Current to power LED

simulate this circuit – Schematic created using CircuitLab

The Circuit is of 2 alternately flashing LED's (BXRA-C0402 -- max i = 500mA). Each of the transistors(TIP120) power 15LED's (going on an off simultaneous) How do I find out if the transistors can supply enough required current to the load (15 LED's each = 500mA*15) and if not, how much current is the circuit short of?? How can I over come this??

• Please fix your schematic. The dots indicate connections and these show one transistor shorted C-E and your 5V supply directly across the other transistor C-E. – DoxyLover Dec 30 '13 at 20:33
• Why have you used an opamp to invert the signal? I think the same invertion can be achieved with a common emitter transistor. – alexan_e Dec 30 '13 at 21:32
• Are the 15 LEDs connected in series or parallel? Your schematic should reflect that, and it looks like there are other problems with your schematics. Please clean that up so we can give you informed responses. – Joe Hass Dec 30 '13 at 21:40

The Circuit is of 2 alternately flashing LED's

Your circuit makes no sense to me. What is the principle of it's operation?

• Your two (sets of) LEDs are in series, they should be independent.
• Why use an op-amp as an inverter?
• Why don't you need resistors to limit base current to the Darlington pairs?
• What is the voltage level of the square-wave control signal?
• What are the values of the resistors?

Your BXRA-C0402 LEDs have a forward voltage of 9.6 V. I don't see where you are supplying that. You seem to have a supply voltage of 1V or 5V depending on which diagram I look at.

How do I find out if the transistors can supply enough required current to the load (15 LEDs)

You

• calculate the current required to drive your LEDs (depends on arrangement in circuit)
• look at the data sheet for the transistor concerned.

If the LEDs are in series you only need 500 mA but you need a higher supply voltage for the LEDs (15 x 9.6V).

If the LEDs are in parallel (each with own current-limiting resistors), you only need about 12V supply but need to handle 15 x 0.5A = 7.5A. A TIP120 has an absolute maximum Ic of 5A - that is not enough.

if not, how much current is the circuit short of?

See above.

How can I over come this?

Change the arrangement of LEDs or use higher powered transistors (or FETs)

For example you could use 15 TIP120s to drive each of your 15 LEDs. Or you could look for a higher powered alternative.

• Hello, The reply was quite helpful, I have added another image of my circuit with out the op amps where I am using a MATLAB code and arduino, However, I am relatively new to electronics and was wondering if not an op-amp what else could I have used to invert the signal? – ShP Jan 2 '14 at 2:00