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Topic: pwm npn mosftet and rgb led with common cathode (Read 1 time) previous topic - next topic

bogomips

Hello guys, I'm an electronics newbie and I have lots of doubts about
project that I'm slowly doing a lamp with RGB LED.

I have led with common cathode and three anode (rgb), everything works perfectly
connecting each anode to a PWM output, problems arise when I want to light so many LEDs, using an 'external power supply.
I bought mosfet P10NK60ZFP (datasheet http://pdf1.alldatasheet.net/datasheet-pdf/view/157200/STMICROELECTRONICS/P10NK60ZFP.html) n channel.
I know that the LEDs, I know that  I should buy the LEDs with an anode and three cathodes so as to control them easily.
Unfortunately, I bought 100 LEDs with common cathode, so I have to overcome? I can reverse the tension with an auxiliary circuit?
I also have the tlc5940nt that I can not use  for the same reason!
I have to throw the led or have any hope?
I could solve a driving pnp moftet using my npn mosfet, it might work or is it crazy?

Thanks to all

MarkT

Firstly MOSFETs are n-channel or p-channel (sometimes called NMOS and PMOS), 'PNP' and 'NPN' are bipolar transistors.

Back to the question - how are you proposing running these LEDS - all in parallel?  You would need a much better spec'd MOSFET for that, the only you link too is a high voltage device (with high Ron).  For low voltage switching go for a MOSFET with 50 milliohm or lower Ron.

A p-channel MOSFET might solve your problem - think about reflecting your circuit top-for-bottom - all the positive and negatives swap over.

If your supply is more than 5V you'll have to interface to the MOSFET with a transistor.  If the supply is 5V choose a logic-level MOSFET (the Ron will be specified at 4.5V as well as 10V for a logic-level device).

By the way the MOSFET you mention is not a logic-level device.  It requires 10V to drive the gate. Don't be confused by the 'threshold' voltage spec - this is not the voltage it turns on at, its the voltage it starts to conduct miniscule currents.  Logic level devices have thresholds about 1V or so.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

bogomips

thanks for your clarification about the names of the mosfet.
yes,i want use the leds in parallel,
so i can't use my mosfet togheter with new p channel mosfet ?
do you can  recommend me  mosfet type ?

thanks


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