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

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Apr 5, 2002
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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.

you can use mosfet instead of bipolar
also i doubt abt 555 driving as mosfet need large transient current in switching

you should look 4 PWM circuit
i have many of them but i don't know how to send or upload them

I have posted only one message.... Now the are 2 post... BOH... Thanks to all for the help. This night I try to search some stuff about PWM...

Problem with 2N3055 may be 60V m@x and low dc hfe (beta). I don't understand what your supply voltage is. Might want to use MOSFET instead. The 555 should be fine with small gate resistor and duty cycle variation as described. Make sure MOSFET has sufficient Vds, Id and low Rds, and gate drive is ok. Should run cool if set up properly for 5W load. Remember that lamp resistance varies greatly with light output.

i think this circuit will fit

**broken link removed**
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I would recommend to connect to your 555 a smart MOSFET that Infineon makes. They are protected against overcurrent (if your lamp goes into short-circuit), against overvoltage, overtemperature etc. Infineon do have many to choose from that will suit your requirements on low conducting resistance of the MOSFET.

Also, keep in mind that if you will have long cables to your lamp, then they will act as inductanses and you will get high-voltage spikes over the MOSFET when it stops conducting. If using a non-smart MOSFET you will have to take care of that possibly problem.

Recommended switching frequency for the PWM would be about 60-120 Hz. If using much higher frequency you will get too much switching losses in the MOSFET. If using to low frequency your lamp will flicker. Other makers of smart MOSFETs are ST, Philips.


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