Current-driving a powerful IR-illuminator array

Not at all, and this also lets you avoid unequal LED voltage drops while driving all the LEDs in parallel. You would have 40 resistor + LED branches all tied in parallel with a medium power MOSFET switching the whole thing ON/OFF, and a common voltage regulator with large output filter capacitor supplying the power.

You will be running only 500mA through the resistors at 3% duty cycle.

No- keep it simple.

Put the 40 LEDs in serie, so you only need one power supply, say

Jacques

BW wrote:

I once used the following approach to drive 25 IR leds in sync with a camera frame sync: From a 12V power supply I drove 5 strings of leds, each string having 5 leds in series. Each led string was connected anode to +12V, cathode to the collector of an NPN transistor, the emitter was connected to ground through a 10ohm resistor. By driving the base from 5V, the circuit behaves like a current source of (5-0.7)/10 = 430mA. Need more current, increase the drive voltage. All bases were connected together though a 10ohm resistor. The total current draw is 2.2 A while each led gets 430mA.

Meindert

Thankyou, I guess the powerloss through the resistors is worth the gain in simplicity in this case. The other solution I was contemplating was to assume the hfe (current gain) of a darlington NPN was fairly stable, and I could feed a resistor-derived small known current into the darlington and get a fairly known high output current. But then again, I would have to contend with the unequal voltage drops in a LED series configuration and that could be worse (I hadn't thought about that) as well as temperature/batch varying hfe.

Any recommendation on a standard MOSFET suitable for this kind of power ?

Regards,

Bjorn

There is no such thing as a standard MOSFET. You want to select a MOSFET with RDS,ON such that RDS,ON*4 Amps< 0.05*(VDC,supply-VF,LED) supplying the circuit- i.e. contributes less than 5% error in your current calculation, it must achieve this RDS,ON with a gate-source voltage drive that you have available, and it's VDS ratings must be compatible with your circuit voltages. See Fairchild, International Rectifier, Philips and whomever selector guides to pick a candidate form the selector chart, and then iterate through availability from your suppliers.

I still toying with a system to flash Led's in sync with a camera. Edmund Optics makes a flasher, but its not heavy enough for my case.I don't see much of a problem making my circuit, even though its going to peak at 65 watts. I probably will use Fet switching to make the common connection at ground.

greg

--- From:

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Transistor beta is notoriously variable and not suitable for that purpose. If you want to vary the current level, it would be better to make your DC power supply variable and adjust current that way. Put a big honking capacitor at its output so that most of the pulsed current derives from that and not the active elements of the supply. You could get away with a small 250mA capable supply that way. You want C so that

John

JF,

the schoolmarm thing is annoying. His response was perfectly intelligible.

John

I designed a high efficiency LED flasher that has all the LEDs in series, so they all shared the same current. The pulse is formed (regulated and time limited) by an LC series circuit. The capacitor is precharged to a specific voltage, prior to each pulse by a flyback switching power supply. This capacitor is discharged through the LED string by a MOSFET, that turns full on for a little more than half of the duration of the pulse. But there is also an inductor in series with this discharge. A diode carries the inductor current through the LEDs, after the MOSFET turns off, till the inductor current runs down to zero.

The values of the capacitor and inductor determine the pulse duration, and the capacitor starting voltage sets the peak current.

You don't want to drive 40 LED/R strings in parallel - it's terribly wasteful. And, as you know, LEDs are current operated, and you can't depend much of Vf being matched. Do as others have suggested - use series strings, maybe 4 strings of 10 or 5 strings of 8, then drive each string with a current driver of some kind - for this, I prefer an ordinary NPN with a calibrated voltage on the base and an emitter resistor such that when (Vb - .7)/R = Iled. I've been known to use two series silicon diodes go get about 1.4V on the base, so then there will be about .7V across the emitter resistor - just use Ohm's law to give you the proper LED current (R = .7V/If). The NPN will regulate the current and the collector gives you the voltage compliance you need.

Like I said, for regulated current, I'd use an NPN, that can handle the voltage and current you need. I don't have a specific recommendation - I think the last one of those I did I used a TIP34C or so.

Have Fun! Rich

LED strobe lights should be readily available at much lower cost than building one yourself. For example in Australia, electronics hobbyist supplier Jaycar

You might also find this project

He's not going to do 20Amps. That 500mA Ipk is for a 100us pulsewidth not exceeding 20% duty. The 50-100mA pulse is more realistic and operation can be continuous. I would not bother with RMS current computations. In the case of 40 LEDs in parallel he has 40*Iled*Vf total dissipation in the LEDs and 40*Iled*Vdc power delivered from the supply, leaving Iled*(Vdc-Vf) dissipation in each resistor, a peak multiplied by

Well, he said he was, about 20 lines up. I just wanted to make sure he didn't size the series resistors based on average current.

John

John, I'm missing something here. How does rms enter here, and how did you get an rms current of 3.5A?

Say we have a 40 volt PWM supply and a 2 ohm resistor. The current is 20 A.

When the duty cycle is 100%, the power dissipated in the resistor is 40^2 /

So for 3% duty cycle, the power is 0.03 * 800 = 2.4 W.

P = E * I, so the average I = 2.4 / 40 = 0.6 A

But the duty cycle times the average current is also 0.03 * 20 = 0.6 A.

So where does rms enter into this?

Mike Monett

Ok I'm back after some actual lab-work ;)

I studied the current-source design with an NPN with an emitter resistor (Re) to GND and a string of 5 leds between Vcc (12V) and the collector. I tried to make the dimensions so that Ve is around 500 mV (to leave space for a Vce of over 1 V and the voltage drops across the LED's which is around 2.1 V per LED at the currents I'm interested in), so for a LED current of Ic=250mA, I chose Re=2.2 ohm. Given the drop over the base-emitter port of the NPN of 0.7V, I'd have to have a Vb of

Somewhere here I realised that at a beta of say 30-40, I would have to support an Ib of around 10 mA (I actually measured this), which is quite much. The trigger source is 5V and I have difficulties of getting those 5V down to the required 1.2V. A resistor-based voltage-divider is not good enough since choosing small R forces a too heavy load on the trigger buffer, and choosing a high R makes the 15 mA create a too big voltage drop (pulling the NPN out of the "easy" feedback model). So I switched the transistor to a darlington NPN with beta > 750 giving an Ib of about 0.2 mA (measured). Still this did not create a sane configuration with the voltage divider model (the 0.2 mA creates a too big voltage drop nonetheless).

I also tried various combinations of 1N4148 strings to drop the voltage, but it does not work as I think it does (I get voltage drops of only 530mV over each 1N4148 can that be right?). I tried both simply putting the diodes in a series from the trig buffer (5V) down to the transistor base. I also tried a resistor from the trig buffer to the base, and a diode string from the base to GND, but this configuration didn't work as expected either :)

I'd be happy to have some thoughts on this seemingly simple circuit :) Perhaps adding a voltage follower (another buffer) at the input port to support the higher currents is enough ? I'll try that in the lab I guess...

Regards,

Bjorn

Simple solution would be: SMPS from your source voltage to 80V with 40 LEDs in series in line with switchable current source to the ground.

On 11 Apr 2006 10:53:04 -0700, "BW" wrote:

--- Here's the LTSPICE circuit file for a circuit that works:

Version 4 SHEET 1 1308 680 WIRE -848 416 -848 352 WIRE -848 560 -848 496 WIRE -736 352 -848 352 WIRE -736 416 -736 352 WIRE -736 560 -848 560 WIRE -736 560 -736 496 WIRE -672 352 -736 352 WIRE -544 352 -592 352 WIRE -480 -160 -480 -208 WIRE -480 -32 -480 -80 WIRE -480 304 -480 -32 WIRE -480 560 -736 560 WIRE -480 560 -480 400 WIRE -432 -32 -480 -32 WIRE -304 -32 -352 -32 WIRE -240 -208 -480 -208 WIRE -240 -80 -240 -208 WIRE -240 48 -240 16 WIRE -240 176 -240 128 WIRE -240 320 -240 176 WIRE -240 560 -480 560 WIRE -240 560 -240 384 WIRE -176 176 -240 176 WIRE -16 176 -96 176 WIRE -16 176 -16 -896 WIRE -16 320 -16 176 WIRE -16 560 -240 560 WIRE -16 560 -16 400 WIRE 96 -736 96 -928 WIRE 96 336 96 144 WIRE 144 -928 96 -928 WIRE 144 -896 -16 -896 WIRE 144 144 96 144 WIRE 144 176 -16 176 WIRE 176 -944 176 -1280 WIRE 176 -512 176 -880 WIRE 176 -208 -240 -208 WIRE 176 128 176 -208 WIRE 176 560 -16 560 WIRE 176 560 176 192 WIRE 272 -912 208 -912 WIRE 272 160 208 160 WIRE 432 -912 352 -912 WIRE 432 160 352 160 WIRE 496 -960 496 -1008 WIRE 496 -736 96 -736 WIRE 496 -736 496 -864 WIRE 496 -688 496 -736 WIRE 496 -512 176 -512 WIRE 496 -512 496 -608 WIRE 496 -464 496 -512 WIRE 496 112 496 64 WIRE 496 336 96 336 WIRE 496 336 496 208 WIRE 496 384 496 336 WIRE 496 560 176 560 WIRE 496 560 496 464 WIRE 496 608 496 560 WIRE 576 -1008 496 -1008 WIRE 576 64 496 64 WIRE 704 -1008 656 -1008 WIRE 704 64 656 64 WIRE 832 -1008 784 -1008 WIRE 832 64 784 64 WIRE 960 -1008 912 -1008 WIRE 960 64 912 64 WIRE 1088 -1008 1040 -1008 WIRE 1088 64 1040 64 WIRE 1248 -1280 176 -1280 WIRE 1248 -1008 1168 -1008 WIRE 1248 -1008 1248 -1280 WIRE 1248 -208 176 -208 WIRE 1248 -208 1248 -1008 WIRE 1248 64 1168 64 WIRE 1248 64 1248 -208 WIRE 1248 304 1248 64 WIRE 1248 560 496 560 WIRE 1248 560 1248 384 FLAG 496 608 0 FLAG 496 -464 0 SYMBOL Opamps\\\\LT1006 176 96 R0 SYMATTR InstName U1 SYMBOL res 368 144 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R1 SYMATTR Value 2700 SYMBOL res 512 480 R180 WINDOW 0 36 76 Left 0 WINDOW 3 36 40 Left 0 SYMATTR InstName R2 SYMATTR Value 1 SYMBOL npn 432 112 R0 SYMATTR InstName Q1 SYMATTR Value 2N4401 SYMBOL res 672 48 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R3 SYMATTR Value 8.4 SYMBOL res 800 48 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R4 SYMATTR Value 8.4 SYMBOL res 928 48 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R5 SYMATTR Value 8.4 SYMBOL res 1056 48 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R6 SYMATTR Value 8.4 SYMBOL res 1184 48 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R7 SYMATTR Value 8.4 SYMBOL voltage 1248 288 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value 12 SYMBOL res -32 304 R0 SYMATTR InstName R8 SYMATTR Value 240 SYMBOL res -80 160 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R9 SYMATTR Value 2200 SYMBOL res -256 32 R0 SYMATTR InstName R10 SYMATTR Value 3300 SYMBOL pnp -304 16 M180 SYMATTR InstName Q2 SYMATTR Value 2N4403 SYMBOL res -336 -48 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R11 SYMATTR Value 10k SYMBOL res -496 -176 R0 SYMATTR InstName R12 SYMATTR Value 10k SYMBOL References\\\\LT1009 -240 352 R0 SYMATTR InstName U2 SYMBOL npn -544 304 R0 SYMATTR InstName Q3 SYMATTR Value 2N3904 SYMBOL res -752 400 R0 SYMATTR InstName R13 SYMATTR Value 10k SYMBOL voltage -848 400 R0 WINDOW 3 -3 207 Left 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V2 SYMATTR Value PULSE(0 5 0 1e-6 1e-6 .001 .01 10) SYMBOL res -576 336 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R14 SYMATTR Value 1k SYMBOL Opamps\\\\LT1006 176 -976 R0 SYMATTR InstName U3 SYMBOL res 368 -928 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R15 SYMATTR Value 2700 SYMBOL res 512 -592 R180 WINDOW 0 36 76 Left 0 WINDOW 3 36 40 Left 0 SYMATTR InstName R16 SYMATTR Value 1 SYMBOL npn 432 -960 R0 SYMATTR InstName Q4 SYMATTR Value 2N4401 SYMBOL res 672 -1024 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R17 SYMATTR Value 8.4 SYMBOL res 800 -1024 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R18 SYMATTR Value 8.4 SYMBOL res 928 -1024 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R19 SYMATTR Value 8.4 SYMBOL res 1056 -1024 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R20 SYMATTR Value 8.4 SYMBOL res 1184 -1024 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R21 SYMATTR Value 8.4 TEXT -858 648 Left 0 !.tran 0 .1 0

It's basically a switched reference driving (if you want 40 lamps) eight five-lamp constant current drivers.

With 5 lamps dropping 2.1V each that's 10.5 volts out of 12, which gives you 1.5 volt of headroom to play with, which the 2N4401's and current sense resistors soak up nicely.

I've modelled the LEDs as 8.4 ohm resistors, since:

I've only shown two drivers and their loads, for convenience, but the other six just connect across the 12V supply, with the + input of all the opamps going to the reference voltage at the junction of R8 and R9.

It simulates beautifully, as you'll see when/if you run it.

Post back if you have any questions. :-)

-- John Fields Professional Circuit Designer