high power leds and switching

Hi all

I am trying to switch 3 high power leds in series .

1 microsecond high and 100 microseconds low.

I have a NPN Epitaxial Silicon Transistor http://www.mouser.com/ds/2/149/KSD526-59826.pdf

Now the problem is when i am switching them my current draw is about 80mA where it should be about 700mA or more.

What is causing this and how can I get it to work probably?


Without a schematic the post is almost useless. “3 high power leds in series” doesn’t cut the mustard…

And how do you measure the current? It’s pretty hard to “just” measure the current in that 1us.

Ok heres a quick schematic.

I measure it with a multimeter but I can also see the difference in brightness when turned fully on and when switching them.

LEDs require a constant-current power supply. With regular small LEDs you can use a resistor to limit/control current, but high power LEDs usually require a special constant-current power supply.*

I measure it with a multimeter

You can't accurately measure the current of a short-term pulse with a multimeter.

but I can also see the difference in brightness when turned fully on

How much current do you measure when fully-on? The current should be the (approximately) same during the 1 microsecond pulse.

1 microsecond high and 100 microseconds low.

Assuming high is on... That's 1/100th of the average current and 100th of the average power and it will appear dim to the human eye (even though it's running a full-power and full-brightness for 1 microsecond.) That's how normal PWM LED dimming works...

* Constant current (or controlled current) power supplies are usually switching designs, but they are NOT a simple transistor or MOSFET switch. They use an inductor to store energy and "smooth" the current, and a feedback circuit to measure the current and hold it constant.

Thanks all I found a constant current driver that i had with these specs that they say you can connect 1 to 3 leds in series:

Input Voltage: 10-20V DC
Output Current: ~750-850mA
Output Voltage: 9-11V

I am switching the negative side of the output of the driver, when I just connect 1 led to the driver and switch it the current draw is about 650mA which is about right but when I add another led the current draw drops and when ive connected all three it drops to about 300mA.

My voltage input is 12v and the drivers output voltage is around 10v, I think the leds forward voltage is 3v.

Why would it do that?


Edit: Another thing that i wanted to ask is if I could use a 555 timer to switch my transistor, Can a 555 timer switch at that speed?

So why not post a link to that driver? And a schematic? Just posting specs is a bit useless on it's own.

1 microsecond "on" is too short for most CC LED drivers.
The ones I know switch at ~500kHz.
You're asking the driver to turn "on" shorter than it's own switching frequency.
LED dimming is done at a much lower "base" frequency, so when you dim, the "on" pulse is at least a few pulses of the switch frequency.
Some datasheets give a confusing max frequency of 50kHz.
They should give a minimum "on" pulse width, so you can find the maximum base frequency for 10- or 12-bit dimming.

So I cant seem to find a driver that can switch at that speed any suggestions?

Would it not work if I use a digital pot as a resistor for the leds and read the voltage of the battery and adjust the pot to keep the current constant?


Why do you want to switch LEDs on/off in 1/1000000 of a second.
It surely has nothing to do with human vision.
Is this a science experiment?

Its certainly possible to switch SOME leds at that speed but not all.

Post a link to your led specsheet.

Its certainly possible to switch SOME leds at that speed but not all.

The problem is the CC LED driver.
You tell it to start, but before it has started, you tell it to stop.

Just put a resistor in series with it, sized so you get the right amount of current.

It won’t be ideal, but it will keep the current on the LED within spec. It’s not like you have to worry much about the thermals, since you’re keeping it off almost the whole time.

1 microsecond is really fast though. You need to use a 'scope or something to take a look at the actual waveform to see what’s going on, not a multimeter.

CC LED drivers are nice, but you can limit current to bigger LEDs with resistors - it’s inefficient, and you need to be a little more careful about the thermals (there’s a touch of positive feedback there, since Vf falls as T increases), but you’re not even leaving it on for long enough to have temperature issues.

What the hell are you making?


There is a way to pulse drive though often used on pulsed lasers.

It uses voltage rather than current drive and is fraught with problems.

Can get them on a chip these days.


Not sure of availability though.

Why do you want to switch LEDs on/off in 1/1000000 of a second.
It surely has nothing to do with human vision.
Is this a science experiment?

No its not human vision, its camera vision.
I need to switch at that speed to get rid of the sun interference, it all works well but I would just like to get the most out of my Leds that I can.

Post a link to your led specsheet.

The one that i get off the suppliers page is incorrect because it says they have a forward voltage of 1.4v which is not the case its 3v, but the leds switch fine on there own.

Just put a resistor in series with it, sized so you get the right amount of current.

Do you think it will be fine if I put a slightly smaller resistor to compensate for the voltage drop of the battery, I think I need about a 4ohm resistor, I think the leds can run at a slightly higher current because of the switching they dont get that hot?

Or I could add a voltage regulator to take my 12v battery down to 10v so when the voltage drops its still the same so then the current should say stable?

What the hell are you making?

Im trying to detect ir leds outdoors with a small camera and trying to not detect the sun which is working, I just would like to run the leds at maximum efficiency.

As no one has pointed this out that I can see I thought I would.

You can not connect an Arduino pin output direct to the base of a transistor, you need a resistor in line.

The forward voltage drop for three LEDs is 9V the emitter / collector saturation voltage is likely to be 1.2V so with 10V you do not have enough voltage to drive three LEDs in seriese.

If you are only getting 10V out of a 12V supply then the supply does not have the current drive capability for your circuit.

Thanks mike forgot about that resistor.

I found this circuit on instructables.com do you guys think it will do the job because I know some instructables are absolute crap.

schematic attached.


Yes that looks OK. Mind you you need a logic level FET.
Also R3 needs to be right.
The problem with this sort of circuit is that the current is defined by the value of R3 and this needs to be a power resistor as all the LED's current goes thorough it.

Ok thanks

On the datasheets what do I look for to tell me its a logic level FET is it the gate source voltage?

And then how do I work out what R3 must be?

Logic level MOSFET normally have VGS(th) at about 1V and RDS(on) with 5V Gate voltage.

Gate Threshold Voltage VGS(th) min 1.0V
(VDS = VGS, ID = 250 Adc)

Static Drain−to−Source On−Resistance (Note 2) RDS(on) typ 12.4 m Ohms
(VGS = 5.0 Vdc, ID = 30 Adc)

R3 = 0.7V/Current In your case for 700mA , you have about 1 Ohm