I want to use an Arduino to control some 1A-rated LEDs. This is for a photographic device. The sensor does not have a global shutter.
This means that I cannot control the exposure time of the sensor (takes too long to reprogram), and I cannot use PWM to control the intensity of the LED.
Since the intensity of the LED can be varied by current (100mA up to 1000mA), I would like to find a circuit that would allow me to use an Arduino to set a given current.
You need a controllable current source. A simple solution could be an Arduino with a DAC and a BJT power transistor that converts the DAC output voltage into a current.
Well you will still need to use PWM output, but filter it and then use a power Transistor like a TIP120, to actually restrict the current.
Value of C1 will influence the response time <-> ripple, and C2 as well, which should probably be bigger if you intend to drive something that can have current up to 1A
For still photography PWM should be OK. Typically, you should avoid a linear dimmer-driver because they are inefficient and for 1A you need a high-power transistor/MOSFET with a big heatsink.
Most switchmode constant current power supply will put-out "smoothed" DC just like a regular constant-voltage power supply. But it's not an easy thing to build yourself.
Most constant current LED power/supplies/drivers (or dimmable "controlled current") put-out DC, although they can be controlled by PWM or variable DC. They are usually high-enough voltage that you can run several high-power LEDs in series from one power supply.
These are normally switchmode power supplies and they are not easy to built yourself.
The industry standard is 0-10V DC or 10V PWM for dimming control so you (may) need a 10V voltage-source and a little (low-current) transistor-driver circuit. But sometimes 10V is built-in so it can be dimmed with a pot and in that case you just need the transistor driver to "boost" the Arduino's output.
Would a suitable appraoch be to set the current level with a PWM running continuously and switch the LED with seperate digital on/off output?
If so there are a number of simple circuits that will allow this. Come back here for further advice on that front.
The Meanwell LDD-1000L does AFAIK voltage dimming (and also PWM).
Dim range is 0.5volt to 2.5volt for 20-100% output current.
A varying voltage can be made from an Arduino PWM pin with a resistor and a capacitor.
Or from a voltage source and a pot (no Arduino needed).
Leo..
What LEDs? How many? In series or parallel? Do you already have a power source? Which one?
You're making me curious; can you share a little bit more?
FYI; I've done some work involving LEDs and photography. Specifically I've made a color enlarger light source (well, a few versions in fact) with all the necessary controls etc. So I'm fairly comfortable around the subject matter of modulating LED output for specific purposes.
Yeah, can be done; you could use a series pass device (typically a mosfet), a shunt resistor for measuring current and one or two opamp stages to control the current through the LED. The Arduino could, by means of a DAC or perhaps even RC-filtered PWM output (yielding a variable level DC voltage), set the comparator level on the opamp circuit. It's not insanely complicated in a simple implementation, but still a lot more complex than just PWM-ing the LED(s) using a suitable LED driver IC. So I would always seriously consider the possibilities for PWM before trying to do it the hard way.
@DrDiettrich & @Deva_Rishi Yeah, something like that...however, linearity and usability for higher current levels (i.e. efficiency), combined with responsivity: I have some concerns about your simple solutions in this regard. Depends on the application, which we know virtually nothing about...
Not sure why you're mention linear regulation for 3watt (1Amp) LED strings.
Low voltage CC LED drivers are just current limited DC/DC converters, working on ~500kHz.
Driving the LED with (almost) pure DC.
That current can be controlled/set with a voltage, or the driver can be switched on/off with PWM.
The Picobuck (1-2watt) is another example. Read the text about voltage control.
Study the PT4115 datasheet to understand how they work.
Leo..
Here's the standard programmable current sink, using an opamp for linear response to input voltage, and with LPF for PWM on the input.
A 1 ohm sense resistor means 1V in gives 1A out, but the divider on the input means that's more like 5V in for 1A out.
The LED supply voltage is probably going to be needed to be more like 6V if this is a white LED due to the various voltage drops with the Darlington and shunt resistor.
(The 100pF and 100 ohm resistor ensure stability of the opamp if you were wondering why this complex, it might work without them, you'd have to experiment.)
Depending on the driver used and within certain limitations. I.e. not all drivers support linear regulation and if they do, it's usually only for currents over 10% or 20% of the maximum set current...but you're right, that's the most straightforward solution especially since the lower limit of 10% seems to be just fine in this case.
With about 2V on the Darlington that leaves only 2V for the LED from a 5V supply. Either higher voltage or lower resistor value or higher divider ration is recommendable.
Yes I reckon 6V to 7V might be needed in practice. You could use a smaller shunt resistor too, but 1 ohm is nice an simple to calculate with! 0.33 ohm for example would also have the advantage of less power consumption (the 1 ohm would clearly need to be at least 1W rated)
I think this is overcomplicating the matter. The OP needs just a way to control current through a circuit, but can not use PWM because the LED's used do not have a lot of after-glow, and the flickering will have an undesired effect with the exposure. It doesn't need to have a fast response time, and i guess it doesn't even need to be linear. (though i think my circuit actually more or less is) Even a small ripple is probably fine, as long as it is less than a 5% deviation (which really is a lot, i manage less than 1%)
The Op-Amp is of course a more precise system, but overkill for this application in my opinion.
Response time can be really slow, it is meant for a controlled, but constant amount of current.
The question i still have is why there is a need to digitalize the whole thing. Just a potmeter controlling a Power transistor will probably be just as effective.
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