Most practical way to drive 4 strings of 20 LEDs (600mA a string at 3.5V)?

[Triode]

A friend has asked me for help with a dance costume, it involves 80 LEDs, in 4 independently controlled parallel groups of 20. The LEDs I'm looking at draw 30mA, and about 3.5v. I can handle the controller part, but since I've only been working with driver circuits for a little while I don't offhand know the best chip or transistor to use. I figure each string of 20 will draw 600mA when turned on. I have some ULN2803 chips and my first idea was just to gang the gates on several together to make triggers for 4, 600ma chains. But can someone suggest a better option? The switching doesn't have to be rapid by electronic standards, so transition speed isn't a concern.

While I'm at it, am I right in thinking that I can just put one resistor at the positive lead into the parallel LED chain, I calculate that it would only need to be 2.2 ohms with a 5V supply, possibly no resistor at all if I use a battery pack at 3.5V. But all the chains I see have a resistor on each LED. So I thought I might have this wrong.

Edit: By the way, sorry I might have put this in the wrong section, I was thinking microcontrollers because that's what I'm mainly working with, but my question doesn't really mention them now that I think about it.

 

[Wp100]

Hi,

Have a look at pages like this one to see whats needed.

Also consider having your leds in series.

http://www.hebeiltd.com.cn/?p=zz.led.resistor.calculator

 

[AtomSoft]

You can place 1 resistor with parallel leds but then you dont know which LED will draw the right amount of current. Current just doesnt split itself so you might get a led with 30mA and another with 100mA... its not safe or pratical.

I would listen to Wp100 and either place the LEDs in series which would require more Voltage or simply use a single resistor per led.
Series LED will use less current tho. Which is good if using a normal NPN transistor. Most transistors can switch 60v @ 100mA or something...

If you parallel the bunch you will need to switch more current which will require something stronger.

 

[PICMICRO]

I would suggest to use groups of 10 leds in series. If you need you can combine two groups of 10.
That way you would be handling only about 35 volts, safe and easy. But, do you have 35Volts source?
Use one resister and one transistor per each group of 10.

 

[Triode]

I know series is usually better, but I'm not sure how I would supply something that a woman is going to be wearing on a costume and dancing in with 70 or even 35 volts. I would even be willing to split them into smaller groups, as long as there are 4 independent groups I can control. If anyone can recommend a good light weight cell to supply that kind of voltage I'd be all for it.

What if I do 8 parallel sets of 10, I resistor per LED, I pair those in series. Then I get 4 groups that consume 300mA @ 7V each. Each group would require 6 of the 25mA gates of a ULN2803, and each ULN2803 has 8 gates, so if I use 3 of the chips, which just happens to be what I have on hand, and gang the total of 12 gates into 3 300mA sets, I should be able to control the whole thing and run it off of 5 AA batteries in series.

But I'm open to your ideas if I can figure out a way to supply 35 volts.

 

[AtomSoft]

I say parallel is better but sucks on current.

If using series and 1 LED dies the rest in that series will be OFF

But the cool thing is you can simply make a boost converter to turn 3v to 35v

Boost converter is simple go here:

**broken link removed**

The values i would use are:
Vin 3.3V
Vout 35V
Iout 40mA
Vripple 100 mV(pp)
Fmin 100 kHz

 

[Triode]

That's clever, the chip looks pretty cheap too. If I want to go with series strings I think that would be the way to go. I still like mainly using parallel. What do you think about the arrangement I outlined above? I still could use a boost converter though, so that I don't need to put so many batteries on it.

 

[AtomSoft]

its kind of confusing.. can you draw it out ? I have a headache so bare with me pls

 

[Triode]

Yeah, written descriptions are pretty ineffective for this kind of stuff. I'll sketch up a plan.

 

[AtomSoft]

heh Thanks a ton.

 

[Triode]

Yeah my math was wrong there, besides miscounting gates I forgot that I need to do 4 strings not 3. Is it even practical to try to run and control 4 sets of 20 LED's off of a small battery?

I have a few more details that could help now that we've picked out the LED's, three of the colors are only 20mA, one set will be ultra brights that are 30mA, but when they're on the others will be off. Maybe I started this out with too much assumed. It doesn't have to be parallel like I first said, any setup that lets me separately activate the 4 groups of 20 with a PIC and power it from a few small batteries is good, if that is possible. It seems like it would be, I've seen flashlights with 20 or far more LED's run off of a few AA or AAA batteries.

 

[colin55]

You need to pulse the LEDs with a high current for a very short period of time to conserve energy.
Do you want them flashing?
20 high-brights with the right circuitry will take 50 milliamp from 9v battery.

 

[Triode]

The ultra brights will only be used as strobes, the 3 other groups will glow continuously for short lengths of time.

 

[Pommie]

What is your idea of a small battery pack?

Assuming 100% efficiency, your LEDs will use around 8W. Two alkaline AA cells have an ideal capacity of 6Wh, so you can expect (at best) them to last 45 mins.

Mike.

 

[colin55]

8 parallel sets of 10. You need 6v or 9v supply. Use a converter chip out of a car phone charger for $2.00 from a $2.00 shop. Your micro can give you PWM and flashing to reduce the current. Use 8 transistors to switch each string.

 

[be80be]

80 leds at 20mA = 1.6 amps / 4 = 25 % on time = 400mA converter chip out of a car phone charger good for about 1amp But you can boost with them to so you use 4 alkaline AA cells you'll still run out of power fast and them converter chip from a car phone charger waste about 20% of the power.

 

[colin55]

be80be You are so totally wrong.

 

[be80be]

What part? The number 1 chip used in a car charger AMC34063 nice chip

Thanks for the kind words LOL I'm not wrong You don't find to many things that has 80 leds running off small AA cell for long

20 leds is doable.

 

[wkrug]

The most energy by weight can deliver LiPo Cells.
2 cells with 600mAh have a weight of about 30g and a Voltage of 7,4V.
This Cells are rechargeable, so you can use it about 1000 Times.
But you need a Charger that's useable for LiPo Cells.

You can also use a Step up Converter to get higher Voltages at less Cells and the output Voltage is regulated.

 

[AtomSoft]

Ok here is my thoughts. Use LIPO batteries but in parallel to get more Ah , use the MC34063A like i said to boost the power then to charge the LIPO use the MCP73811 or better the MCP73812