Electrical – Transistor driving LED

This should work: -

Rset and Vset determine the current that flows through the load resistor: -

\$I_{LOAD} = \dfrac{V_{SET}}{R_{SET}}\$

However, the op-amp needs to be able to work down to 1.8 volts or lower and will need power connections (not shown above) to V and ground. It also must have rail-to-rail inputs and also preferably outputs.

If Vset is made to be 0.15 volts and Iset is required to be 15mA then Rset is 10 ohms.

LEDs are 23.5 Vf and 2.1 A typical current.

Typical isn't good enough in this case. A look at the I/V curve shows that \$ V_f \$ varies at 2.1 A from about 20.8 V to > 26 V.

Figure 1. \$ V_f \$ for the 5300 series lamps.

There are four curves given from MAX Vf through "typical" to MIN Vf all at 25°C. Then there is the red line at 70°C.

The green load-line represents your 1 Ω resistor. It will drop 1 V/A so at 4 A the voltage will have dropped to 20 V. The voltage and current for each LED curve can be estimated by the intersection of the curve with the load-line.

1. With worst-case maximum Vf you can expect 0.4 A.
2. The typical case would draw 1.3 A.
3. The minimum Vf at 25°C would draw 2.1 A.
4. As the LEDs heat up to 70°C the current increases to 2.6 A. This, of course, will make the LEDs hotter, the Vf will drop and the current increase until something gives up - the PSU, the resistor or the LEDs.

The problem is that you haven't enough headroom for a simple resistive current limiter. If you had several more volts available the load-line wouldn't be so steep and the current wouldn't vary so much.

Note also that you are assuming your batteries will give constant voltage. They won't!

You need dimmable constant current power supplies.

Figure 2. A typical mains-powered constant current power supply.

Figure 3. The dimmable constant-current power-supply control circuit. Source: Dimmable mains PSU control | LEDnique.com.

Many of the LED power supplies such as those by Mean Well, etc., offer three modes of control: output constant current level can be adjusted through the control input

• by connecting a variable or fixed resistance to the control terminals,
• using a 0 ~ 10 V DC control signal,
• using a 10 V PWM signal between.

In your case you would purchase a PSU model that would supply the required peak current and run it normally in dimmed mode (reduced current).

I haven't checked all the details for you so you have some engineering to do. In particular, make sure that the current can be controlled over the range of Vf voltages you are interested in.

I've written more on the subject in the article lined above. Load-line resistance graphic tool may also be of interest in explaining in more detail how to use the load-line - although the article deals with lower voltage circuits.

^{simulate this circuit – Schematic created using CircuitLab}

Figure 4. Op-amp control.

Try something like Figure 4. The 2.1 A input can be connected to 3.3 V permanently. The 4.0 A input can be connected to a GPIO. Select the resistors to give you about 300 mV at the non-inverting input per amp of LED current.