I want to use a UDN2981 8-channel source drivers IC (see UDN 2981 datasheet) for a matrix multiplex light.
I connected the UDN2981 this way:
- pin 9 (VCC) to 5V
- pin 10 (GND) to GND
- pin 6 to VCC with a 10K ohm resistor
- pin 13 to a 3mm white LED to a resistor (220 ohm) to GND
And I connected another LED not via the UDN: from VCC -> white LED -> 220 ohm resistor -> GND
What I see is
- that the 3mm white LED via the UDN gives only 0.1 mA (my multimeter here does not have more accuracy)
- that the 3mm white LED directly from 5 V gives 1.7 mA
My calculation for the latter would be
V = I * R <=> (5 - 3.3) = I * 220 <=> I = 1.7 / 220 = 7.7 mA
This is still a lot more than 1.7 mA, but maybe the forward voltage is higher
However, I'm more interested to know why the LED via the UDN gives only 0.1 mA... I tried different resistors to pin 6 but this does not affect the current through the LED.
(I think it has something to do with the internal diodes of the UDN giving a voltage drop... this means I should use a higher voltage to VCC or less current; I hope I can calculate this easily).