Summary of (GETTING RID OF) THE GHOST IN THE LED
This article details how high-efficiency LEDs' internal capacitance caused "ghost lighting" in a multiplexed Arduino project. When one LED bank was active, current leaked through the off-bank's LEDs, dimly lighting them. The author solved this by leveraging the diode properties of LEDs to prevent unintended current flow between banks.
Parts used in Multiplexed LED Project:
- Arduino
- High efficiency LEDs
- Transistors
- Diodes
Multiplexing is a very old technology in which control signals are intermixed for the sake of being able to control more devices than there are control signals. For [mihai.cuciuc], the problems started when he multiplexed some very efficient LEDs.
The problem? In two banks of six LEDs each, both LEDs connected to a single Arduino pin would light, even when only one bank was turned on at the ground side. The LED In the bank that was switched on lit brightly, and its corresponding LED in the bank that was off would also be very dimly lit. [mihai] was able to determine that the problem was not due to a leaky transistor, but rather due to a quality of the LEDs themselves.
What is an LED but a diode, and it’s well known that diodes also have capacitance. In fact, this quality is exploited in varactor diodes, a specialty diode whose capacitance can be changed by varying the voltage on the cathode. [mihai] deduced that this capacitance was causing current to flow in the bank that was off. Where was the current going? From the Arduino pin that was on, through its attached LED, and then into the rest of the bank of LEDs, charging them like capacitors. [mihai] hasn’t seen this before, but theorizes that for the latest batch of high efficiency LEDs, this minute current is enough to light the LED through which the current is flowing.
[mihai]’s solution is an elegant hack which he’s made available for your perusal. You might also enjoy this introduction to diode basics by W2AEW. If you have any great diode or LED hacks of your own, be sure to drop us a line!
Source: (GETTING RID OF) THE GHOST IN THE LED
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Why did an unpowered LED bank light up dimly?
The internal capacitance of high-efficiency LEDs allowed current to flow from the powered bank into the off bank, charging the capacitors enough to create a faint glow. -
Is the ghost lighting problem caused by a leaky transistor?
No, the author determined the issue was not due to a leaky transistor but rather a quality inherent to the LEDs themselves. -
How does the capacitance in LEDs affect multiplexing?
Capacitance allows current to pass through the off bank when the other is on, effectively charging the LEDs and causing them to emit a dim light. -
What type of diode exploits variable capacitance for voltage control?
Varactor diodes are specialty diodes whose capacitance can be changed by varying the voltage on the cathode. -
Why do modern high-efficiency LEDs show this leakage effect more than others?
The minute current flowing through the capacitance is sufficient to light these specific high-efficiency LEDs, whereas it might not affect older or less efficient models. -
What is the path of the stray current in this scenario?
Current flows from the active Arduino pin, through its attached LED, and then into the rest of the LED bank that is supposed to be off.
