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    Driving multiple high power IR LED with sine wave

    Greetings,

    I'm building an obscuration meter which uses multiple high power IR LED to illuminate sample, then a photodiode is used to measure transmittance. Since external IR-sources and noise are distorting results significantly, I've utilized phase sensitive detection to minimize errors. However due to this I've got to drive IR LEDs in sine(actually half sine).

    Now the problem is that I can't think of any ways to drive these IR LEDs without distorting sine waveform. Also I don't have any idea how to set a maximum peak voltage level for the signal.

    IR LEDs are consuming 400mA at 1.8V Vf, and I'm using 5 of them to achieve sufficient radiating power. Sine wave comes from a function generator, actually audio output from my PC and frequency is 1kHz. I'm using a diode to remove negative sine, so resulting is 0V to 1V half sine. I thought I use high current amplifier, but amps that I found usually had stable gain at over 10. Also I thought using audio amplifier, but their output current are rather small and output voltage rather high. MOSFETs and BJTs I've played with usually distorts the signal.

    Another problem with audio output as function generator is that I've experienced some fluctuations with input signal in terms of Vpk-pk. Amplitude fluctuations weights quite a lot in the final results. Maybe some automatic gain control or voltage controlled resistor? Zener diodes would just clip the signal if it exceeds threshold.


    Could anybody give me some ideas that I could investigate?


    Thanks

    •   Alt18th November 2011, 10:54

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    Re: Driving multiple high power IR LED with sine wave

    Hi,

    If you're distorting the signal using transistors then you're using them incorrectly. The way I'd do it is to use current mirrors, use a good quality op amp to generate a sinusoidal voltage, convert that into a current, then either use transistors with differing current gains to generate the level of current you need to drive the LEDs or boost the current provided by your op-amp by attaching a BJT/FET at it's o/p.
    You say your signal is 0-1V yet your led has a 1,8V forward voltage. It isn't a good idea to work down near 0V when driving led's and detecting optical signals, there are numerous reasons for this.
    The audio output from a PC is always going to be noisy, but there are maybe a couple of things you can do to minimise such stuff. If you can buffer the output so that the impedance seen by the output is always constant (this is probably done on the card, but it shouldn't hurt if you use high quality components), and of course buy a better quality sound card.



    •   Alt18th November 2011, 16:47

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    Re: Driving multiple high power IR LED with sine wave

    You're encountering problems because it is not trivial to deliver a precise waveform of just a volt or two amplitude, at 1/2 amp. Furthermore the sudden load of diode junctions all turning on at a certain volt level...

    The device that could provide this must have a minimum 'on' resistance in terms of fractions of an ohm. Its controlling waveform must be adjusted to a fraction of a volt.

    There's a narrow window of driving voltage between lighting an led and frying it. So another hard part is to drive parallel led's equally.

    It would be much easier if you could string the led's together in series, and run them at several volts with lower current. Is this feasible?

    It would make it easier to produce a purer sine waveform. It would make it easier to adjust to the proper volt level.



    •   Alt18th November 2011, 17:20

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    Re: Driving multiple high power IR LED with sine wave

    I think it is more appropriate to talk about driving LEDs in terms of current rather than voltage. It is generally thermal management problems that causes LED's to fry, the voltage across them is going to be their forward voltage regardless of what you make the driving voltage. I shouldn't have thought driving a sinusoid at 1/2 an amp would cause much problems unless it's a very high frequency and slew becomes an issue, assuming he's not switching everything off and on (as it stands, the way he describes the signal the diodes are never conducting).
    There is a trade off to consider between parallel and serial driving of LEDs, if you drive LEDs in series it is the same current flowing through every LED so they have the same drive, however creating a modulated current source becomes more difficult as the power it has to supply/dissipate has multiplied by the number of diodes. I find current mirrors are a good way to keep the drive current through diodes pretty darned equal, which makes the driving circuit much easier to design. As Brad mentions, going from no current draw to 1/2 an amp is going to make matters more difficult, but as I suggested it's better all round to keep the diodes on, this improves things at you sensor and the driving side of your system.



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    Re: Driving multiple high power IR LED with sine wave

    Hi,

    Thank you both for reply.

    Actually driving them in series is a great idea! Why I didn't think of this before? So when using this method, I only have to use 400mA(+headroom) op amp.


    Sine Signal ---> High-pass ---> 0.1Vf diode ---> 400mA op-amp (non-inverting input) ---> Using 9Vpk to drive five 1.8Vf IR Leds in series

    Op-amp will have gain of 10, so output voltage swing will be 9V, which is equal to 5*1.8V. Then I'd use simple peak voltage buffer and voltage feedback to automatically adjust gain so that output peak will stay at 9V. What do you guys think?

    My IRs forward current is a non-linear function of forward voltage (1V will be threshold), however I'd expect some tolerances when driving them. In this case wouldn't it be harder to drive them equally compared to having a current source with fixed max 1.8 Vf? Will they be equal in series?


    Thanks



    •   Alt18th November 2011, 17:52

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    Re: Driving multiple high power IR LED with sine wave

    5*1.8 = 9V. 1.8 V is your forward voltage right?
    You'll struggle to find an op-amp which will supply that much current, you'll need to use a transistor at the output to supply the current, or employ a current mirror with transistors with differing current gains to multiply the driving current to a sufficient level.
    You'll also need to configure it to supply a voltage which is in excess of the combined forward voltages (current source if driving with the op-amp output or a 9V or higher dc supply if using a current mirror to drive them). Both fairly straightforward circuits.

    I'm unsure of what you mean here,
    "Then I'd use simple peak voltage buffer and voltage feedback to automatically adjust gain so that output peak will stay at 9V. What do you guys think?"

    Are you intending to have the diodes turn off every half cycle? Is this necessary? If it is it's probably worth switching the current to a dummy load so that your supply isn't having to jump from almost no current to 400mA every cycle, (a comparator, a logic gate or two, a power resistor and a load switch would do). I'd try and keep the diodes on if the purity of your sinusoid is important. How are you detecting the signal, different methods of detecting modulated signals are more forgiving than others- if you're using a lock-in amplifier I wouldn't stress too much about a bit of distortion.

    I'm a touch unsure of what you're asking here too.
    "My IRs forward current is a non-linear function of forward voltage (1V will be threshold), however I'd expect some tolerances when driving them. In this case wouldn't it be harder to drive them equally compared to having a current source with fixed max 1.8 Vf? Will they be equal in series?"
    I wouldn't expect the forward voltage to vary that much, this should be covered in the data sheet though, so the only variation in brightness will be process/manufacturing tolerances, if you drive them in parallel you'll still have this mismatch plus any mismatch in the parallel legs (however you drive them). I usually use 2 or 3 in series with a number of parallel legs in the drivers I build (the LEDs I use have forward voltages of 4V typically so more than a few in series and voltages start to get higher than is sensible for my application.



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