreases, the pulses become further apart. The brightness remains indistinguishable until the voltage falls near 2volts. (The circuit does not function well below 2volts) I doubt the PVC flashlight has this efficiency near the end of its life.

The extended battery life

According to Electronic Design, this circuit consumes an current equivalent to about 17mA. Powered by our ‘C’ cell in the PVC, this circuit could run for 265 hours! This is ten times the original PVC design.

Another advantage to LEDs is their operating temperature. Most available today can be function for hours and remain cool to the touch. Directionality is another key property. They only emit light over a relatively small angle. This can be advantageous for reading lights, but hinders performance when attempting to light an entire room.

If the advantages of LED lighting interest you, Myths Busted, LED Lighting is an excellent article by a researcher in outdoor lighting solutions. (And the source of most my information.)

Powering your LEDs with a DC source

Warning: Driving your LEDs with too much current will permanently disable them.

If you attach LEDs directly to an unlimited power source, they naturally draw enough current to blow themselves out. Therefore, the driving current must be limited with a resistor. The relationship as described by Ohm’s Law, is V = I*R where V is the voltage over the resistor, I is the driving current, and R is the limiting resistor. Two example circuits are shown below. These particular LEDs are rated at 25mA and are powered by a 12 volt regulated supply.

Each white LED gives a voltage drop of 3.6 volts. As an example for a 12 volt light, you can run a maximum of 3 white LEDs in series at full power (3.6 x 3 = 10.8 volts drop). Subtract this from your supply voltage of 12 volts to get the additional voltage that must be dropped (in this case, 12 - 10.8 = 1.2 volts of additional drop needed). In this case, 1.2 volts of additional drop / .025 amps (25 ma) = 48 ohms… resistors are rated in watts. So in this case, 1.2 volts x .025 amps = 0.03 watts. A 1/4 watt resistor would work fine.

The tutorial above also explains how to construct a 12 volt voltage regulator from any 12+ volt DC source. Voltage regulation is highly recommended because large shifts in your driving voltage can cause the driving current to increase and burn out your LEDs!

If you are in doubt of your calculations, use one of the many LED resistor calculators.

Using an AC source to drive LEDs

It is also possible to convert an AC source to DC. My favorite way to accomplish this is with a bridge rectifier as shown below. As an added bonus, R is easily calculated using the method above.

In operation, a DC voltage of around 170 is produced from the bridge rectifier and 50uF capacitor. The capacitor value is not critical and can be anything from 20uF or more… To find the resistor value and wattage, multiply the number of LEDs by the individual LED voltage. Then subtract this number from 170 and divide the result by the desired current (usually 20 milliamps).

Dimming your LEDs (with PWM)

Using pulse width modulation (PWM) to dim your LEDs is extremely important! Simply decreasing the input voltage yields unreliable results and potentially reduces their life-span. Pulse width modulation basically pulses the source voltage on and off so quickly that your eye is unable to distinguish the difference.

PWM can be implemented using a variety of methods. The simplest is switching on/off the output of a microcontroller. There are also several circuits that implement PWM. My favorite method uses two comparators. The details are excruciatingly painful and may deserve their own article someday.

The first example uses the standard op-amp oscillator circuit to generate a triangular waveform which is level-shifted and fed to a comparator (e.g. LM339) to give the PWM waveform.

Purchasing LEDs

LEDs are available all over the Internet. Recommending a single source or particular LEDs is hard as prices, projects, and the LEDs themselves may vary. If you are planning on starting an LED lighting project, it is best to do some research. For smaller applications, like a reading light, it may be more cost-effective to skip out on the top-of-the-line and buy a few more (relatively) cheaper LEDs. If you want to build a moonlight for your fish-tank, then you don’t need all those lumens blinding your fish!

Digg! this article…

Creating a battery from pennies

In order to turn pennies into batteries, another electrode and an electrolyte are needed. In this case, dimes (zinc) are used as the positive electrodes and salt water is used an electrolyte. Copper wire, galvanized nails, and lemon juice are also popular and cheaper solutions. Such a battery produces a differential of about 0.5 volts.

Finding ample power for an LED

Unfortunately, this battery is not enough to light an LED. In order to string eight of these cells in series, an ice cube tray is used. Metal paperclips hang the pennies and dimes into the electrolyte banks. Because the paperclips are conductive, the eight cells are automatically connected in series forming a more powerful battery. This provides a differential of about two volts.

As you may notice, 0.5 volts * 8 != 2 volts. Not all of the banks produced a reliable voltage. In fact, one bank seemed to be working against me.

Lighting a LED with pennies

Generally, LEDs require a resistor to prevent excessive current flow from blowing them out. This project does not require a resistor because the battery simply cannot provide that amount of current.

Connecting the short end of the LED to the penny and the long end to the dime lights up the LED! Everything works as planned. The penny batteries provide about 110 micro-amps of current in series. At two volts, this is only about 220 micro-watts of power!

It does in fact ‘cost’ pennies to power an LED.

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