Question Driving BiStable Relay

Why are you still using the 2803? You'll probably need more than 12V for it to work with a 12V relay.

Blueroom .. As I said earlier . I loved the simplicity of your posted design but I am stuck with this one. I must use ULN28003 , regardless of this being an inefficient design. Once the concept is proven then I will move in the direction efficiency.

Well get unstuck, it's a poor design.

Hi xeeshan74,

as Bill already stated this is a very poor design. Connecting a 12V relay in series with a 20Ω resistor the relay won't work at all if its coil resistance is almost the same as the series resistor.

The ULN2XXX don't switch ground directly, but they contain darlington transistors, so I'd assume at least 1.4V voltage drop between emitter and collector leaving 10.6V for the relay, which could work properly if there wasn't an additional voltage drop across the resistor of at least 0.5V. (assuming 400Ω coil resistance)

Considering you have to drive up to four relays that makes an unneccessary pin count of 32 pins overall. (Compared with 4 MosFet drivers of 8 pins and 4 ULN2003 of 16 pins). This method is not only unreliable, but requires much more board space.

You should convince your boss, or whoever is ordering you to use ULN type transistor arrays to control the relays, that the already suggested method will be the much better choice. (Or have him build the circuit with 100% repeatability.)

The suggested way is absolutely reliable and requires less board space!

Regards

Boncuk

Thanks both of you.. There is no Boss ..
I am the Boss. I will order the parts tommorow.

Any help with the symantics is greatly appriciated.

will get L293DNE (couple)MC34151PG(couple) to try out and see..

The superior SN754410 may be slightly cheaper. They're pin compatible.

will order that as well.. look cheap .. thanks for the suggestion...

Hi xeeshan74,

disregard part of my latest post.

That circuit won't work at all!

Electricity is pretty lazy and always uses the shortest path. In that particular case why should it detour through the relay coil if you offer it a 20Ω resistor?

It would work using two relay coils, but never with one and relay relay coil terminals permantly connected to +12V via 20Ω resistors.

Connecting an ammeter between +12V and the resistor should show you a reading of 565mA if the relay is supposed to be energized.

This current corresponds to +12V - 0.7V voltage drop of the ULN and the given resistance.

Boncuk

I see your point . but as taken blueroom and your suggestions I am droping it completely. Though I took this Design off internet.. Just check my first entry and you wil find the PDF link there. Probably you already saw it.

xeeshan74 said:

I see your point . but as taken blueroom and your suggestions I am droping it completely. Though I took this Design off internet.. Just check my first entry and you wil find the PDF link there. Probably you already saw it.

Click to expand...

I saw it already. Not everything found in the internet must necessarily be good.