18 replies to this topic

[chocamo]

I am new to the world of electronics and I want to tap into a signal to tell when something is on or off. When the external switch is on the current is 30amp and I want to sample it using netduino. I am thinking that using a relay and a resistor will be the best bet but I'm not really sure what to look for in one to be sure I don't break the netduino. Any tips would be great

[Mario Vernari]

Maybe it's me, but I have no clear what's your goal... Could you post any schematic about the switch (and what you're expecting on)? Cheers

[Chris Seto]

relays are for switching things on from the mcu, they cannot provide input like you seem to want to use them...

[chocamo]

Sorry I know I could have phrased that all a lot better. Here is a paint picture of what I want to do, basically tell when that switch has been flipped

[]Diagram

[Mario Vernari]

OK, is that current alternating or direct (AC/DC)? How much is the voltage on that line (approx)? Cheers

[chocamo]

DC and I believe its 12V

[Mario Vernari]

30Amps are 30Amps and I'd avoid to place anything is series, unless strictly necessary. Is all right to monitor the voltage after the switch, i.e. when the switch is closed also the voltage is 12V, otherwise is zero? That would be much better, if it is possible. Cheers

[chocamo]

I actually need to do this twice, once with a 5amp source and one with the 30amp source. With the 30amp source that is the only way to tap into it. With the 5amp source I could probably follow your suggestion. If you have a solution for either case I'd be happy to try them

[Nevyn]

I am new to the world of electronics and I want to tap into a signal to tell when something is on or off. When the external switch is on the current is 30amp and I want to sample it using netduino. I am thinking that using a relay and a resistor will be the best bet but I'm not really sure what to look for in one to be sure I don't break the netduino. Any tips would be great

Current clamp?

Regards,
Mark

[chocamo]

Current clamp?

Regards,
Mark

That's actually a good idea, how would I connect this to netduino though?

[Nevyn]

That's actually a good idea, how would I connect this to netduino though?

There is an energy monitoring project for the Arduino which you could use that as a starting point. Your original post suggests you just want to detect on or off. I'd think about calibrating the clamp and determining the on / off output readings. You could then use either the analog ports or if you wanted digital then maybe a transistor as a switch.

Hope this helps,
Mark

Edit: Remember to take into consideration that Arduino is 5V but Netduion is 3.3V.

[Mario Vernari]

Current clamp?

Stop guys: the current clamp needs AC current!
That's the way I asked for the kind of current...

The easier way is using a shunt.
Let's say you break the circuit and insert a resistor having a very small value in series.
Take a look at the schematic attached.
I supposed that:
the common ground of the Netduino can be connected to the battery negative wire;
the shunt (10milli Ohm resistor) is connected as the schematic.
At this point you may use an analog input to measure the voltage on point "A", and the circuit rightward this point could be discarded. Otherwise you may consider that additional circuit to use a digital input.

Note that 30Amps flowing through a 10milli Ohm shunt produces a voltage drop of about 0.3V. It means that your LOAD will be powered at 12-0.3 = 11.7V.
Cheers

Attached Files

•  current-detector.JPG   17.67KB   17 downloads

[Nevyn]

Stop guys: the current clamp needs AC current!
That's the way I asked for the kind of current...

Good catch.

BTW - Would it be advisable to use opto isolation?

Regards,
Mark

[Mario Vernari]

BTW - Would it be advisable to use opto isolation?

Absolutely.
However the circuit will turn more complex, although I'd prefer any kind of insulation at all.
Cheers

PS: "isolation" or "insulation"?

[CW2]

However the circuit will turn more complex, although I'd prefer any kind of insulation at all.

How about using (only) the optoisolator in parallel with the load? It will output digital signal according to the internal LED state, i.e. when the load is switched on/off.

[Mario Vernari]

How about using (only) the optoisolator in parallel with the load? It will output digital signal according to the internal LED state, i.e. when the load is switched on/off.

I asked it about yet, but it seems that's not possible for the 30A load. Take a look above.
Cheers

[CW2]

I asked it about yet, but it seems that's not possible for the 30A load. Take a look above.

Could you be more specific, please? Why it is not possible to add the optoisolator in parallel with the load?

[Mario Vernari]

@CW2: I don't know why it is not possible. I asked whether it is possible to monitor the voltage across the load, but (4th post) the poster say's not possible. I assume that's true, otherwise the task will turn very simple of course.

[CW2]

@CW2: I don't know why it is not possible. I asked whether it is possible to monitor the voltage across the load, but (4th post) the poster say's not possible.

I see, I missed that post, thanks. Unfortunately, the 'diagram' seems to be no longer available.