PWM on High Power Loads

I am starting my bread board design for my RGB LED clock and need to start ordering parts. The question is how can I power each channel over PWM? With out multiplexing the shift registers I would need around 2A for each color for the 88 LEDs. With multiplexed shift registers it would be around 300mA. What are your PNP transistor or FET sugestions?

I am not sure I understand that circuit. What are the circles and the blocks with R, G & B?

With out multiplexing the shift registers I would need around 2A for each color for the 88 LEDs. With multiplexed shift registers it would be around 300mA

That is also strange as if you are multiplexing the display you generally boost the current to compensate for time it is off.
Are you trying to use PWM along with multiplexing? If so it is impossible to synchronise the two without writing software that performs both tasks, and doing PWM in software takes a lot of resource.

I know crappy sketch from a while ago.

LED display is done like a 7 segment Display with 2 LEDs per segment with the common cathodes of the LEDs connected to a pin on the ULN2803 with each of the color anodes connected to a transistor.

As for the shift registers I meant like how John_Ryan did.
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1185507207/45#45

Ok on the seriese LEDs and pull down with a 2803, but
still not sure what are the blocks with R, G & B? Are they mosfet top switches?

That link did not mention using PWM just multiplexing. But yes that is a way to multiplex a matrix. However unless those blocks are switches you will need one output per LED pair, which is doable but resource heavy.

Have you seen my matrix:-
http://www.thebox.myzen.co.uk/Hardware/Econo_Monome.html

or the one I did with multiplexing and PWM fading:-
http://www.thebox.myzen.co.uk/Hardware/Mini_Monome.html
Although the PWM bit is handled in hardware.

They are a MOSFET

The RGB monome is nice. Like how you made the buttons.

OK, not sure then how you have common cathode RGB LEDs in series?

Just the grounds are connected.

Better Schematic of the first one. Click on it to make it more clear.

Clock model. Click for big.

Thanks:-

not sure then how you have common cathode RGB LEDs in series?

That is because you have not connected the LEDs in series but in parallel. :wink:

The PNP transistors are the wrong way round. That is the collectors and emitters need to be swapped over. Also the PWM signals going to them need to be boosted to +9V in order to turn the PNP transistors off. You can do this with an NPN transistor on each output. You also need a current limiting resistor in each LED anode. Finally add a few 0.1uF supply decoupling caps around IC1.

Good luck.

Attach a NPN to each PNP.

Can you show a sketch of that.

You have been very helpful.

Thanks, ROB

I’m still learning how to use KiCad for schematics.

Not got and drawing facilities here at the moment but see if you can follow this.

PWM output of arduino to a 1K resistor other end to the base of an NPN. Emitter of this transistor to ground. Collector of this transistor to the base of your PNP. Also 10K resistor from the base of the PNP up to 9V (this is the same as the emitter of the PNP).

Little update

I have gotten this working now.

schematic

Well done. :slight_smile:

Not sure what the PNP transistors in the schematic bring to the party though.