I was originally trying to use a RIGOL DG952 function generator (50 Ω out) to modulate a 10 mW UV LED requiring a 7.2 V forward voltage and 100 mA... which I now understand my the DG952 can drive the LED directly!
Now I want to upgrade my UV LED to a 18 mW model requiring 7 V forward and 180 mA current... the RIGOL won't suffice for this one as 7/50 = 140 mA.
Rigol DG952, accordong to the specs,
https://www.tequipment.net/Rigol/DG952/Function-Generator/#description , can do
Max Amplitude Vpp @ 50 ohms 10 V
which means, internally it has a 20Vpp in series with a 50 ohms resistor, or else said, +/-10Vdc in series with 50ohms. Since the LED needs DC, the max you can set in the generator is +10V, but there is already an internal 50 ohms in series, inside the generator.
Your 10mW LED requires about 7V, so if you connect it directly to the generator, the max current through the UV LED can be no more than (10V-7V)/50\$\Omega\$ = 20mA. So, the generator alone can not drive the 10mW LED at its max power.
Another trick you can do, is to connect the LED between the CH1 and CH2, and synchronize the 2 channels to output pulses of opposed polarity: CH1 between 0/+10V, CH2 pulses between 0/-10V. Yhe LED is connected between the center of the BNC connector at each channel, with anode at the middle pin of the BNC out at CH1, and the cathode at the middle pin of the BNC out from the second channel. This will give you max 40mA, so still not enough to drive the 10mW to its max power.
If you want to drive the LED to the max, you will need a transistor and a power source, even for the 10mW LED, let alone for the 18mW one.
- Beware at the LED polarity, they got damaged easily if you connect them in reverse.
- Beware of the LED with 3 pins (with optical power photodiode included), they got damages if the optical feedback diode is neglected. For those 3 pins "LEDs" you'll need a dedicated control loop if you want the max power without damaging the LED.
- Most of all, beware of the LED power, it can do permanent damage to the eyes, yours or others.
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