boylesg
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Because I have a bunch of salvaged ATX PSU and I may as well make use of them I figure.but why a ATX power supply?
Hi,
I´m confused. please clarify:
* I can´t find MCP23017 in your datasheet. So how is it related to your circuit, or should we just ignore it?
* "I must use a darlington transistor". Are you talking about the BC373 (this is the only darlington I see..)? It is rated for 1A only, but you calculated up to 8A of current.
* You talk about "TLC4950", I can´t find the datasheet, please post a link.
* in the schematic there is a "TLC5950", I can´t find the datasheet, please post a link.
* in the schematic there is a "MCP2007", I can´t find the datasheet, please post a link.
* "actual RGB LEDs would see 4V". What RGB LEDs are you using? Please post a link to the datasheet.
* "With the TIP120 the LM317lz can supply a peak of 8A". If you drop voltage from 12V down to 7V then there is a power dissipation of 5V x 8A = 40W. You need a fan and a big heatsink.
* "So I would need 8 of those LM317 circuits to supply 64 columns of 8 RGB LEDs.", If you need 8A for 8 columns you need 64A for 64 columns. multiplied with 12V this gives more than 750W of heat. This is in the range of a hairdryer. You may heat your room in winter with that application..
Klaus
Sorry but I am a bit dyslexic when it comes to those two components. They are meant to be MCP23017 and TLC5940 but I have mislabelled them as MCP2007 and TLC5950
And I don't have a datasheet for the RGB LEDs - got them from ebay. They are commonly available, e.g. these ones: **broken link removed** i
I have just assumed the recommended resistor values (plus bit) for red, green and blue LEDs, powered with 5V) on the Jaycar website
100R for blue and green and 180R for red (and they are quite bright enough)
How do did you calculate?I have calculated that 8 columns of 8 RGB LEDs can draw a max of around 8A.
Hi,
The most important electrical values are : voltage and current. Both is given at that site.
* Forward Voltage: R: 1.8-2.2 G: 3.0-3.4 B: 3.0-3.4
* Electric Current: 20mA
--> There´s nothing to "assume". You can calculate it.
Additionally this means: With "4V" across the LED you will kill them.
Klaus
Added:
How do did you calculate?
This is the wrong way of calculation.1 RGB led will consume about 127mA assuming all cathodes are GND'ed simultaneously.
Hi,
This is the wrong way of calculation.
Each color in the LED must consume not more than 20mA. Thus the max current per complete LED device is 60mA. Don´t overload them. Don´t ever use 127mA or in that range.
Even if you pulse them you must never get more than 20mA average current per LED. Indeed you may pulse them maybe with 100mA, but then the duty cycle needs to be less than 20%.
I assume about 15..17% to get equal dissipated power.
I assume they calculated with total voltage of 5V = V_LED + V_R
Let´s try:
V_LED (red) = 2.0V--> V_R = 5V - 2V = 3V, 3V / 180R = 16,7mA
V_LED (green, blue) = 3.2V--> V_R = 5V - 3.2V = 1.8V, 1.8V / 100R = 18,0mA
Both values look reasonable
Klaus
Hi,
TLC5940 datasheet says it includes LED current regulation.
If so, then you don´t need the resistors at all. Then I recommend to omit them.
Klaus
an ongoing problem with teaching myself electronics haphazardly through the internet - I am still filling in gaps in my understanding of things.
Why do you use BJTs so much in designs that really are quantatively and qualitatively better with MOSFETS? It seems wasteful, besides archaic - a good circuit is designed by a Scrooge-like mind "You want how many milliamps?!!! No, no no, I can't afford that."
Otherwise uneven sharing of current in parallel LEDS...
For example I am trying to keep to TO92 packages to keep board real estate to a minimum.
But try and find a 1-2A logic FET in a TO92 package - they don't exist as far as I can see.
You are trying to use a const current source/ sink as a logic to turn on a transistor? That may be or may not be a good idea.
The you have 24 LEDs in three groups. Each group has 8 leds and the intensity depends on the current. You are using three channels of the controller as logic signal.
Each LED needs 20mA and 24 LED (if lit simultaneously) will consume 480mA; at 12V you will dissipate 6W. Out of this the LEDs will dissipate 1W (approx).
My suggestion: use the const curr feature of the IC and use 2 or 3 or 4 leds (I suggest 2 each) in series and more channels to control the lights. You will get more flexibility in the software.
Hi,
Sadly you refuse to accept the warnings about dissipated power.
--> No way to use a TO-92 case bjt for your application.....because of the dissipated power.
You need BJTs that have to be screwed to a heatsink.
In best case a single TO-92 case can dissipate 0.5W of power...free standing in 25°C air.
But the combinalion of a lot of these BJTs, each with maybe 2W of dissipated power, will make your complete PCB hot enough that the solder melts.
Without heatsink and without fan you risk fire. Don't burn down your house.
Klaus
Yes. You only talk about current.Have I made any errors here?
Not really. I have looked but I cannot find any of those LED power supplies on ebay with an adjustable voltage...
The LM317 in this schematic is simply an inexpensive way of supplying my construct with enough voltage such that the actual RGB LEDs see 5V despite the transistors dropping some of the voltage..
If I understand correctly, your TLC5940 can take LED voltage upto 17V and can regulate current from 5mA to 130mA. And you have 16 such currents. And the currents can be set by a single resistor...
You do not need the regulator but you will need a microcontroller to program the driver...
Anyway, the LM317 will need a heatsink...
ATX does have 12V regulated output (as well as 5V);
Just use a bigger (and heftier) dropping resistors with the LED and run them off the 12V. Anyway, you will dissipate the same heat but resistors can take abuses better.
It gets better if you use LED pairs in series. Less wastage of heat (not really but so what...)
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