While Bill's circuits with one transistor per 3 LEDs might be a bit much, I don't think anyone is suggesting a single transistor for the whole 5K array.

3 LEDs plus a small ballast resistor = 1 string. 20 strings = 100 mA, or 1.1 W dissipated in the combination of 20 resistors plus 1 transistor. 84 groups = 5K LEDs. If you have known good airflow, you probably can combine more strings per transistor.

ak

 

I made a 768 LEDs matrix and I use a 12V 1A wall adapter + one LM2596 Module, the chip gets a little warm, but besides that nothing heats up and no resistors at all, just PICs and digital MOSFETs.

OK only 1/6 is on at any time, but it is not a panel for illumination.

For this purpose voltage should be high, 60 volts or so. And you rather should use power LEDs not small discrete LEDs.

So you would need a 12V 8 Amps supply.

I doubt this project is for real until I see these 5000 LEDs or just 1000.

 

To use a BJT (bipolar junction transistor) as a saturated switch requires 1/10th of the desired collector current flowing through the base; so if you had 10A continuous collector current, you'd need 1A continuous base current.

With MOSFETs, you simply need to charge and discharge the gate(s); once charged or discharged there is no current required to maintain the state.

There are MOSFETs available that have very low gate charge such as an http://www.irf.com/product-info/datasheets/data/irlr7821.pdf IRLR/IRLU7821; 12.5m Ohm max when drain current = 12A and Vgs = 4.5v; Qg=10nC (gate charge). That is a VERY small gate charge! Now, the peak gate charge/discharge current may be high, but that's for a small fraction of a second, and the average is considerably lower than would be required to saturate BJT's.

I was thinking of an emitter follower. The voltage at the emitter follows the base minus 0.6V drop.

 

LEDs, 555s, Flashers, and Light Chasers

Actually you could do it with one transistor, and lots of resistors...

Transistor Drivers

I have another image in the link provided.

The common emitter and MOSFET are the cleanest (the MOSFET is the best IMO due to high gate impedance) because the transistors are very close to ground indeed when they are turned on.

 

Nightmare of a project.

They give you the sack or did you quit Bill?

 

When driving LOTS of LEDs, the key is managing voltage drops. Having too many devices per circuit makes the whole thing very sensitive to voltage drops.

No need to go crazy with complex current limiting circuits unless efficiency is a priority, wasting power in the limiting resistor buys you generous tolerance and simplicity.

Make each LED it's own voltage driven unit, by including a single dropping resistor of the correct value for the applied voltage. This unit is then rather tolerant of voltage drops because the limiting resistor sees ~ 10V so a few volts of drop in the external wiring causes imperceptible changes in the current.

You can then connect these units to multiple branch circuits to provide power.

If you need to dim the whole thing, use a PWM dimmer.

The attached image shows a project that I engineered for the Whitney museum in NYC- 1000's of LEDs.

 

Large numbers of LEDs can be. You could always use LED ribbons.

As to the latter comment, very astute, but not pertinent to this thread.

 

The attached image shows a project that I engineered for the Whitney museum in NYC- 1000's of LEDs.

Neat. How much you charge 'em? 20 grand?

 

I do consulting work for an LED artist- Erwin Redl, he does tons of big LED art installations all over the world.

http://www.paramedia.net/index.php