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# Stupid question for this forum... but I dont say where find

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#### masterx81

##### Member level 1
Hi, I am new in this forum and I am a beginner with electronics... I wont to pilot and variate the power a 12 volt 5w lamp. If I use a bd 649 it becomes very hot. I have also tryied win a 2n3055 (15 ampere.......). I wont to pilot the final with a 555 with a variable duty cicle, for slowly power off the lamp. Is the best thing? How conntect the 555?
Sorry for my ugly english.

Hi masterx81,

if your power source is DC, what you say (variable duty cycle with 555) is OK (for AC you could use a phase control wuth triac).
A power MOSFET (as IRF520 or 6N10 among others) will be better than a bipolar transistor. It has very low ON resistance and very fast switch, so no power consumption. The gate can be driven directly from de output of the 555 using a power supply of 12V.
Regards

Z

masterx81,

below is a sample I did design very quickly.

The NE555 works in a usual astable configuration, where C1 is charged through the series of R1+R2 and discharged through R2. The ratio of R1/R2 is calculated in such of 50% of duty cycle at Q(pin 3) is achieved when about 6V are on center tap of RV1 (center position).

The frequency square wave at Q is depending from duty cycle in a ratio near to 1:3. The minimum frequency, about 50Hz (20ms), is achieved when center tap of RV1 is near to 12V and so the m@x duty (100%). The lamp is on its m@x power.

The frequency in opposite condition is near to 150Hz, where duty is at its minimum and Lamp is off. The frequency range from 50Hz to 150Hz should be good enough to avoid lamp flickers. In case you may change value of C1 with smaller one.

I used 2N3055 as you mention it, but I agreed about power MOSFET. Smaller ON Drain/Source resistance means less power loss, so less heat.

Hope this help

james

But I have one more problem. I wont that the lamp goes off slowly, in automatic, i have build a circuit with a cap and var resistor on the base of a prefinal, that drive a final, and the lamp goes off slowly in base of the value of the res. But the final goes too hot. There are a mode to implement this work with 555? thanks

masterx81,

If you place a cap accross 2N2222 base and ground (I'm afraid big capacitor value), you will integrate the square wave, so you will have again a dc value proportional to 5V * Duty(%/100). It is bad, as you will drive the Darlington couple (Q2+Q1) again as linear regulator and you lost all beneficts from 555 and the final will goes hot.

You've to use the above circuit, just a bit modified. Let explain how.

1. Remove RV1
2. Wire capacitor between pin 5 of 555 and ground.(10uF will enough depending from how much slow you want variation)
3. Use switch deviator (NO/NC), with center tap wired to pin 3 of 555.
4. Wire the switch NO contact to R5 (other side of R5 is still connected to 12V)
5. The switch NC contact to a 330k resistor.
6. The second wire of 330k resistor to ground.

With 330k and 10uF you will have about 3 seconds. If too fast increase cap from 10 to 100uF.

Switching on NO (R5), the 10uF cap is quickly charged through R5=100ohm. Switching on NC (330k res) the 10uF cap is slowly discharged through the 330k.

The Duty from Q (pin 3 of 555) will slowly reduced, but Darlington couple will still driven in switching mode.

james

I've been not too much accurated this time.

The answer of mine did not consider the 2/3 Vcc voltage at pin 5 (Control Voltage) of 555. Sorry...

Attached is modified sample I've verified to be working. LM358 output will sink current to the internal 555 ladder resistors and that will reduce slowely the potential at pin 5 as you required, as well as the duty cycle at pin 3 (Q).

If you require differents time slope, change C2 or R5.

Use of LM358 is due to its near 0V output when single rail power has used.

james

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