I have an LED light bulb ratted 13watt that stopped working, so I decided to disassemble it to know what could be the problem. I found out that most of the LEDs have blown out and since the total of the LEDs are 15pcs connected in series I decided to couple another one from the spare one that I have. When I solder it back it turns on and is bright. Here is where the confusion starts. When I checked the output voltage of the LED driver with a multimeter it was reading 300v but when I connected a single LED to the output the voltage was reading 2.677v but the light is not really bright. I connected a white COB light and it was showing 23.17v on the multimeter. In short, the output voltage of the LED driver automatically adjusts itself to the voltage of whatever I input. Honestly, this is the first time I am trying this out but I am confused as to why the voltage is switching. Any help will be appreciated. Here are the attached pictures
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5\$\begingroup\$ An LED driver, a constant current regulator, is a device that adjusts voltage to keep the current constant. What you are seeing is this regulation. \$\endgroup\$– user1850479Commented Dec 4 at 16:44
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\$\begingroup\$ Does that mean the output does not have a specific voltage ? \$\endgroup\$– UpInCommented Dec 4 at 17:10
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4\$\begingroup\$ the output has a specific current \$\endgroup\$– jsotolaCommented Dec 4 at 17:34
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\$\begingroup\$ Note that you have a linear regulator, meaning it operates as a variable resistor. Since mains voltage will rectify to about 311V, connecting a 3V LED means 308V is dropped across the regulator, and for every 1 mW the LED receives the regulator will have to dissipate 102.6mW of heat. You probably want to avoid connecting loads under 150-200V to it for more than a few seconds. \$\endgroup\$– user1850479Commented Dec 4 at 20:15
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\$\begingroup\$ What looks like one LED might be several LEDs in one package. \$\endgroup\$– Andrew MortonCommented Dec 4 at 21:28
1 Answer
LEDs have a very nonlinear current-voltage characteristic. Let me dust off my whiteboard.
The I-V curve for a LED, like most diodes, has an exponential dependence on voltage.
If you connect a voltage source across the LED, you get something like this. The voltage source tries to maintain a constant voltage across its terminals regardless of how much current it provides and the intersection of the vertical voltage source line and the exponential LED line is the operating point.
This looks good for now, but LEDs - like all devices that aren't made of superconductors - heat up as current goes through them. And LEDs have a negative temperature coefficient; their forward voltage (the internal voltage barrier that must be overcome to get current to flow through them) drops by about 2mV for every degree Celsius rise. On the graph, that looks something like this:
The voltage source is still trying to maintain the same voltage across the LED, so it allows more current through, which causes the LED to heat up more, which allows more current through, until the LED is destroyed. This is called thermal runaway. We sometimes prevent this by inserting a current limiting resistor in series with the LED:
So now, instead of moving along the green voltage source curve, the operating point moves along the orange I-V line of the resistor. The LED still heats up, but far less than before; losses to ambient are able to remove the extra heat produced, and the operating point settles at a reasonable value.
For a couple small LEDs operating at a low voltage, this is probably just fine, but if you are making a system for lighting a room where you might have dozens or hundreds of individual LEDs to drive and a supply voltage of potentially hundreds of volts, it makes sense to put them in series since you can apply a high voltage across the series combination and run a relatively low current through the string. In addition, LEDs don't play well together when they're in parallel since the thermal runaway effect means that the hottest LED takes more and more current for itself until it burns out, and the next one, and the next one, etc.
If you are driving a lot of LEDs in series, a resistor isn't a great way to control the current since the forward voltage changes of the individual LEDs will add together which means that the combined forward voltage of the the entire string could change by a lot. With a simple resistor, this means a big change in brightness, but you also need enough supply voltage overhead to ensure that the whole string will be adequately bright if it's operating at -20°C or at 50°C. A large voltage overhead means possibly a large voltage dropped across the resistor, which is very inefficient and engineers love efficiency, so the solution is to use a constant current driver:
A constant current driver adjusts its output voltage to maintain the same current into/out of its terminals. Now, it doesn't matter what the LED curve does or even (within reason) how many LEDs are in the string; they will all have the same current through them.
Constant current drivers have a "compliance range", which is the range of voltages they are able to apply to the load to get the desired current to flow. You have measured this directly. 300V appears to be the maximum and the minimum is somewhere below 2.6V.
