Power Supply Regulation

Question

This is a simple question and I just want confirmation before I buy the wrong power supply.

I am trying to test some LEDs which need about 2-20mA of current at 3.3v. I am gonna be testing a bunch so I need a variable power supply. The problem is that most power supplies have a current regulation of about 0.3% + 5mA. Looking at it I'm guessing that would cause some fluctuations but then I realized it also has a voltage regulation of about 0.3% + 5mV. If I use the voltage regulation at 3.3v and use a fixed resistor that should mean technically my current should only change at a value of VariableVoltage / FixedResistor and thereby essentially giving me a near constant current supply of around 2-20mA depending on resistors used right?

Answer 1

You are testing a 'bunch' of LEDs, so what are you testing:

1. Go-NoGo
2. LED voltage at a constant current
3. LED current at a constant voltage
4. Optical output at 2,3 above

As Marcus pointed out you can regulate (make constant) either Voltage or Current but not both.

I'd suggest you pick the parameters you want, and build either a well regulated voltage source or a well regulated current source.

Since you have a reasonably well regulated power supply (one assumes it's a lab supply, you could simply create a small fixture and use an external INA226 board to capture the LED current and voltage, that would probably be accurate enough for rough batching. Here you would alter the voltage to achieve whatever current level you require and calculate the LED Vf

Answer 2

Just use a common inexpensive constant voltage (fixed) 5V supply with a 1/8 W (or greater) resistor for each of the various currents desired.

It is important to use the actual measured V_f for each type of LED when calculating the resistor value.

Calculator Source LED Series Resistor Calculator

For 2 mA a 850Ω 1/8 W:

For 20 mA 85Ω 1/8 W:

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I made a PCB to do something similar.

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I used a Vishay Dale SR3R0100FE66 0.01Ω 1% shunt resistor to measure the current with a volt meter.

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Not shown in the schematic I have 8 LED outputs.
Originally I used a 2 row header rather than a switch.
The 0.01Ω shunt resistors go in the center row of the PCB (right of dime).
Just got the boards yesterday.

Answer 3

You can make this work with an adjustable voltage supply and a fixed resistor of 1 K 1 W 1% or better, in series with the LED. Now you know already that you need to source a voltage above 3.3 V for the LED to even come on dim.

The resistor limits the current to 1 mA per volt above 3.3 volts. So 4.3 volts will put 1.0 mA into the LED. 13.3 volts is 10.0 mA, 23.3 volts is 20 mA = .466 W, which is why I suggested you use a 1 watt resistor. If you have a DVM you can measure the Vdrop across the resistor, which gives you the current in mA.