After some time lurking, I'm here to ask for help
To make it short, I'm VERY new to electronics, and I'm working on a project that involve some sensors, motors and a sd logger, using (as you can see) the same power supply to power both the arduino and the LED...but my problems rotate around the led part:
I understand (at least i think) that the constant current driver is basically a step down that outputs a fixed current.
This one that came with the led has an output voltage range from 8 to 12v, so I thought I could use a second step-down to change the voltage and dim the led.
Got one, even with a 7 segment display to show the voltage. nice.
BUT (I still havn't tried) since I read an old post in this forum about pwm and constant current drivers not going well togheter: is this thing really possible? or I'm going to damage the driver/led?
Now, IF it's possible, second problem:
nice to have the display, but would be better to save the voltage value in the sd logger, but how? I explored the idea of retro engineering the signal sent to the display, but nope, too much of a hassle.
So voltage divider here we go!
BUT I really don't understand how to choose a resistor...I mean, the resistance value ok, but the power rating? In this forum I came across a post suggesting for this application 1/2W resistors for a 35/50W power supply
so the question here is: am I missing something?
also, if it's actually possible, should I really connect the GND to the divider, even if the power supply is shared?
If this whole thing is a problem, from the step-down/driver to the voltage divider, I'll just remove everything and go straight from power supply to driver
The power supply is largely un important .
You have 8.6 k across nonminally 12v .
Work out the voltage across each resistor .
The Power dissipated by each resistor is the voltage drop squared divided by the resistance ( ohms).
I don’t think varying the input voltage to the constant current supply will dim it , it’s unsure how it would react to a PWM signal too, really need a dimmable led driver
Here is the easy way to calculate: Ohms Law Calculator It will give you voltage, current, Power, and Resistance. Resistors have a wattage rating, if you do not exceed about 1/2 of that rating it should be relative cool. Here is a bonus: Voltage Divider Calculator
Do you have the specs for the constant current driver? And the specs for the LED (current and/or voltage)?
You probably don't need the step-down between the power supply and it's probably causing trouble. Usually constant current drivers are happier with "extra" voltage.
If the constant-current driver doesn't have a dimmer input there's no way to properly dim it because it's trying to put-out constant current, which means constant brightness.
There are two kinds of dimmable "constant current" drivers/power supplies and they have a dimming input/connection. Both usually work with PWM. Some are actually controlled-variable current (so it's "constant" at whatever it's set for) and the other kind passes-through the PWM, blinking the LED on & off faster than the eye can see to make it appear dim.
Some dimmable drivers can be driven from 5V PWM but there is an industrial 0-10V dimming standard which can use 0-10VDC, or 10V PWM. Usually the driver/power supply has the 10V built-in so you can use a pot to get the variable voltage, or you can use a transistor to "boost" 5V PWM.
I've seen a clever hack for dimming a constant-current supply - They used a MOSFET with PWM to short-out the constant current output (for the low or high part of the PWM cycle). That's something you should NEVER do with a regular constant-voltage power supply but if the constant-current supply is "well behaved" the voltage falls to (nearly) zero while the power supply keeps happily pumping the constant-current through the MOSFET. In theory this can be very efficient because with (nearly) no voltage there is (nearly) no power (wattage). But it's a risk shorting-out the driver... Either it will work or you'll fry the driver.
thank you gilshultz, the think is that I already tried these calculators, but were missing one part: wich values should I insert?
Now, thanks to hammy, I know I should use the voltage and the resistor resistance, but with my lack of knowledge I thought the needed values were my voltage and current, resulting in the 36W and in an almost impossible resistor to find
The thing is that I want the led with driver and dissipating fan to be used as a "stand alone" in a way that if I want to connect it to the arduino block I can do it, measure the voltage sent to it and all the other stuff with the other sensors , but I also want to be able to detach it to the main block and power it with a random 12v power source without the sensors
mmmh, thanks for the suggestion, but I think I'll leave the clever hack to someone else
btw, I sent all the links in the answer to Delta_G
but now, if there actually is no way to dim it...can I bypass the driver and power the led directly using a stepdown to regulate both tension and current?
something like this one
so that i can control the led with this when connected to the main block and with the driver when powered with an external power source
lol, I appreciate the frankness
I suspected it, gave it a chance because I wanted to leave the led block on the lightweight and this driver 1 I already had it and 2 is pretty small
I tried searching for other dimmable drivers, but are pretty big for my application
That LED driver uses a PT4115 chip, which can be dimmed on pin8 (next to the word "out").
The pin is likely not connected to anything now. Check that.
Solder a thin wire to that pin, and connect it directly to a PWM pin of the Arduino.
You should then be able to dim the LED.
Leo..
saw it in the linked datasheet, and this is really helpful
so I guess i can actually use the driver without an external step down
now, just for personal understanding, the fact that I have 3amps is a problem, right?
also, to use the pwm signal, now i should connect pin 8 straight to a pwm arduino pin or something is needed in between? should i connect GND? and if yes, is there a difference between pin 7 and 6?
Yes. If you have the square LED with nine dots, then 12volt is the absolute minimum for the driver.
Because those LEDs have three LEDs in series the LED already needs 10volt.
A LED with nine dots couldn't have worked on 5volt.
Not a problem. The supply could be 100Amp. The driver only takes what it needs.
I already said "connect it directly to a PWM pin".
Driver ground (supply ground) must of course also be connected to the Arduino.
Pin7 is not a PWM pin on an Uno.
Google "Arduino Uno PWM pins".
Leo..
Pin 6 and 7 are likely joined on the board, and connected to power ground.
You're fine as long as power ground is connected to Arduino ground.
You might want to use a direct ground connection in case the connection via the supply fails.
I have used hundreds of these PT4115 chips, and usually on a 24volt supply, driving six LEDs in series. With the PWM pin driven by a PCA9685 at about 500Hz.
Leo..
