Simple Voltage to Current Converter Drives White LEDs

You sometimes need to drive a white LED from one 1.5V battery. Unfortunately, the forward voltage of a white LED is 3 to 4V. So, you would need a dc/dc converter to drive the LED from one battery. Using the simple circuit in Figure 1, you can drive one white LED or two series-connected green LEDs, using only a few components. The circuit is a voltage-to-current converter, which converts the battery voltage to a current that passes through the LED.

You can adjust this current and, thus, the brightness of the LED, by varying resistor R3. If you turn on switch S1, resistor R2 feeds base current to transistor Q2. Q2 turns on, and its collector current, via R3, turns on Q1. Now, the current through inductor L1 increases. The slope of the increase is a function of the value of L1 and the battery voltage. The current through L1 increases until it reaches a maximum value, which depends on the gain of Q1. Because the value of R3 sets the base current drawn from Q1, Q1s collector current is also limited.

White LEDs Circuit Diagram

Once the current through L1 reaches its maximum value, the slope of the current through L1 changes. At that instant, the voltage on L1 switches to a negative polarity forced by the changed slope. This negative voltage traverses capacitor C1 and turns off Q2, which in turn turns off Q1. The negative voltage on L1 increases until it reaches the forward voltage of the LED. The peak current through inductor L1 now flows through the LED and decreases to zero. Now, Q2 switches on again, via the current through R2, and the cycle starts again.

By adjusting resistor R3, you can set the peak current through L1 and the peak current through the LED. The brightness of an LED is a linear function of the current through the LED. So, adjusting the value of R3 also adjusts the brightness of the LED.

It doesnt matter which LED you use; the forward voltage on the LED always increases until the peak current through L1 flows through the LED. Different forward voltages of the LEDs yield different on-times (duty cycles) but the same peak current through the LED. With the values shown in Figure 1, the circuit oscillates at a frequency of approximately 30 kHz and delivers a 20-mA peak current through the LED.

The duty cycle depends on the ratio of the battery voltage to the forward voltage of the LED. One advantage of this circuit is that it requires no series-limiting resistor for the LED. The peak current through the LED is a function of the value of R3 and the gain of Q1.[via]

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Step Up Converter For 20 LEDs

The circuit described here is a step-up converter to drive 20 LEDs, designed to be used as a home-made ceiling night light for a child’s bedroom. This kind of night light generally consists of a chain of Christmas tree lights with 20 bulbs each consuming 1 W, for a total power of 20 W. Here, in the interests of saving power and extending operating life, we update the idea with this simple circuit using LEDs. 

Power can be obtained from an unregulated 12 V mains adaptor, as long as it can deliver at least about 330 mA.  The circuit uses a low-cost current-mode controller type UCC3800N, reconfigured into voltage mode to create a step-up converter with simple compensation. By changing the external components the circuit can easily be modified for other applications. To use a current-mode controller as a voltage-mode controller it is necessary to couple a sawtooth ramp (rising from 0 V to 0.9 V) to the CS (current sense) pin, since this pin is also an input to the internal PWM comparator.

Circuit diagram :

Step-up Converter For 20 LEDs Circuit Diagram

The required ramp is present on the RC pin of the IC and is reduced to the correct voltage range by the voltage divider formed by R3 and R2. The RC network formed by R4 and C6 is dimensioned to set the switching frequency at approximately 525 kHz. The comparator compares the ramp with the divided-down version of the output voltage produced by the potential divider formed by R6 and R7. Trimmer P1 allows the output voltage to be adjusted. This enables the current through the LEDs to be set to a suitable value for the devices used. The UCC3800N starts up with an input voltage of 7.2 V and switches off again if the input voltage falls below 6.9 V. The circuit is designed so that output voltages of between 20 V and 60 V can be set using P1.

This should be adequate for most cases, since the minimum and maximum specified forward voltages for white LEDs are generally between 3 V and 4.5 V. For the two parallel chains of ten LEDs in series shown here a voltage of between 30 V and 45 V will be required. The power components D1, T1 and L1 are considerably over specified here, since the circuit was originally designed for a different application that required higher power. To adjust the circuit, the potentiometer should first be set to maximum resistance and a multimeter set to a 200 mA DC current range should be inserted in series with the output to the LEDs. Power can now be applied and P1 gradually turned until a constant current of 40mA flows. The step-up converter is now adjusted correctly and ready for use.

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Circuit modifications for operating 150 LEDs

Parts List

R1 = 220 Ohms, 1/2 watt
R2 = 100Ohms, 2 watts,
RL = All 22 Ohms, 1/4 watt,
C1 = 100uF/25V,
D1,2,3,4,6,7,8 = 1N5408,
D5 = 1N4007
T1 = AD149 or similar,
Transformer = 0-6V, 500mA

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