sixeyes:
magnethead794:
So if it's dropping 2.9 volts, I is 19.3 mA ,
Can you verify that this is the voltage being dropped, using a multimeter?
Passing 2.1 volts to the LED resistor, which is then dropping current to 14mA. So how many volts are being dropped by the second resistor/how many volts are the LED's seeing?
The LEDs should drop about 2 volts but that may not be the case at such small currents. I don't know what LED you're using and even if I did the manufacturer may not test it for such low currents.
Can you measure the voltage across one of the LEDs when it's in the dim condition and the voltage across "its" resistor?
If you're actually getting 19.3mA through the single "dimming" resistor, this will be split between all 39 LEDs and their resistors, so it should be about 0.5mA each. This would give a volt drop across the 39 LED resistors of about 75mV. Again any actually voltages you can measure would help.
Iain
Vs = 5.050V
Vdimmer = 3.219V
Vprimary = 160.7mV
VLED = 1.667V
21.35 mA flowing between Vs and the dimmer resistor
Pdimmer = 0.02135^2 * 150 = 0.068 watt
Vs = 4.96V
Vprimary = 2.927V
VLED = 1.945V
341.6 mA flowing between Vs and primary resistors.
measurements taken with 20 LED's turned on. Each LED is getting 1.667V @ 1mA dimmed and 1.945V @ 17.08mA at full brightness.
Vs = 3.310V (NOTE: Will be 3.8 volts in application)
Vdimmer = 1.391V (NOTE: 150 ohms, same as 5V circuit)
Vprimary = 207.6mV
VLED = 1.709V
9.22 mA flowing between Vs and the dimmer resistor
Pdimmer = 0.00922^2 * 150 = 0.0127 watt
Vs = 3.310V
Vprimary = 1.403V
VLED = 1.889V
59.34 mA flowing between Vs and primary resistors.
measurements taken with 4 LED's turned on. Each LED is getting 1.709V @ 2.305mA dimmed and 1.889V @ 14.835mA at full brightness.
Strangely, both sets of LED's were about the same brightness at both full brightness and reduced brightness. I found that somewhat peculiar since I used a 150 ohm resistor on the lower-voltage set of LED's.