ULN2003 connection problem

[donrecardo]

Hi
I need to control 6 small 5v relays with a Picaxe.
The relays draw too much current ( around 25Ma each) for the picaxe so
I need to boost it up a tad

In my parts draw I have a ULN2003 which claims to supply 500Ma and has
7 outputs ( more than enough for my 6 relays )

I have never used this device before and after looking at the data sheet
I am still not 100% sure how to connect up .

Obviously my Picaxe outputs will feed the ULN2003 inputs ( are resistors needed ? )

I assume I then connect one side of each relay to 5v and take the other side of the coils to the ULN2003 outputs so they will sink the current . Now there is another pin ( pin 9) which is labeled common and I believe from looking at the data sheet that this is for the reverse protection diodes when the relays are turned off , to dump the back emf . My problem is I am not quite sure where to connect this common pin . I am guessing to the 5v line , but is that right or have I missunderstood.

Lastly , will the relays still turn on with a high on the picaxe input pins or will it reverse the action so that a low turns the relay on

Regards
Don

 

[MartinM57]

Your pretty much right in everything:
- no resistors needed (datasheet shows a 2k7 resistor inside each driver)
- one side of relay to +v, other side to darlington output
- common pin to +5v
- high on darlington input turns darlington pair on, so current is sunk and relay turns on

Manual 3, page 6...

 

[donrecardo]

Thanks Martin
I thought it was right but something was nagging away
in the back of my mind giving me doubts .
I think my brain was having a senior momment , I seem to have a
lot of those these days

Don

Your pretty much right in everything:
- no resistors needed (datasheet shows a 2k7 resistor inside each driver)
- one side of relay to +v, other side to darlington output
- common pin to +5v
- high on darlington input turns darlington pair on, so current is sunk and relay turns on

Manual 3, page 6...