[SOLVED] How to Prevent Leakage Led Lighting Between ULN2803 and 24V?

[apsua]

Hello,

I have a circuit shown below:



When the ULN2803 doesnt pull down to GND, because of the voltage difference between 24V and ULN output pin, the led lights a little. It seems like it is on. What can I do by only changing the components?
Note: The 24V supply also powers the ULN2803.

Thanks.

 

[d123]

If pin "1C" is the input/output, and it is active high, why not try a low value resistor to ground on the line where it connects to the pin - about 1k or so, so the 2803 knows when to be off. Or would a higher value resistor on the LED help? I'm guessing the first option.

 

[betwixt]

The ULN2803 doesn't have a power pin - it's a transistor array.

The design isn't good, it looks like you are relying on R3 to protect the 74HC166 from damage. However, the problem is almost certainly a combination of leakage into the ULN2803 and through the static protection diodes at the HC166 input pin. The simplest fix to try is replacing R3 with a silicon diode (1N4148 for example), cathode toward the LED and adding a pull-up resistor (~10K) from U1 pin 2 to U1's supply pin. It will ensure U1.2 is at the correct logic high voltage when the LED is off and still pull low when the ULN output goes low.

Brian.

 

[KlausST]

Hi,

Connect a 47k resistor in parallel to the LED.

Klaus

 

[apsua]

Hello,
Thanks for your help. I have solved the problem by connecting the supply of the led to +5V and changing the resistor to 470R.