Circuit knowledge

Showing posts with label lamp. Show all posts

Friday, November 14, 2014

PWM LAMP DIMMER

A simple and efficient PWM lamp dimmer using timer IC NE555 is discussed in this article. Earlier linear regulator based dimmers can only attain a maximum efficiency of 50% and are far inferior when compared to the PWM based dimmers which can hit well over 90% efficiency. Since less amount of power is wasted as heat, the switching elements of PWM dimmers require a smaller heat sink and this saves a lot of size and weight. In simple words, the most outstanding features of the PWM based lamp dimmers are high efficiency and low physical size. The circuit diagram of a 12V PWM lamp dimmer is shown below.

Fig. 1

As you can see, NE555 timer IC which is wired as an astable multivibrator operating at 2.8KHz forms the heart of this circuit. Resistors R1, R2, POT R3 and capacitor C1 are the timing components. Duty cycle of the IC’s output can be adjusted using the POT R3. higher the duty cycle means higher the lamp brightness and lower the duty cycle means lower the lamp brightness. Diode D1 by-passes the lower half of the POT R3 during the charging cycle of the astable multivibrator. This is done in order to keep the output frequency constant irrespective of the duty cycle. Transistors Q1 and Q2 forms a darlington driver stage for the 12V lamp. Resistor R4 limits the base current of transistor Q1.

Understanding the variable duty cycle astable multivibrator

As I have said earlier, the variable duty cycle astable multi vibrator based on NE555 forms the foundation of this circuit and a good knowledge on it is essential for designing projects like this. For the ease of explanation the timing side of the astable multivibrator is redrawn in the figure below.

Fig. 2

Upper and lower halves of the POT R3 are denoted as Rx and Ry respectively. Consider the output of the astable multivibrator to be high at the starting instant. Now the capacitor C1 charges through the path R1, Rx, and R2. The lower half of POT R3 i.e. Ry is out of the scene because the diode D1 by-passes it. When the voltage across the capacitor reaches 2/3 Vcc, the internal upper comparator flips its output which makes the internal flip flop to toggle its output. As a result the output of the astable multivibrator goes low. In simple words, the output of the astable multivibrator remains high until the charge across C1 becomes equal to 2/3 Vcc and here it is according to the equation Ton = 0.67(R1+Rx+R2)C1.

Since the internal flip flop is set now, the capacitor starts discharging through the path R2, Ry into the discharge pin. When the voltage across the capacitor C1 becomes 1/3 Vcc, the lower comparator flips its output and this in turn makes the internal flip flop to toggle its output again.

This makes the output of the astable multivibrator high. To be simple, the output of the astable multivibrator remains low until the voltage across the capacitor C1 becomes 1/3 Vcc and it is according to the equation Toff = 0.67(R2+Ry)C1. Have a look at the internal block diagram of NE555 timer shown below for better understanding.

Fig. 3

How does the frequency remain constant irrespective of the position of POT3 knob?

What ever may be the position of POT3 knob, the total resistance across it remains the same (50K here). If anything decreases in the upper side (Rx) the same amount will be increased in the lower (Ry) and the same thing gets applied to the higher(Ton) and lower(Toff) time periods. The derivation shown below will help you to grasp the matter easily.

With reference to Fig 2, we have:

Ton = 0.67(R1+Rx+R2)C1

Toff = 0.67(R2+Ry)C1

Total time period of the output waveform “T” is according to the equation :

T = Ton + Toff

Therefore, T = 0.67(R1+Rx+R2+R2+Ry)C1

T= 0.67(R1+2R2+Rx+Ry)C1

We know that Rx + Ry = R3

Therefore, T = 0.67(R1+2R2+R3)C1

Therefore, frequency F = 1/(0.67(R1+2R2+R3)C1)

From the above equation its is clear that the frequency depends only on the value of the components C1, R1, R2 and the over all value of R3 and it has nothing to do with the position of R3 knob.

Monday, September 1, 2014

20W 40W Fluorescent Lamp Inverter Wiring diagram Schematic

This is certainly a low-cost 20W/40W fluorescent lamp inverter schema project. Even so the maximum effective would be to make use of a 40 watt tube (or two 20 watt tubes in series). It is a schema you may put collectively from junk box parts or assemble from a kit. It certainly is quite easy to construct and no need printed schema board. The transformer is hand-wound on a ferrite rod (from an old transistor radio) and the winding wire could be salvaged from an old transformer. We’re intentionally maintaining bills down to confirm how cheaply it could be put together.

20W/40W Fluorescent Lamp Inverter Schematic

Dependant upon your stock of components, the cost could vary from $3.00 to about $15.00 and in case the wires from the transformer are soldered to the ends of the tube(s), you are able to make a free-standing light that can be set-up in the garden to light up a dark place without having the worry of operating 240v wiring. The cost of powering the schema is about 22 watts and this will make the same light output as a 60 watt globe.

Using a regular fluoro operating on the 240v mains, a ballast (or choke) is required in series with the tube to limit the current after the tube has “struck”. This ballast dissipates about 10-20 watts for a 20 watt tube and lowers the effectiveness of this schema. In case the ballast is replaced with an electronic schema and high-frequency transformer, the losses are lower than 5 watts. In addition, if we tend not to drive the tube as tough as the 240v type we are able to get even better effectiveness.

Parts List:

1 – 2R2 1/4watt (for testing)
1 – 47R 1/4watt
1 – 180R 1watt
1 – 47k
1 – 100k mini rim pot
1 – 100ngreencap
1 – 100u 16v electrolytic
1 – BC338 transistor
1 – TIP 3055 transistor
1 – on/off switch
1 – 12mm bolt and nut for transistor
1 – heat-sink 5cm x 10cm
1 – ferrite rod 10mm dia x 8m long
1 – 30m winding wire .28mm dia
1 – 4m winding wire .61 mm dia (wire diameters are NOT critical)
1 – insulation tape either sticky tape or masking tape
1 – interlayer insulation – paper

PCB Kit Design:

Wednesday, August 13, 2014

220V AC Lamp Toggle Switch Circuit

Due to the low current drawing, the schema can be supplied from 230Vac mains without a transformer. Supply voltage is reduced to 12Vdc by means of C1 reactance, a two diode rectifier cell D1 & D2 and Zener diode D3. IC1A, IC1B, R2, R3 and C3 form a reliable bounce-free toggle switch operated by P1. R4 and C4, wired to pin #6 of IC1B reset the schema (lamp off) when power supply is applied. IC1C and IC1D wired in parallel act as a buffer, driving the Gate of the Triac through R5.

Parts:

R1 = 470R
R2 = 10K
R3 = 100K
R4 = 100K
R5 = 1K
C1 = 330nF-400V
C2 = 100uF-25V
C3 = 100nF-63V
C4 = 10uF-25V
D1 = 1N4007
D2 = 1N4007
D3 = BZX79C12
D4 = TIC206M
IC1 = 4011 NAND Gate

Notes:

* The schema can be wired permanently to the mains supply as current drain is negligible.
* Due to transformerless design there is no heat generation.
* Low Gate-current Triacs are recommended.
* Obviously, other appliances can be powered in place of a lamp, provided their power dissipation does not exceed about 400W @ 230V
* 110-120Vac operation is easily obtained by simply changing C1 value to 680nF 250V. No further changes are necessary.
* In some cases, e.g. when the controlled device is far from the toggle switch, a pilot LED could be necessary for monitoring purposes. If so, disconnect pin #10 of IC1C from pin #11 of IC1D and wire a LED and its 1K series current limiting resistor across pin #10 of IC1C and negative supply.
* Warning! The schema is connected to 230Vac mains, so some parts in the schema board are subjected to lethal potential! Avoid touching the schema when plugged in and enclose it in a plastic box.
* P1 will SPST Pushbutton