Driving LEDs at 1.4A, 10-500Hz

Blammo

Hi all,

For all my projects up until now, I've used a ULN2308 for powering things. My current project uses big flashy LEDs that draw much more power than I'm used to, and I'm a bit stumped. Pardon the clear newbishness here, but what should I use to deliver 1.4A at frequencies ranging from 5-500Hz, possibly higher?

Thanks for any pointers. Learning links appreciated!

Blammo

Mike Green

Theoretically, you could use the ULN2803 by paralleling 4 sections since they're rated at 500mA each and can be paralleled. Practically, you'd be better off using an ordinary power transistor (junction or MOSFET). An IRL510 is rated at 4A and is used just like a section of the ULN2803. You connect the gate to the I/O pin, the drain to ground, and the source lead to the load. With MOSFETs, it's best to use a resistor between the gate and ground to make sure the transistor stays off when the I/O pin is in input mode. A 100K resistor should work fine.

Nuts and Volts Column #6 discusses the use of transistors to control high current loads and the Industrial Control tutorial has a chapter on this.

Leon

Are you building a stroboscope?

Leon

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Amateur radio callsign: G1HSM
Suzuki SV1000S motorcycle

Blammo

Thanks for the responses. I'll have a go with some IRL510s. Out of curiosity, how high a frequency can you put through them before they start having problems? Is there any practical limit? Also, do MOSFETs work all the way up into the hundreds of watts, or is something else required at that point?

I'm trying to build a 3 LED (R,G and light that adds up to white, but does so by rapidly flashing each colour in succession. It should give some realy·interesting effects at the edges of moving objects. I'm going to throw a plain white stroboscope mode in too.

Blammo

Mike Green

MOSFETs work all the way up into the thousands of watts, but you need care in that range. In terms of frequencies, this type of MOSFET has been used well into the high frequency range. The higher the frequency and the higher the amount of power, the more gate drive (voltage & current) you need. There are graphs included with the datasheet that give some specific information.

Leon

MOSFETs are often used in radio transmitters; ordinary ones are OK up to 10 MHz or so and some work up into the UHF region.

Leon

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Amateur radio callsign: G1HSM
Suzuki SV1000S motorcycle

Timothy D. Swieter

There are prebuilt modules you can buy to driver LEDs at those currents.· You could also go fancy and use one of the many ICs out there along with their external circuitry.· I am betting the IC route at the moment isn't the best route.·

Trying searching for "Buck Puck".· That is a brand of module I have used in the past for driving higher wattage LEDs.· The modules vary in price, but can be from US$12 to US$20 if I remember correctly.· Maybe this is too costly for your project, but an option I thought I would throw out.

Here is one link, but there are similar products from others.· http://www.ledsupply.com/buckpuck.php

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Timothy D. Swieter, E.I.

www.brilldea.com·- Prop Blade, LED Painter, RGB LEDs, uOLED-IOC, eProto fo SunSPOT, BitScope
www.sxmicro.com - a blog·exploring the SX micro
www.tdswieter.com

StarMan

Timothy brings up a good point.· The Buck Pucks and similar devices are great for the high power LEDs because of the current levels involved.· With smaller LEDs (20mA), limiting the current from a constant voltage source with a resistor is no big deal.· You can usually use 1/4 watt resistors and combine the LEDs (series and/or parallel) to minimize the power loss in the resistor.

Now, say you try to drive a high power LED that draws 350mA at 3.3 volts with a regulated 5 volt power supply.· A 5 ohm resistor will limit the current to 340mA but the power draw from the resistor will be a whopping 1.7 watts.· I've done it before with five 1 ohm, half watt resistors in series but it's a pain.· I wouldn't even consider it for 1.4 amp LEDs.

The Buck Pucks are much easier.· Also, many of them have a "control" pin that will accept a PWM input for dimming.

If you want to design your own, National Semiconductor has some schematics and component calculators to fit the schematic to your particular system.· I highly recommend watching the "Analog by Design" web TV shows about driving high power LEDs·on the National site http://www.national.com/nationaltv/.· They're informative and I found them very entertaining as well.

Chris I.