Constant Current Drivers

My intent is to use my arduino to control some LEDs via PWM. I have the PWM signal part down but Im having trouble understanding how to make the driver circuit. I want to use 12v from a car and the LEDs foward voltage is 3.7v @ 700mA. If I use the circuit below I wouldnt be able to put more than 3 LEDs in series before exceeding my input voltage. (3.7v x 3=11.1 voltage drop) So if I wanted to use 9 LEDs for example could I make 3 driver circuits with 3 LEDs each and power them all from the same 12v source? Does this sound right or am I completely off base?

Sounds reasonable.
Car voltage running ~14.7V

Yes, that is correct; to drive 9 LEDs you would need to replicate the entire circuit to provide three channels.

Note that you must use a "logic level" FET as you only have 4.3 v to switch it on. In this circuit, it is operating in linear mode and the only advantage of using a FET is its extremely high gain allowing a very low drive current. In this case that is only relevant when you add the two extra driver circuits as a bipolar transistor would be quite adequate to control 700 mA with the 20 mA available from the Arduino (R1 being 220 Ohm, R3 1 Ohm).

Paul__B:
Note that you must use a "logic level" FET as you only have 4.3 v to switch it on. In this circuit, it is operating in linear mode and the only advantage of using a FET is its extremely high gain allowing a very low drive current. In this case that is only relevant when you add the two extra driver circuits as a bipolar transistor would be quite adequate to control 700 mA with the 20 mA available from the Arduino (R1 being 220 Ohm, R3 1 Ohm).

Sorry, I dont fully understand. Are you clarifying the type of FET I need to use or are you suggesting I make a change to the circuit?

The FET doesn’t offer any efficiency advantage over a bipolar transistor, but if you do go with a FET then you have to make sure it will turn on with a VGS of just 4.3V.

Sorry to bring this up again but if I use an RGB LED I would have to triple the amount of drivers, which seems excessive. Is there a different circuit that can handle more LEDs in a row? Ideally I’d prefer to just have three drivers, one for each color.

PRXVII:
Sorry to bring this up again but if I use an RGB LED I would have to triple the amount of drivers, which seems excessive. Is there a different circuit that can handle more LEDs in a row? Ideally I'd prefer to just have three drivers, one for each color.

A suitably specified driver can handle as many LEDs in series as you provide voltage for, but clearly you need a driver for each series chain, so three drivers for RGB to start with.

There are different RGB assemblies. Those with six pins rather than four permit colours to be chained in series.

Paul__B:
A suitably specified driver can handle as many LEDs in series as you provide voltage for, but clearly you need a driver for each series chain, so three drivers for RGB to start with.

There are different RGB assemblies. Those with six pins rather than four permit colours to be chained in series.

Yes. The LEDs I'd like to use have 6 pins, a positive and negative for each color. I'd like to connect each color in series (or parallel) and only have three drivers controlled by PWM. One for each color. Is this doable with the 12-14v?

As explained before, you can put up to three LEDs per chain to operate at 12V. How the LEDs are physically assembled is immaterial as long as you can connect them in series.

You might use an additional transistor to drive the FET and then use a
resistor from the 12V to provide the gate voltage.
That would make selection of the FET easier.
You can drive multiple controls for the same color from a single uP output.
To bad you don't have a higher voltage to work with. A real simple current control
is the LM317 voltage regulator. wired as a constant current. You need about 3.5V
of extra head room.
Dwight

PR17,

I fixed that circuit for a guy last year.