How can I power three LEDs in series with shift register?

[Mbira]

Hi folks,
I am using this circuit using 595s:

http://arduino.cc/en/Tutorial/ShiftOut

If you scroll to the bottom, you see where he adds a second set of LEDs.  My project has four sets of LEDs.  I have gotten that to work just fine.  My question is that I want to have one of those sets power three "ultra bright" LEDs from each leg of the output. 
This is the datasheet for the LEDs I have.  They draw 50mA each.  So wired in series each leg of the shift register would need to output 150mA I think. 

Is there a chip I could use to replace one of those 595s that would work?  Should I be wiring those LEDs in series? 

The reason for the bright LEDs is these are for stage use and are under translucent rubber.  Yes I need them that bright.  Thanks!

 

[JohnRoberts]

There are some nice dedicated LED driver/latches, but AFAIK they do not support 150mA (typically top out around 50 mA). Your best shot might be to use the 595 and something like an ULN2003 darlington array. The darlingtons will drive 500mA each, (7 per package), but will invert the signal so you may need to flip polarity for on/off.

JR

 

[Richard Crowley]

All integrated circuits have definitive data sheets which are easily available online. These documents are made specifically to answer questions like yours.  According to the 74HC595 data sheet the output pins are only rated for 35mA, and the data sheet for your high-brightness LEDs want 50mA.  So the 74HC595 won't directly drive 50mA loads like you bright LEDs.

As Mr. Roberts suggests, you can use a multiple current driver chip like the ULN2003, or you can use discrete transistors (such as the ubiquitous 2N2222)  Remember that if you connect the LEDs in series, you will need a higher-voltage power supply.

Or, you can use a high-power version of a shift-out register like the TPIC6B595 which is rated for 150mA on each output pin (500mA total)
https://www.sparkfun.com/products/734

 

[Harpo]

... I want to have one of those sets power three "ultra bright" LEDs from each leg of the output. 
This is the datasheet for the LEDs I have.  They draw 50mA each.  So wired in series each leg of the shift register would need to output 150mA I think.

50mA If is the abs.max.parts value. Three in series is still abs.max.If 50mA at three times Vf (typical 2.1V each up to 2.7V each). Use a current limiting resistor to stay below parts limits. Are you sure about wanting a 4° viewing angle part ?

 

[Mbira]

Hi Robert.  Yes indeed.  That's why I said:

you can use a high-power version of a shift-out register like the TPIC6B595 which is rated for 150mA on each output pin (500mA total)
https://www.sparkfun.com/products/734

Nice find!  That's what I'm looking for.

Harpo-yes-my bad I said series and I meant parallel...Regarding the viewing angle, these are under translucent rubber so that should diffuse the light enough.  There will be three of them under a 2.5" square of rubber (1/4" thick).

50mA If is the abs.max.parts value.

Check that data sheet again.  These are super bright LEDs where it's 50mA at 2.1V.  Not typical ones.

 

[Richard Crowley]

Hi Robert.  Yes indeed.  That's why I said:

you can use a high-power version of a shift-out register like the TPIC6B595 which is rated for 150mA on each output pin (500mA total)
https://www.sparkfun.com/products/734

Nice find!  That's what I'm looking for.

But remember that you are asking to put a 150uA load on EACH output (if we understand you properly).  You can not do that without violating the 500mA total limit on the chip.
OTOH, if you had a higher voltage supply, you could connect the 3 bright LEDs in SERIES on each output as that would be only 50mA per output pin = 400mA total.

 

[PRR]

> They draw 50mA each.

I think you know this and I'm just being a fuss-pot, but.....

They don't "draw" 50mA. They would be happy to draw anything you offer, until they burst.

You *pick* how much they will get. In simple systems with ample steady voltage you pick a resistor to pass an appropriate current. 12V supply, 2V LED, 10V to blow. If 50mA is your goal, 10V/50mA= 200 Ohms 0.5 Watts. With wobbly or low supply voltage you turn to current limiting tricks.

For multiple LEDs at the same current-- SERIES is a natural thing. Drawback is that voltages stack. Four 2.1V LEDs series needs 8.4V. If you only have 5V, no-go. With 9V you need a really sharp-pencil current limiter. At 15V it's just a 6.6V/50mA= 132 Ohm 1/3Watt resistor.

 

[JohnRoberts]

If they draw 50mA each, three in series is still 50 mA.  50mA is around the upper limit for the dedicated LED drivers. The good thing about the dedicated LED drivers is that they are generally current source outputs (adjustable with one resistor for entire package) with decent voltage compliance so they simplify several aspects of the design, while I wouldn't expect the small SMD package to be very happy driving say 16x50mA. Which comes back to something more practical like a darlington relay driver.

Whatever... you'll figure it out.

JR

 

[Harpo]

50mA If is the abs.max.parts value.

Check that data sheet again.  These are super bright LEDs where it's 50mA at 2.1V.  Not typical ones.

I did and you don't want to exceed the 'Absolute maximum ratings' of If 50mA as long as you keep the ambient temperature rise below 85°C. Max.If must not exceed 30mA if ambient temperature rise should reach upper limit 100°C with Tamb 25° when you take a look at Fig.1 on pg.3.

 

[Mbira]

50mA If is the abs.max.parts value.

Check that data sheet again.  These are super bright LEDs where it's 50mA at 2.1V.  Not typical ones.

I did and you don't want to exceed the 'Absolute maximum ratings' of If 50mA as long as you keep the ambient temperature rise below 85°C. Max.If must not exceed 30mA if ambient temperature rise should reach upper limit 100°C with Tamb 25° when you take a look at Fig.1 on pg.3.

That's true.  Luckily our stage does not get to 185 degrees Fahrenheit....though we do melt faces. ;-)

 

[Mbira]

Right on guys-yeah, I meant parallel and not series....sorry for that type John and PRR. 

This will not have more than three rails lit at once (max) which would be 450mA, but 99% of the time it will be shifting between two rails.  (It's like drum pads that are lit up in a sequence when hit), so I think those high power 595s will do the trick....plus I grabbed a few free samples from TI to try them out.  ;-)

PRR- no need to appologize for being picky on terminoligy.  You're totally right of course, I could blow those LEDs up good and fast if I so choose!

 

[Richard Crowley]

They don't "draw" 50mA. They would be happy to draw anything you offer, until they burst.

As with most components.  But I was assuming that you were practicing sensible circuit design and using the recommended nominal design values.

 

[Rochey]

spend a penny on a pn2222 transistor per LED. probably cheaper than the LED.

 

[Mbira]

Rochey, your company is sending me samples of the high power 595s. ;-)