pranaych Posted June 11, 2007 Report Share Posted June 11, 2007 I need a 1.2 V PWm Signal To dim my LEDs which I am driving through zetex Part number ZXLD1350.The Dc dimming they have proposed is not sufficient for me.Please help me and tel;l me so as how to generate PWm signal having duty cycle of 1.2 V Quote Link to comment Share on other sites More sharing options...
audioguru Posted June 11, 2007 Report Share Posted June 11, 2007 You don't want a 1.2V PWM signal. The datasheet shows an open-collector transistor that is fed from any current-limited voltage signal. The open-collector is used so that the ADJ pin voltage does not exceed the reference voltage that has a tolerance.The IC is supplied from a higher voltage that might be able to power the PWM oscillator. If the voltage is 15V or less then I would use a 555 or a Cmos 555 oscillator with two diodes making it do PWM, then it feeds a resistor to the open-collector transistor like this: Quote Link to comment Share on other sites More sharing options...
pranaych Posted June 11, 2007 Author Report Share Posted June 11, 2007 I am currently using LM 324 op amp Ic for generating PWM waveforms from a 9 V supply.Do you mean to say that say taht at the output of the MOSFET I just have to have a 200k resistor and a general purpose BC 547 transistor.please advice Quote Link to comment Share on other sites More sharing options...
audioguru Posted June 11, 2007 Report Share Posted June 11, 2007 I am currently using LM 324 op amp Ic for generating PWM waveforms from a 9 V supply.Do you mean to say that say taht at the output of the MOSFET I just have to have a 200k resistor and a general purpose BC 547 transistor?There is no Mosfet. The 200k resistor is inside the Zetex IC. It is the collector load of the new transistor.A BC547 transistor is fine.The output of the LM324 goes up to about 7.8V when the supply is 9V and up to 4.8V when the 9V battery is nearly dead. So the base resistor should be about 100k.The old LM324 is awfully slow to be used for PWM. Keep the frequency below about 500hz. Quote Link to comment Share on other sites More sharing options...
pranaych Posted June 11, 2007 Author Report Share Posted June 11, 2007 I am using the following circuit for PWM Dimmer.Please advice what should I do! Quote Link to comment Share on other sites More sharing options...
audioguru Posted June 11, 2007 Report Share Posted June 11, 2007 That is a good PWM circuit. I made it for my electric model airplane but I used more modern and faster MC33172 dual opamps instead. I used a frequency of about 2kHz because I like to hear the motor whine.Connect it to your Zetex IC like this: Quote Link to comment Share on other sites More sharing options...
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