High Current Through LED Help

mctrivia · 2009-03-02 07:13

my problem is I have a 60x14 grid of LEDs. Each row is trigered on by positive and each column by negative.

I have a counter IC, and four 4 to 16 decoder ICs triggering which of the columns is being lit

I have two 3x8 decoders and 14 inverters lighting 2 leds on each row.

With this setup I have about a max duty cycle per led of 1/420 and the screen is pretty dim.

I believe the problem is the inverter IC's are not providing much current to the LEDs. Any suggestions how I can increase current to each led when it is suppose to be lit without increasing wasted current when leds are off?

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virtuPIC · 2009-03-02 07:52

First have a look at the LED datasheet. Usually they have a current limit around 100 mA giving you an average current of some .25 mA. They won't shine very brightly.

If you want to drive more than the maximum output current you can simply add a driver of one resistor and one transistor to allow for higher current. If you take low power transistors like 2N2222 / 2N2904, BC547 / BC557... you can drive at least 100 mA or even up to 1 A. Please look at the transistor datasheets, too.

                     3V3
                      +
                      |
                      |     2k2
                       >|   ___
                        |--|___|-
                       /|
                      |
                      |
                      V ->
                      -
                      |
                      |
                     .-.
                     | | adapt to duty cycle, VCC...
                     | |
                     '-'
                      |
              2k2     |
              ___   |/
            -|___|--|
                    |>
                      |
                      |
                     ===
                     GND

For this simple driver V_CC must be the same as for the logic. Otherwise you have to add another resistor and transistor... Please note that the transistors are inverters in this schematic.

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Post Edited (virtuPIC) : 3/2/2009 8:10:45 AM GMT

Brian Fairchild · 2009-03-02 07:55

You're going to have to reduce your duty cycle. 1/420 is way too low. With the arrangement you have, you have two easy choices - you can go to 1/60 which is still a bit low or better yet 1/14. To do this you'll need to somehow latch the required column data (60-bits) and then drive the right row (1 of 14). Both these options will need more hardware.

Store the 60-bits in something like some shift registers or latches (74xx574 or similar) and then select the row.

Your other problem is the limited maximum current if driving LEDs directly from chips. 20mA is about all you can expect from a normal chip. While your average currents will be less than this, your peak currents, needed to maintain brightness in a multiplexed scheme, will be much higher.

The reason you can't simply increase the current through the LEDs is that an LED has two current ratings - an 'average' one and a peak one. The average one determines how bright it is and the peak one is the maximum current it can stand. In your case, for average brightness, you'll want around 10mA of current. With a duty cycle of 1/420 that means you'll need a peak current of 4.2A per LED which is more than the device will take.

I'd suggest that you are going to need to add some devices to increase your current drive capabilities (FETs or transistors) or move to using some LED driver chips.

mctrivia · 2009-03-02 08:06

1 in 60 could be done with 16 io lines or 4 + 14 latches from the prop. I only have 9 free so it would have to be with latches. I am sure there is a relatively cheap ic that could do 8 or 16. 1 in 14 could be done with 60 latches but that is a little impractical.

virtuPIC for that schematic I would need a Vcc of at least 3.7V(diode drop is 2V, transister of 2x0.7v) but my Vcc is only 3.3V and can not change.

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mctrivia · 2009-03-02 08:20

2x 74LVX594TTR would do the job for 1/60.
8x of them could do 1 in 14 think I will go for the 1 in 14. Saves me four 4 to 16 and an inverter ic.

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Need to make your prop design easier or secure? Get a PropMod has crystal, eeprom, and programing header in a 40 pin dip 0.7" pitch module with uSD reader, and RTC options.

Brian Fairchild · 2009-03-02 09:08

40 LEDs from one 16-pin chip...

http://www.maxim-ic.com/quick_view2.cfm/qv_pk/3195

mctrivia · 2009-03-02 15:23

not feisable for me as the display is lready built and i have 840 LEDs. I just realised running 1/14 I would need to apply 1.8Amps(30mA * 60) at the positive side if all LEDs were on. Any way to do this like virtalPIC sugested but with a device with lower voltage drop then a transister?

Also the problem is my power supply is only 1A. but if I limit current to 20mA each and add a large supply cap this should be ok. 100% lights will never be on the entire screen.

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Need to make your prop design easier or secure? Get a PropMod has crystal, eeprom, and programing header in a 40 pin dip 0.7" pitch module with uSD reader, and RTC options.

kwinn · 2009-03-02 15:23

mctrivia, be careful with high peak currents through a LED. There is a maximum current an LED can handle regardless of how short the duty cycle is. Exceed that peak and the LED is DEAD. You need to get the duty cycle to around 1/10-20 to get reasonable intensity and peak currents. Lighting up to 60 LED's x 14 per scan would be better. Rewiring the array to have 30 x 28 would work and minimize the number of drivers as well.

mctrivia · 2009-03-02 15:29

rewiring the array in this case is not really possible. for future projects I will keep this in mind. Spec sheet says 90mA max 30mA normal.

I have decided to change it to light 60LEDs at a time 1ith a 1/14 duty cycle. But this does require a high current to drive the positive side of those 60LEDs.

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Need to make your prop design easier or secure? Get a PropMod has crystal, eeprom, and programing header in a 40 pin dip 0.7" pitch module with uSD reader, and RTC options.

Brian Fairchild · 2009-03-02 16:17

I'd suggest you make a test setup with just a few LEDs and write a bit of software to simulate the duty cycle you are going to drive it at. That way you will be able to see how much current you need to achieve an acceptable brightness. Even one LED will give you an idea of what is required.

To get low voltage drops across the drives you could use FETs.

mpark · 2009-03-02 16:42

virtuPIC said...
First have a look at the LED datasheet. Usually they have a current limit around 100 mA giving you an average current of some .25 mA.

Does the max current listed on the datasheet mean that the LED limits current to that value, or does it mean that current should be limited to that value before it gets to the LED, so to speak?

The quote above and things I've heard other people say imply the former, but I always thought it was the latter. There's some ambiguity. Can anyone give me a definitive answer?

mctrivia · 2009-03-02 16:43

I have several p channel mosfets used with an ic driver. I am not sure how to drive myself though they don't function like transistors which I know.

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Need to make your prop design easier or secure? Get a PropMod has crystal, eeprom, and programing header in a 40 pin dip 0.7" pitch module with uSD reader, and RTC options.

Brian Fairchild · 2009-03-02 17:09

mpark said...

Does the max current listed on the datasheet mean that the LED limits current to that value, or does it mean that current should be limited to that value before it gets to the LED, so to speak?

The LED will limit current to the peak value in the same way a fuse does! Exceed it and the LED goes open circuit.

The serious answer is that it is the value that you should not exceed. The construction of an LED is no more robust than any other semiconductor device. Exceed the maximum rating and you risk burning it out.

virtuPIC · 2009-03-02 19:27

mpark: The datasheet tells the maximum current the LED can stand. They state different values for continuous and pulse current. Don't exceed the currents! Recently I've burnt a few LEDs like fuses (they simply don't work anymore) but years ago I held an LED to the contacts of a model rail poser supply. Don't try it, the LED will explode into two parts!

mctrivia: Already thought about adding MCP23016 / -17 / -18? They demultiplex and latch from I2C to 16 lines per chip. They are fast. If you add several of them you can share the clock since you are driving them together. And you only use them for output which makes handling even simpler. Oh yes, Microchip has a generous sample policy.

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Post Edited (virtuPIC) : 3/2/2009 7:32:45 PM GMT

mctrivia · 2009-03-02 19:35

well I will look into those. could run the entire display off 5 of those assuming I can figure out how to use a fet to drive the positive side of the led anyone have a schematic for that?

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Need to make your prop design easier or secure? Get a PropMod has crystal, eeprom, and programing header in a 40 pin dip 0.7" pitch module with uSD reader, and RTC options.

mctrivia · 2009-03-03 04:35

think EEC-F5R5U105 would fix the problem of 1.8A draw from 1A supply?

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Need to make your prop design easier or secure? Get a PropMod has crystal, eeprom, and programing header in a 40 pin dip 0.7" pitch module with uSD reader, and RTC options.

Ken Peterson · 2009-03-03 18:19

If you look closely at an LED, you will see a fine wire jumper going from one of the terminals to the top of the die (chip). This wire pretty much acts as a fuse, so if you exceed the rated current it will vaporize this little wire. It's not a fuse by design, but rather a side-effect of the construction. However, I suppose if you didn't have this little fuse in there, you could have LEDs exploding when they get too hot.

I'm working on a project that uses multi-channel serial LED drivers (Allegro A6279 for example). Four of these would drive your columns and then you only need some P-FETs for the rows. You can clock data into these drivers using a SPI driver (is there an SPI object out there??). The A6279 handles up to 90mA per channel (not all at the same time), and it has a serial-out pin so you can daisy-chain them together to increase the number of channels.

www.allegromicro.com/en/Products/Part_Numbers/6278/

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Post Edited (Ken Peterson) : 3/3/2009 6:24:22 PM GMT

yarisboy · 2009-03-04 02:58

On a stupid day I had one pop in my fingers. Ouch, mini burn. I peeled the two parts out of my skin. My application was much much simpler but the math remains the same. My LED was built to draw 20 ma at 2 volts continuous. Much like your transistor conversion I used quad pack OP-AMPs in a unity gain configuration with current limiting resistors in series with the LED. Without the impedance matching unity gain amps, the BS2 could only drive 4 LEDs before they started getting dim. With them I have one input pin and 15 LED output pins. With the hardware, I don't have to bother with duty cycle in the code. Op amps are over kill but both NPN and PNP functions are available on the same chip. Layout is simplified too.

mctrivia · 2009-03-04 03:05

never thought about op-amps. would work if I had the space. I can barely fit 14 SSOP3 pack p-chanel mosfets. I believe have figured it out. I can hook up the source of the FDN306P to 3.3V drain to my leds. a logic 0 will allow up to 2.6A through to run the LEDs. in software I need to alter my duty cycle between 1:14 to 1:28 depending on number of leds on in each row. Anything more then 50% in 1 row needs to be split into 2 halfs or it will over power the power supply.

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Need to make your prop design easier or secure? Get a PropMod has crystal, eeprom, and programing header in a 40 pin dip 0.7" pitch module with uSD reader, and RTC options.

Cluso99 · 2009-03-04 04:19

One of the above posts suggested you try one LED to check on the brightness. I really suggest you do that as lots of the newer LEDs are Super Bright and Ultra Bright. I always use at least Super Bright. I am driving them off the Propeller with a 10K pullup (to 5V on the mouse port) and the brightness is OK when fully on (not that bright in daylight but certainly visible). This means that I have 5V - 1.8V (assumed) across 10K = 0.3mA approx. So you see, finding out the current required may be important for your total current requirements.

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mctrivia · 2009-03-04 04:37

they are super bright. spec sheet says 30mA current draw. There is no natural light in my workshop. under florescent even 1/840 looks good. I can software dim easily so my goal is to make as bright as possible through a light senser on there to detect room brightness and dim accordingly.

I thought about using pull up resisters but then your current draw is higher when leds are off then on and I want as energy efficient as possible because this will run on battery and wall power(my power supply module auto switches between 2 sources.)

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Need to make your prop design easier or secure? Get a PropMod has crystal, eeprom, and programing header in a 40 pin dip 0.7" pitch module with uSD reader, and RTC options.

Cluso99 · 2009-03-04 18:32

??? The resistor is in series with the LED. If the LED is off, there is no current drawn through the resistor.

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Links to other interesting threads:

· Home of the MultiBladeProps:· SixBladeProp, TriBladeProp
· Prop Tools under Development or Completed (Index)
· Emulators (Micros eg Altair, and Terminals eg VT100) - index
· Search the Propeller forums (via Google)

My cruising website is: ·www.bluemagic.biz·· MultiBladeProp is: www.bluemagic.biz/cluso.htm

mctrivia · 2009-03-07 01:41

with a pull up when you send a low out to turn off the led current draws through the pull up then the ic to ground.

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Need to make your prop design easier or secure? Get a PropMod has crystal, eeprom, and programing header in a 40 pin dip 0.7" pitch module with uSD reader, and RTC options.

High Current Through LED Help

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