QUANTSUFF's LED Pages

QUANTSUFF'S CIRCUIT PAGE.

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Regulated LED Circuits 1
The HAK-LITE
The 'Dollar' Light The Fake Fluorescent

Here is our circuit with a simple change: the base drive comes off the rectified LED output. This ensures that there is more than enough drive to saturate the transistor.
  When power is applied, current flows from the battery through the diode, the resistor to the Base, starting the oscillation. After that, the LEDs act as a zener to maintain a relatively stable supply to the base.
  The circuit requires a higher starting voltage to overcome the Vf of the diode, so it is useful to have a Schottky here. But once started, the circuit will run until the battery drops to Vce, which could be .3V or even less!
  Another advantage to this circuit is that its output is fairly well regulated from 1 to 2 Volts and it is readily adaptable to 2- or even 3-cell circuits, because the base drive is less dependent on input voltage than other designs.

The circuit can even drive a 1-watt Lumiled at full output! Just remember the drain on the battery is over 1-Amp! A word of caution:- if the circuit does not start, INCREASE R! If the drive current is too high, the transistor will not switch off and the saturated coil becomes just a short circuit!

By using a second transistor to act as a timer, the circuit on the left uses an off-the-shelf inductor, but is able to drive a 1-watt LED to full brightness with a 1.5-volt battery. Again, we can get much higher drive by utilising the 'bootstrap'.
    The circuit operates at the maximum limits of the FJN965 transistor and the LED, so it is important to stay with the values given. You can use a 100-ohm resistor paralleled with a 470-ohm if you do not have the 82-ohm part.
    Parts will run warm, but heat-sinking is only required for the LED - it gets as hot as 42C even then! And, if the paint on the inductor starts blistering, it's a darn good sign that you don't have one that can handle the 1-amp surges!

A simple installation driving 6-10mm LEDs (in 2 x 3-LED chains). These LEDs (available here on eBay) have a +/- spread of only 6 degrees, which makes them a good replacement for a flashlight - without needing a reflector. When driving multiple LEDs in series, care must be given to the transistor's Vceo and V rating for the output capacitor (especially tantalums). The peak voltage may be 50% higher than the measured voltage across the LEDs!

Another simple 6 LED setup with all the components mounted on a 1.5 inch square perfboard. Ultra-efficient 80%+ with 20mA into the LEDs and only a 300mA draw from an AA battery!

Next to a 3W Luxeon it has virtually the same throw and superior brightness, but uses less than 1/2-Watt!

	The "Hack" Circuit.
	This circuit uses the boost circuitry we have been using to lower (buck) a supply by placing it in series with the load (the LEDs) and feeding the "extra" voltage back to the smoothing capacitor. This 'hack' of the boost circuit can be used as a buck regulator to run a reading light in the car - and it's 90% efficient! The voltage difference between the car battery and the LEDs, about 3 volts, is stored in the coil (seen as the flat part of the 'scope display), and periodically dumped back into the lights through the schottky diode (the sharp 14v spike). Or, use this concept to make a highly efficient flashlight for 9-volt batteries? Check out the HAK-LITE
This circuit is ideal for the latest Lithium- (and Potassium-) based batteries. As shown, the design will operate from 3 to 5 volts. The optional 1N914 small-signal diode acts as a very simple voltage regulator and lets us use this device for supplies up to 8 volts. With the 10k resistor chosen this circuit will drive a pair of 25mA LEDs in parallel between 15 and 30mA each. In fact the circuit will work off virtually any NPN transistor; the output is only limited by the gain of the transistor, which can be offset by changing the value of the resistor (27K will halve the output while 6.8K will double it). I did a random sampling of transistors at hand, and found the 2N4401 and MPS651 solid performancers, and even the lowly PN2222 (the plastic version of 2N2222) managed 75% output.
Here are pictures of a Dollar-store "camping-light" which had the circuit inserted free-form through the (removed) bulb socket. It should give you 800 hours of use from a set of alkaline AA-cells. The LEDs used are 4.8mm 25mA bright-whites with a 120-degree coverage, available here. In this assembly, the LED wires are bent to form a trellis to support the 5uF capacitor and the other components "hanging" underneath. After everything is assembled (and tested), the coil and components are slipped through the neck of the light. The LEDs wires are then melted into the plastic rim by gently heating with a soldering iron. The inductor consists of two coils of 10-15 turns using 30-g wire-wrap on a small (1/3" od) toroid, but anything from 200 to 1000uH can be used. Click here for a detailled description on making your own.


The simplicity of design lets us retrofit another 3-cell Camping light (from DealExtreme) and we can reuse the existing SPDT switch to control the 2 lights, each with different output levels!

The Universal Hack Circuit
This same circuit, with the addition of a small timing capacitor, becomes virtually 100% self-regulating from 3-volt up to almost 20-volts, the Vce limit of the transistor we have chosen. The waveform across the transistor shows how it works: up to about 4-volts, the coil charges up through the LEDs, then, when Q1 shuts off, the coil discharges through D1, keeping the LEDs bright. Then the cycle repeats. However, when the supply rises over 4-volts, the capacitor extends the time Q1 stays off in proportion to the input voltage, seen as the flat plateau on the second graph. This limits the current through the LEDs to a safe level.
This circuit is good for LEDs using up to 50mA; after that, we will have to look at regulated circuits, starting here.

Another take on the Solar Garden Light. With the addition of a solar cell, a simple diode, and some clever rearrangement of our circuit, we can devise a circuit which will charge a NiCd battery when it is light and automatically turn itself on after night falls. Can you explain how it works?
	It was implied that the first transistor circuit shown was the simplest possible, but substitute a FET for the transistor, and you won't even need a limiting resistor! Most FETs do not switch on until 2 or more volts, and even with the BS108, you may have to do some selecting before you get one with a low enough Vgs(on). But the Rds(on) for this device is as high as 8-ohms, which means we will not get full performance out of our circuit. However, it will drive up to 3 LEDs in series easily.
Another approach: a hybrid circuit by Watt-sun:
If you want to experiment, here is another circuit using a FET - which uses it as an On-OFF toggle switch.

Questions? Suggestions? Send me email!

	Here is our circuit with a simple change: the base drive comes off the rectified LED output. This ensures that there is more than enough drive to saturate the transistor. When power is applied, current flows from the battery through the diode, the resistor to the Base, starting the oscillation. After that, the LEDs act as a zener to maintain a relatively stable supply to the base. The circuit requires a higher starting voltage to overcome the Vf of the diode, so it is useful to have a Schottky here. But once started, the circuit will run until the battery drops to Vce, which could be .3V or even less! Another advantage to this circuit is that its output is fairly well regulated from 1 to 2 Volts and it is readily adaptable to 2- or even 3-cell circuits, because the base drive is less dependent on input voltage than other designs.
The circuit can even drive a 1-watt Lumiled at full output! Just remember the drain on the battery is over 1-Amp! A word of caution:- if the circuit does not start, INCREASE R! If the drive current is too high, the transistor will not switch off and the saturated coil becomes just a short circuit!
	By using a second transistor to act as a timer, the circuit on the left uses an off-the-shelf inductor, but is able to drive a 1-watt LED to full brightness with a 1.5-volt battery. Again, we can get much higher drive by utilising the 'bootstrap'. The circuit operates at the maximum limits of the FJN965 transistor and the LED, so it is important to stay with the values given. You can use a 100-ohm resistor paralleled with a 470-ohm if you do not have the 82-ohm part. Parts will run warm, but heat-sinking is only required for the LED - it gets as hot as 42C even then! And, if the paint on the inductor starts blistering, it's a darn good sign that you don't have one that can handle the 1-amp surges!
A simple installation driving 6-10mm LEDs (in 2 x 3-LED chains). These LEDs (available here on eBay) have a +/- spread of only 6 degrees, which makes them a good replacement for a flashlight - without needing a reflector. When driving multiple LEDs in series, care must be given to the transistor's Vceo and V rating for the output capacitor (especially tantalums). The peak voltage may be 50% higher than the measured voltage across the LEDs!
Another simple 6 LED setup with all the components mounted on a 1.5 inch square perfboard. Ultra-efficient 80%+ with 20mA into the LEDs and only a 300mA draw from an AA battery!	Next to a 3W Luxeon it has virtually the same throw and superior brightness, but uses less than 1/2-Watt!