ULN2803 as a relay driver - LED/Spike diode question

xanatos

Hi all, I'm driving relays from a BS2SX via some 74HC595s and ULN2803s. My question is - I need to add status LEDs AFTER the ULNs- but that also effectively puts them out of the built-in spike protection fo the ULNs, I am thinking. Would the reverse voltage rating of the average LED handle the spike of the relays, or do I need to add another diode (like a 1N914) to protect said LED... The relays coils operate on 5vdc and I'll be using a 1K current limiting resistor with the LED (dark environment, they don't need to be very bright...) Thoughts?

Thanks!

Dave

Of course, looking at the schematic of the ULNs... maybe that diode WILL still provide the protection... still - rather check with those potentially wiser than I in this issue... :-)

RobotWorkshop

I don't think you'll need to add any other components to protect the LED's. The 1K resistor should be fine. When I use relays I usually add an LED w/resistor in parallel with the relay coil. I've never run into any issues.

The Parallax relay board uses a similar method of adding a status LED as shown in the schematic here:

http://www.parallax.com/Portals/0/Downloads/docs/prod/boards/DigIOBrdSchem.pdf

Robert

xanatos

Hi Robert - thanks, that's fantastic. Turns out that that schematic is pretty much identical to what I've been having fun designing myself here! Only difference is I'm driving 32 relays... it's a relay farm!

Confirmation AND reassurance in one response, thanks very much!!!

Dave