K8055 as a relay driver

Kits PC Related Projects
1

Hello…

I like to use the dig.out from K8055 to switch on relays, somehow i doesn’t work. I hope anyone could help me. the relay coil resistance are 995 Ohm and 156 Ohm.
Has anyone some schematic examples? programming is no problem.

Thanks

Willie

2

The K8055 digital outputs are open collector NPN transistors. You have to use an external voltage source for the relay coil.
Here is an example of the connection:

799×410

3

Hi,

I thought it was standard practice to have diodes protecting from back-emf?
Are they not needed for the motor and/or relay?

Brilliant diagram by the way, very helpful, thanks very much.

Kind Regards,
LLamnuds

4

You are right. The transient protection diodes have to be used.
If you look the internal circuit diagram of the ULN2803 you’ll see that there are internal diodes from each output to the CLAMP terminal (pin 10): datasheetcatalog.org/datashe … s/2801.pdf

This is why the positive pole of the external supply must be connected to the CLAMP terminal of the board.

5

Ah I see, I must look deeper, I understand - thanks very much for your prompt and clear reply

So if I’m reading that right the clamp can take up to 50V, and each output can handle a maximum of 350mA (or is it 100mA like the K8055 manual says)?

Kind Regards,
Llamnuds

6

[quote]So if I’m reading that right the clamp can take up to 50V, and each output can handle a maximum of 350mA (or is it 100mA like the K8055 manual says)?[/quote]Indeed, there is some safety margin in the K8055 specs.

7

OK, thanks very much.