# Why does an UDN2981 lower the current?

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).

• Just off the cuff, are you aware that the UDN2981 uses a high-side Darlington output and may have a voltage drop of $1\:\text{V}$ or more perhaps when ON? If you are expecting full voltage to be applied, it's not a good switch to use.
– jonk
Mar 30 '19 at 17:54
• @jonk I was not really aware of this, but it will not be a problem, I can easily change the resistor values, and since I need a separate power supply anyway, I can as well use a 9V or 12V. Mar 30 '19 at 17:59
• Hi, Michel. Schematics are better than words. Is this more work for the band? Give us a YouTube link sometime to see if the help we are giving is of benefit to humanity or if we should discourage you from playing! Mar 30 '19 at 17:59
• @MichelKeijzers Then that's the approach. Just adjust the resistors. (Just measure the output voltage drop you are getting already. You can work out what the device is actually doing from that and work out the resistors. Or else raise up the supply voltage, which the device can easily handle.)
– jonk
Mar 30 '19 at 18:02
• Please try to reduce the impedance of the resistor from Vcc to pin6. For example 2K ohm. Mar 30 '19 at 20:37