Posts tagged Leds
How many white LEDs in series on 120v AC?
Question by Kevin D: How many white LEDs in series on 120v AC? OK folks, I don’t think my actual question is quite as straight forward as it seems here, but maybe someone here can help anyway. I bought a GU10 48 LED bulb (think track lighting) made to run directly off ~120v AC. After a few months of use (around 100 total usage hours), it failed due to an LED burning out. So, being the inquisitive person that I am, I had to open it up to see how it ticked.
Basically, the simple circuit driving the LEDs contains a couple capacitors, a few resistors, and a full-wave bridge rectifier:
x | C1, C2, R2 (all in parallel) | —-| D — D |ssss| (-)o ———- R3 –o (+) |ssss| D — D —-| | R1 | x
The 120v line voltage is applied at the “x” and the circuit output is the “o” (the ssss is a space….it’s difficult to draw a circuit here). For whatever it’s worth,
R1: Resistor bands are too difficult to read (and I don’t have a multimeter handy), but I think it was around 500 ohm R2: Surface-mounted resistor; 150 stamped on it (assuming 150 ohm) R3: Surface-mounted resistor; 150 stamped on it (assuming 150 ohm) C1: 250v 474J capacitor (unsure of capacitance) C2: 250v 684J capacitor (unsure of capacitance) D: M7 diodes (rectifier bridge)
When I measured the RMS voltage at the circuit output, it was 110v (vs. 120v input). The 48 LEDs are wired in two series of 24 LEDs each (only one string was non-functional). Doing the math tells me that the LEDs are being driven at just over 4.5 volts, which seems exceedingly high (assuming a forward voltage of ~3.5v for each one normally). So, this brings me to my question….I wondered how many more LEDs I could string in a single series off this circuit configuration without going past 3.5v. Again, doing the math would lead me to believe that I could add maybe 10 more LEDs to the series and be OK. But, in experimenting with it, I was able to add a bunch more than that without any discernible decrease in light output. I think I stopped trying at around 25 extra LEDs in series with the original 24. What gives? I realize LEDs are current driven devices, but they would still need a certain forward voltage to operate, right?
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How many white LEDs in series on 120v AC?
Question by Kevin D: How many white LEDs in series on 120v AC? OK folks, I don’t think my actual question is quite as straight forward as it seems here, but maybe someone here can help anyway. I bought a GU10 48 LED bulb (think track lighting) made to run directly off ~120v AC. After a few months of use (around 100 total usage hours), it failed due to an LED burning out. So, being the inquisitive person that I am, I had to open it up to see how it ticked.
Basically, the simple circuit driving the LEDs contains a couple capacitors, a few resistors, and a full-wave bridge rectifier:
x | C1, C2, R2 (all in parallel) | —-| D — D |ssss| (-)o ———- R3 –o (+) |ssss| D — D —-| | R1 | x
The 120v line voltage is applied at the “x” and the circuit output is the “o” (the ssss is a space….it’s difficult to draw a circuit here). For whatever it’s worth,
R1: Resistor bands are too difficult to read (and I don’t have a multimeter handy), but I think it was around 500 ohm R2: Surface-mounted resistor; 150 stamped on it (assuming 150 ohm) R3: Surface-mounted resistor; 150 stamped on it (assuming 150 ohm) C1: 250v 474J capacitor (unsure of capacitance) C2: 250v 684J capacitor (unsure of capacitance) D: M7 diodes (rectifier bridge)
When I measured the RMS voltage at the circuit output, it was 110v (vs. 120v input). The 48 LEDs are wired in two series of 24 LEDs each (only one string was non-functional). Doing the math tells me that the LEDs are being driven at just over 4.5 volts, which seems exceedingly high (assuming a forward voltage of ~3.5v for each one normally). So, this brings me to my question….I wondered how many more LEDs I could string in a single series off this circuit configuration without going past 3.5v. Again, doing the math would lead me to believe that I could add maybe 10 more LEDs to the series and be OK. But, in experimenting with it, I was able to add a bunch more than that without any discernible decrease in light output. I think I stopped trying at around 25 extra LEDs in series with the original 24. What gives? I realize LEDs are current driven devices, but they would still need a certain forward voltage to operate, right?
Read More >>