Electronics > Projects, Designs, and Technical Stuff
RGB Strip has 600 5050 SMD on it, but comes with a 12V 5A PSU?
DW1961:
--- Quote from: ozcar on July 30, 2020, 06:51:46 am ---
--- Quote from: DW1961 on July 27, 2020, 06:08:36 pm ---I made a typo. It was 5V all the way down both the strips. Each strip is 5 meters.
I didn't do a current test. I tested all colors and then set them to WHITE at 100%. Same deal. 5V all the way down, no drop at all. It was actually a little over 5 volts.
The two light strips are split at the controller. Both (I assume) receive 5 volts, which is 10 volts. A 20% headroom would be 12V, right? If so, they did it perfectly.
I got a reply back from the vendor and he said the resistors in the tape limit overall wattage to 55 watts for both strips.
"Dear Customer
We installed a resistor on the light strip to control the overall current of the light strip,
So the actual power of our 10m light strip is about 50-55W."
Also, just to be clear, each SMD diode has its own resistor. That is, 3 resistors per SMD.
--- End quote ---
It’s not clear to me where you are measuring the voltage. If you measure it from the common +12V conductor, to one of the G, R, or B conductors, then you would expect to see close to 12V when the strip is on at maximum brightness white.
Of course, the controller can reduce the power to the LEDs, but it would probably achieve that by means of PWM. That is, rapidly turning the LEDs on and off (too fast to seem them flicker), and varying the amount of time they are on, vs the time that they are off. Trying measure a voltage in that situation can get a bit tricky ( eg see https://www.eevblog.com/forum/testgear/how-a-multimeter-measures-pwm-voltage/ ).
It is possible that the controller does not allow the LEDs to reach their true maximum brightness. You could do a test without the controller, either using another 12V power supply, or with the one you got with the LEDs - you are not going to overload it if you test one channel at a time.
Otherwise, what test equipment do you have other than a DMM or two?
--- End quote ---
All I have is a DMM. I measured the voltage at the end of the strip, both 12+ and the cathode sides of all LEDS when the strip was on 100% and White.
Why couldn't they just reduce power to LEDs suing resistors in the controller, and avoid possible RF interference from PWM?
ozcar:
--- Quote from: DW1961 on July 30, 2020, 07:12:48 am ---
All I have is a DMM. I measured the voltage at the end of the strip, both 12+ and the cathode sides of all LEDS when the strip was on 100% and White.
--- End quote ---
If that is where you measured 5V, then it does seem that the controller limits the maximum power to the strips.
--- Quote from: DW1961 on July 30, 2020, 07:12:48 am ---
Why couldn't they just reduce power to LEDs suing resistors in the controller, and avoid possible RF interference from PWM?
--- End quote ---
That can waste a lot of power, and cause heating of the controller. It can also cause colour shift as brightness is varied, but maybe you would not notice that.
A lot of DMMs can measure frequency, which would allow you to confirm that PWM is being used, and some DMMs can also measure “duty cycle”, which is what varies as the brightness changes.
DW1961:
--- Quote from: ozcar on July 30, 2020, 07:51:01 am --- that.
A lot of DMMs can measure frequency, which would allow you to confirm that PWM is being used, and some DMMs can also measure “duty cycle”, which is what varies as the brightness changes.
--- End quote ---
Sadly, none of mine read Hz or duty.
ozcar:
--- Quote from: DW1961 on July 27, 2020, 06:08:36 pm ---I tested all colors and then set them to WHITE at 100%. Same deal. 5V all the way down, no drop at all. It was actually a little over 5 volts.
The two light strips are split at the controller. Both (I assume) receive 5 volts, which is 10 volts. A 20% headroom would be 12V, right? If so, they did it perfectly.
--- End quote ---
I think you have already all but proved that the controller must be using PWM.
The two strips would be connected in parallel - connecting two 12V strips in series would require a 24V power supply, and would go wrong if the two strips were not identical (voltage would not distribute equally across the two strips - your strips may be the same now, but say you decided one was too long, and cut some of it off).
If you were to connect those strips (all three colours, and without the controller) to a variable power supply (or, as you more-or-less suggested, to a fixed voltage power supply, in series with a variable resistor), and started with the voltage across the strip at 12V. The strip would be at maximum brightness, and with all three colours on, white. If you were to gradually reduce the voltage, then as you would expect, the brightness would reduce. So how bright would it be at 6V? And, would it still be white? There are some clues further back in this thread.
Without a variable power supply, cut one section (three LEDs) off of one of the strips and connect it to one of the 5V power supplies that you probably have lying around.
Another thing you could do is simply switch your meter to measure AC volts instead of DC, and repeat your measurement. While you are about it, try measuring the output voltage of the power supply by itself with the meter set to AC and DC, and see if you can understand what you find (clues this time in the thread I pointed you to before).
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