Hello,
I am new to this forum but seems like I may find some useful input to my circuit designed to drive an LED 8x8x8 RGB cube with common anodes.- they will sink to a series of TLC5940 constant current ICs which are filled with data from a Chipkit Uno
The Power = this is why i need a separate external 5v source!!
The part of the circuit I have designed is the part that drives the current to the common anode layers (8x8x3=192 possible leds on at once @20ma = 3.84A - this will fry my chipkit !!!) so I'll use a separate supply. Note I won't get near the 3.8A as I'll be using dot correction to reduce some of the power requirement, I'll also be very unlikely that all leds and all colours will be on at once. Some rough calculations I made indicate that max power will be in the region of 1 - 1.5A to each layer.
Idea is that the 3.3V + signal from a chipkit source will drive an npn transistor on. In turn this will send the gate of a Pchannel Mosfet low and allow the mosfet to draw current from the 5v to the anode layers.
Note - only one of the transistor/mosfet pairs will be on at any one time as the inputs from the chipkit will cycle through and send each input high in turn before turning it low and then turning on the next input.
Questions>
1. Will the circuit work?
2. Will these switched pairs turn on/off fast enough to support Persistence of Vision displays?. I've read about the capacitance characteristics of the IRF5210s but am not sure at the speeds I need to multiplex this will be an issue. - I'm not too sure also exactly what speeds I will need - I just thought I'd go as fast as the Chipkit would allow me with an ISR driving the inputs.?
3. I've seen so many conflicting articles about the requirements for pull up resistors and also resistors in series with the transistors and the chipkit that I am thoroughly confused about a) whether they are needed and b) what values they should be. I've seen so many variants on these from 100s of ohms to megaohms? Can anyone explain exactly what these do and a rational as to what are reasonable values. (note the 2k2s I've used seem to be about the most common I've seen)
Really hope that someone can provide me with some input to help .
Kind regards
Russell
I am new to this forum but seems like I may find some useful input to my circuit designed to drive an LED 8x8x8 RGB cube with common anodes.- they will sink to a series of TLC5940 constant current ICs which are filled with data from a Chipkit Uno
The Power = this is why i need a separate external 5v source!!
The part of the circuit I have designed is the part that drives the current to the common anode layers (8x8x3=192 possible leds on at once @20ma = 3.84A - this will fry my chipkit !!!) so I'll use a separate supply. Note I won't get near the 3.8A as I'll be using dot correction to reduce some of the power requirement, I'll also be very unlikely that all leds and all colours will be on at once. Some rough calculations I made indicate that max power will be in the region of 1 - 1.5A to each layer.
Idea is that the 3.3V + signal from a chipkit source will drive an npn transistor on. In turn this will send the gate of a Pchannel Mosfet low and allow the mosfet to draw current from the 5v to the anode layers.
Note - only one of the transistor/mosfet pairs will be on at any one time as the inputs from the chipkit will cycle through and send each input high in turn before turning it low and then turning on the next input.
Questions>
1. Will the circuit work?
2. Will these switched pairs turn on/off fast enough to support Persistence of Vision displays?. I've read about the capacitance characteristics of the IRF5210s but am not sure at the speeds I need to multiplex this will be an issue. - I'm not too sure also exactly what speeds I will need - I just thought I'd go as fast as the Chipkit would allow me with an ISR driving the inputs.?
3. I've seen so many conflicting articles about the requirements for pull up resistors and also resistors in series with the transistors and the chipkit that I am thoroughly confused about a) whether they are needed and b) what values they should be. I've seen so many variants on these from 100s of ohms to megaohms? Can anyone explain exactly what these do and a rational as to what are reasonable values. (note the 2k2s I've used seem to be about the most common I've seen)
Really hope that someone can provide me with some input to help .
Kind regards
Russell