Im having a peculiar problem with my circuit, your help is very appreciated :)
I am using an FQP30N06L N-Channel MOSFET to pulse an array of LEDs at 38 kHz. A micro-controller (Arduino Uno R3) is used to send 38 kHz pulses at a 27% duty-cycle to the gate of the MOSFET (I am using a 100 Ohm resistor from the micro-controller to the gate of the MOSFET and a 470 k-Ohm resistor from the gate of the MOSFET to GND). With each pulse the MOSFET connects the circuit from the LEDs to ground thus allowing the current to make a complete proper path.
To power the LEDs I am using an LM317 voltage regulator set-up as a constant current supply with an out-put of 100 mA per string of 7 LEDs (300 mA total for 3 strings of 7 LEDs/string) Later on I plan to upgrade my circuit to have a total of 3 strings of 7 LEDs per string (a total of 21 LEDs). At a 27% duty-cycle the current through one string of 7 LEDs should be approximately 27 mA.
Here is my schematic....
The current flowing through the LEDs is not steady. It starts fine then exponentially drops and levels-off. For example I connected my multimeter between one string of 7 LEDs and the MOSFET. The multimeter starts reading around 27 mA and exponentially decreases to 17 mA where it seems to stable off.
Sometimes when I place my hand the circuit (without touching the circuit) the current significantly changes from a few milli-Amps to 10, 20, 30 mA change. When I remove my hand it returns to normal.
I think the problem lies with the MOSFET. I checked the output of the LM317 and that remains stable at 100 mA. Do you happen to know why the circuit is performing this way and how to remedy it?
This is how I connected the multimeter to verify the LM317's output....
The small green module is the power supply. The TO-220 next to the Arduino with the two terminals and the diode is not used in the circuit. It is disconnected.
The TO-220 on the upper left of the board is the N-Channel MOSFET FQP30N06L used to pulse the LEDs at 38 kHz. The TO-220 on the upper right of the board is the LM317 voltage regulator.
The white wire connected from the Arduino is connected to the gate of the MOSFET. It is pulsed at 38 kHz.
You may also notice the CAT5e cable (ethernet cable) on the upper right. I will be using it to connect a second string of 7 LEDs. As of right now the wires of the ethernet cable are only soldered on but not connected to any circuit.
The larger square of aluminum foil covers the infrared LEDs. The foil is completely covered with clear tape. I use it to prevent the infrared receiver from receiving unwanted signals from with-in the the circuit housing.
The smaller square of aluminum foil covers the rear of the infrared receiver and is also completely covered with clear tape.
The three wires (red, black, blue) are for the infrared receiver. They are completely disconnected from the circuit during testing.
On the left is the infrared receiver. On the right is the string of 7 infrared LEDs
Voltage Regulator datasheet: http://www.mouser.com/ds/2/149/LM317-64104.pdf
MOSFET datasheet: http://www.mouser.com/ds/2/149/FQP30N06L-244344.pdf
BJT Transistor datasheet: http://www.mouser.com/ds/2/149/2N3904-82270.pdf