Transistor Light Dimmer

This article is about a transistor light dimmer that needs only one 1.5 V battery to work. The circuit can be made with two light bulbs connected in series. Then you will need two 1.5 V batteries. However, you can also connect a few 1.5 V bulbs in parallel instead, thus eliminating the need for a second battery.

Components: light bulbs, light bulb sockets, TO-3 BJT PNP transistor, TO220 BJT NPN transistor, 56-ohm high power resistor, 1 kohm low power resistor, 1 kohm potentiometer, matrix board, heat sinks (TO220 and TO-3), heat transfer paste, bolts, nuts, washers, wires, solder.

Tools: pliers, screwdriver, wire stripper, soldering iron.

Optional components: plastic box or cardboard box, blu tack.

Optional tools: multimeter, voltmeter, electric drill.

The design of this circuit is straightforward.

The current entering the base of the Q1 transistor is amplified and supplied to the base of the Q2 transistor from the Q1 collector. The Q2 transistor collector pin supplies current to the light bulb. You might be thinking the following question now. Why not connect the variable resistor in series with the light bulb? This is because the variable resistor cannot handle the high current of the light bulb (300 mA), especially if you are planning to connect a few light bulbs in parallel. You can use a high-current potentiometer that is expensive.

A cheaper alternative to this circuit is using just one transistor. The transistor collector (output current) is equal to:

Ic = Ib * Beta

The typical Beta value is 100. However, the minimum Beta value could be 20. This is why the cheap alternative might not work, especially if you are using a high-current light bulb or connecting a few light bulbs in parallel.

The Rs and Cs low pass filters are used to remove the power supply ripple (this only occurs if you use a power supply from a socket) and prevent possible oscillations in the power supply that might be amplified in the circuit. The high internal resistance of the battery or other power source can cause such power supply oscillations. Higher current and fully charged batteries have smaller internal resistances. AA batteries can provide a higher current than AAA batteries because they are larger in size. I made the circuit and found out that this low-pass filter was not needed. However, I suggest you still implement this filter just in case there are power supply oscillations.

The bandpass frequency of the power supply low pass filter is equal to:

fl = 1/(2*pi*Rs*Cs)

= 1/(2*pi*100*(4700*10^-6))

= 0.33863 Hz

Each light bulb needs 1.5 V or 3 V for two light bulbs connected in series.

The maximum power dissipation when the transistor collector-emitter voltage is at half supply voltage:

Ptmax = (Vs/2)*Icmax

(the light bulb's maximum current is 0.3 A)

Ptmax = 0.75 V*0.3 A = 225 mW

However, at half supply voltage, it is not likely that the light bulb current will be 0.3 A. The maximum current is when the light bulb voltage is maximum, 1.5 V.

The following article explains how you can select an appropriate heat sink for the transistors.

https://www.instructables.com/Component-Heat-Dissipation

Simulations show that the light bulb of the 1.5 V circuit will not receive the full 1.5 V but only 1.3 V. This is because the transistor saturation voltage is not zero but 0.2 V. If you are using a 3 V circuit each light bulb will receive:

Vbulb = (Vs - Vsat) / 2

= (3 V - 0.2 V) / 2

= 2.8 V / 2 =1.4 V

I have made only the 1.5 V light dimmer.

I did not have 56 ohm resistor. I used 68 ohm resistor instead.

The 1 kohm resistor could be smaller. It is low power.

I drilled holes in the matrix board for the heat sink. You will see the Q2 transistor in the next step which is encasement.

In the photo, you can see the 1.5 V D battery and the Q2 transistor with the TO-3 heat sink.

The second photo shows you the 1 kohm potentiometer. The green wire in the second photo is for the light bulb.

Testing showed that the circuit was working.