Electronic – Questions about a simple LED circuit using the ULN2003A

ledtransistor-arraytransistors

I'm a newbie, and am trying to drive several independent LEDs using a transistor array in a 5-volt CMOS circuit. Here's what I've come up with, showing only the first LED:

Schematic

In the schematic, LED_ENABLE is the output of some logic IC in the circuit. I'm assuming that the LED has a voltage drop of 1.8 V and requires 20 mA of current.

Does this seem sane, or have I missed something? Specific questions:

  1. I've tied LED_ENABLE directly to the base pin, because my understanding is that the ULN2003A's internal resistor is designed to let me do so. Is that safe to do without thinking further? It seems like the value of the resistor should need to depend on \$I_{CE}\$ and the transistor's gain, so I'm not sure how this can be one size fits all.

  2. I chose the resistor voltage by calculating \$\frac{5 V – (0.9 V + 1.8 V)}{20 mA}=115 Ω\$ and rounding up, since the data sheet for the ULN2003A says that the collector-emitter voltage at saturation is 0.9 V for \$I_C=100 mA\$ (the lowest listed current). Is this the right calculation?

  3. If I understand correctly, I don't need the COM pin. Is it sane to leave it floating?

  4. And how about leaving the other base and collector pins floating? Maybe they should be tied to ground?

  5. Anything else obvious I've missed?

Best Answer

To answer your questions:

  1. Correct: it is ok to tie directly to tbe base pin, given the internal resistor. The reason one size fits all, is the combined gain of the darlington pair is so high, and that the output pins' current ratings mean any current at the output within specs will be sustained by the internal resistor at the base.
  2. Yes your calculation looks good.
  3. Look at the circuit diagram below which shows how the darlington pairs are implemented. You can leave COM floating.
  4. You can leave the other pins floating. They should have no bearing on your circuit, since as per the diagram below, they are independent (as long as you do not pull COM low).
  5. Perhaps you have not wired it up correctly, or the chip is dead. For the sake of troubleshooting, try connecting COM directly to ground, which would be equivalent to all transistor pairs saturated on. The LED should light. Perhaps the LED's polarity is reversed. Sanity check by connecting pin 1C directly to ground.

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