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QUANTSUFF'S CIRCUIT PAGE.

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As brighter, and more powerful LED devices become affordable, we will need to be able to drive currents over the 80mA or so limits of the basic circuits. Also, we will need to have tighter control over the higher currents going through the LEDs. Fortunately, as we can see below, the basic "hack" circuit can easily be modified to operate in constant-current mode. With LEDs, they are self-regulating once the current is limited.
When power is applied, Q1 turns on and current flows, through the LED, the coil, Q1 and R-limit. The LED lights and any excess voltage over Vf is used to charge the coil. When the current reaches approximately (600 / R-limit) milliamps, Q2 turns on and it turns Q1 off.
    The coil discharges through D1 and maintains brightness in the LED. The 330pF (use 470pf for 100mA or lower currents) capacitor ensures that Q1 stays off until the coil is discharged, at which point the cycle repeats.
    The scope image below shows how it works.

The top (green) trace shows the current through R-limit, which corresponds to the drain (100mA per div) on the battery . The bottom trace (yellow) is the voltage across the transistor.
The start of the cycle is when the top trace current goes up. The battery is driving the LED at the same time it is charging the coil. Notice the current slowly increases - as the coil charges up, its resistance decreases. When the current limit is reached - about 120mA here, Q2 turns Q1 off, seen as the sudden drop in current. The LED is kept bright by the charge stored in the coil and the cycle repeats, 2 to 400,000 times each second. Although the battery drain reaches as high as 120mA, part of the time the coil is supplying the power, reducing battery use to almost 0. At 6v, this saving is about 38% (the "duty cycle"), so that we can drive a LED at 120mA using only 75mA on average.
This design can drive a chain of multiple types of LEDs in series, so long as their Current rating is greater than the Current allowed by R-limit. The power supply must also be at least 1-volt over the combined Vf of these LEDs (4.5v for 1 white LED, 8v for 2, etc).
When the supply voltage drops below that, the circuit will run in direct drive. For 1/2 or 1-watt LEDs, just about any NPN transistor with a current limit of 1-amp will work (FJN965, 2SB2500, BC337, MPS651 etc) - although you may have to adjust R (try 2.2K to 10K) to compensate for gain. The coil can be any value from 150uH to 470uH - make sure it can handle the current.
By the addition of a capacitor, we eliminate the timing winding on our coil, so we can now use standard off-the-shelf inductors.

Here, we've taken our dollar-store sample apart and removed the incan bulb to prepare it for some upgrades. I'd decided to build it with the LED at the top and shining down, thinking it would give a better quality light... It doesn't :(

You can make your own coil by taking 4m (12-ft) of #30 wire-wrapping wire and winding it over a 1-1/4" (3 cm) form - I used the top cover of an aeresol can. Leave enough slack to go around the loops to secure it. This gives us a coil of about 50uH - not ideal, but it works just fine with inputs of 4.5-volts. The LED is an 8mm, 100mA "straw-hat" obtained on eBay. So a 1/4-watt 10-ohm was selected as R-limit for about 60mA.
    The parts can go together as free-form art - just make sure there are no shorts. And, of course, test, before assembly! Here are the parts just at the start of the soldering process.

The whole works stuffed into the top of the lamp. The coil just fits inside. I used the uncut leads of the smoothing capacitor through existing holes to secure the whole package.

AND NOW, FOR SOMETHING COMPLETELY DIFFERENT! (...SORTA)
So I can tell them at the office I'm just packing a light lunch!

The same circuit, but running off 4 x AA - so that I can use rechargeables if necessary.
Again, free-form construction. Taped and epoxied to the lid. Got real lazy - light uses a piece of paper as an on-off switch. I like this design - it can sit as a globe lamp, or left hanging from a pole.

See my other Projects on Instructables.com

Questions? Suggestions? Send me email!

As brighter, and more powerful LED devices become affordable, we will need to be able to drive currents over the 80mA or so limits of the basic circuits. Also, we will need to have tighter control over the higher currents going through the LEDs. Fortunately, as we can see below, the basic "hack" circuit can easily be modified to operate in constant-current mode. With LEDs, they are self-regulating once the current is limited.
	When power is applied, Q1 turns on and current flows, through the LED, the coil, Q1 and R-limit. The LED lights and any excess voltage over Vf is used to charge the coil. When the current reaches approximately (600 / R-limit) milliamps, Q2 turns on and it turns Q1 off. The coil discharges through D1 and maintains brightness in the LED. The 330pF (use 470pf for 100mA or lower currents) capacitor ensures that Q1 stays off until the coil is discharged, at which point the cycle repeats. The scope image below shows how it works.
The top (green) trace shows the current through R-limit, which corresponds to the drain (100mA per div) on the battery . The bottom trace (yellow) is the voltage across the transistor. The start of the cycle is when the top trace current goes up. The battery is driving the LED at the same time it is charging the coil. Notice the current slowly increases - as the coil charges up, its resistance decreases. When the current limit is reached - about 120mA here, Q2 turns Q1 off, seen as the sudden drop in current. The LED is kept bright by the charge stored in the coil and the cycle repeats, 2 to 400,000 times each second. Although the battery drain reaches as high as 120mA, part of the time the coil is supplying the power, reducing battery use to almost 0. At 6v, this saving is about 38% (the "duty cycle"), so that we can drive a LED at 120mA using only 75mA on average. This design can drive a chain of multiple types of LEDs in series, so long as their Current rating is greater than the Current allowed by R-limit. The power supply must also be at least 1-volt over the combined Vf of these LEDs (4.5v for 1 white LED, 8v for 2, etc). When the supply voltage drops below that, the circuit will run in direct drive. For 1/2 or 1-watt LEDs, just about any NPN transistor with a current limit of 1-amp will work (FJN965, 2SB2500, BC337, MPS651 etc) - although you may have to adjust R (try 2.2K to 10K) to compensate for gain. The coil can be any value from 150uH to 470uH - make sure it can handle the current. By the addition of a capacitor, we eliminate the timing winding on our coil, so we can now use standard off-the-shelf inductors.

Here, we've taken our dollar-store sample apart and removed the incan bulb to prepare it for some upgrades. I'd decided to build it with the LED at the top and shining down, thinking it would give a better quality light... It doesn't :(
	You can make your own coil by taking 4m (12-ft) of #30 wire-wrapping wire and winding it over a 1-1/4" (3 cm) form - I used the top cover of an aeresol can. Leave enough slack to go around the loops to secure it. This gives us a coil of about 50uH - not ideal, but it works just fine with inputs of 4.5-volts. The LED is an 8mm, 100mA "straw-hat" obtained on eBay. So a 1/4-watt 10-ohm was selected as R-limit for about 60mA. The parts can go together as free-form art - just make sure there are no shorts. And, of course, test, before assembly! Here are the parts just at the start of the soldering process.
The whole works stuffed into the top of the lamp. The coil just fits inside. I used the uncut leads of the smoothing capacitor through existing holes to secure the whole package.
*AND NOW, FOR SOMETHING COMPLETELY* DIFFERENT! (...SORTA)** So I can tell them at the office I'm just packing a light lunch! The same circuit, but running off 4 x AA - so that I can use rechargeables if necessary. Again, free-form construction. Taped and epoxied to the lid. Got real lazy - light uses a piece of paper as an on-off switch. I like this design - it can sit as a globe lamp, or left hanging from a pole.