Simple 1.2V NiMH Cell LED Flashlight Circuit

This circuit is like a tiny supercharger built into your flashlight.

Regular flashlights with NiMH batteries only use part of the batteries power.

This circuit includes a special booster that takes the weak 1.2V and increases it to a higher voltage usually around 3.3V to make your LEDs shine bright.

This way, you can use all the juice in your battery and get a brighter flashlight for longer.

Circuit Working:

Parts List:

The single cell LED flashlight circuit utilizes a self oscillating boost converter.

A typical white LED operates most efficiently at around 20mA and requires about 3.3V resulting in approximately 66mW per LED.

This circuit features 7 LEDs connected in series, requiring a driver circuit capable of delivering approximately 23V at 20mA.

This can be achieved using a 1.2V NiMH rechargeable cell or a 1.5V alkaline cell.

When the circuit is switched on R1 and D1 bias the transistor into its linear range through the feedback winding on T1.

This causes a current to flow through the 18 turn winding, driving the transistor into saturation due to positive feedback.

The base current is determined by the total of 1.2V from the cell 0.2V induced in the feedback winding and the 0.7V base emitter drop of the transistor resulting in approximately 32mA of base current through the 22 ohm resistor.

At this stage D1 does not conduct significant current because the transistor clamps the base voltage to 0.7V with the 3 turn winding subtracting 0.2V from this, leaving only 0.5V across the diode.

The base current maintains the transistor in saturation until its collector current reaches about 1A while the transformer is loaded.

At this point, the transistor begins to exit saturation causing a drop in the feedback voltage, quickly driving the transistor into cutoff.

The collector voltage rises as T1 continues to force current flow until D2 starts conducting and discharges the transformer into C2 with a narrow pulse.

During operation, this pulse is approximately 24V high causing the feedback winding to develop 4V, which results in applying about -3.3V to Q1s base enough to switch it off rapidly but not enough to cause reverse conduction.

Once the transformer fully discharges into C2, the voltage on it breaks down and the transistor enters conduction to initiate a new cycle.

The oscillation frequency is 30kHz and the transformer operates at a peak flux density of 0.1 tesla well below saturation resulting in low loss.

C2 is tasked with handling the load pulses starting at about 1A and maintaining a relatively constant voltage to provide the LEDs with almost smooth DC.

The given value for C2 works effectively.

For continuous operation over 30 years, it is advisable to select a capacitor with low equivalent series resistance ESR and a relatively high ripple current.

However, for flashlight use, a standard 47µF 35V electrolytic capacitor works well.

C1 is not essential, the circuit operates similarly without it saving costs.

However, including C1 improves circuit performance as the cell nears full discharge and its internal resistance increases.

Formulas:

The main equations and relationships for a self oscillating boost converter with a BJT and a transformer are usually as follows:

Duty Cycle D:

The BJTs duty cycle in the boost converter is provided by:

D = V_out​​ / V_in​

where,

• where V_out represents the intended output value and
• V_in represents the input voltage.

Output Voltage V_out:

The output voltage V_out can be expressed as:

V_out​ = N_secondary​​ / N_primary ​× V_in

​where,

• The numbers in the secondary and primary windings of the transformer are, respectively, N_secondary and N_primary.

Peak Current through the BJT I_peak:

One may estimate the peak current flowing through the BJT at switch on time by using:

Ipeak​ = Vin​*D​ / Rload​

where,

• R_load​ is the load resistance.

Energy Stored in the Inductor EL:

The principal winding of the transformer, the inductor stores energy that is provided by:

E_L​ = 1/ 2​*L_primary​*I²_peak

where,

• L_primary​ is the inductance of the primary winding.

Note:

These equations give a basic grasp of the parameters and workings of a self oscillating boost converter that uses a transformer and a BJT.

In practice, design and computation may take other factors like feedback loop dynamics, component tolerances, and losses into account.

How to Build:

To build a Simple 1.2V NiMH Cell LED Flashlight Circuit follow the below mentioned connections steps:

Connect the Transistor:

• Connect the NPN transistor Q1 to the circuit board, ensuring proper orientation base, collector, emitter.

Add the Feedback Winding:

• Wind the transformer T1 with an 18 turn winding for the main circuit and a 3 turn winding for feedback.

Install Diodes and Resistor:

• Connect diode D1 and resistor R1 in series to bias the transistor and provide feedback.

Connect the LEDs:

• Connect the seven white LEDs in series ensuring they are oriented correctly anode to cathode.

Add the Capacitors:

• Connect the electrolytic capacitor C2 to smooth the output voltage.
• Optionally, add a smaller capacitor C1 for additional filtering.

Complete the Circuit:

• Connect the components according to the circuit diagram, ensuring all connections are secure and insulated.

Test the Circuit:

• Insert the battery and test the circuit by switching it on.
• Verify that the LEDs light up and the circuit operates as expected.

Adjustments:

• If necessary, adjust component values or make minor changes to optimize the circuit’s performance.

Note:

• Remember to follow standard safety practices when working with electronics such as soldering in a well ventilated area and avoiding short circuits.

Conclusion:

A 1.2V NiMH cell LED flashlight circuit is a simple and efficient circuit that uses a single NiMH rechargeable cell to power a series of LEDs.

The circuit includes a boost converter to step up the voltage from the cell to the required level for the LEDs providing a stable and reliable light source for a flashlight.

References:

LED lights on 1.2 V battery