Current/voltage sensor

[lilimike]

It is for a 12 Volt application more specifically for a boat. As an example I'd like to know when the bilge pump is running or any other device such as a light, a motor, a TV. I don't need to know how much current is flowing, I just want to sens if the device is on or off. I don't want to have to cut wires, If I had a way to make something that looks like this it would be perfect. I am obviously also looking for the cheapest way as in under $10.

Any suggestions?

Mike

 

[alec_t]

You could use a Hall-effect current sensor next to a feed wire of the device being monitored.

 

[lilimike]

For some reason all the hall-effect sensors I have looked at requires to cut the device wire and have it go through its coil. I have seen some which the coil (or ferrite core) can open like an ammeter but those were $100+

Mike

 

[alec_t]

In the case of a motor-driven device (e.g. pump) you could fit a simple piezo element to it to sense vibration.
A thermistor could probably be used in contact with an incandescent lamp to sense it is on.

 

[dougy83]

For some reason all the hall-effect sensors I have looked at requires to cut the device wire and have it go through its coil. I have seen some which the coil (or ferrite core) can open like an ammeter but those were $100+

I'm not sure of the availability of these sensors, but they don't require cutting the wire **broken link removed**

You can probably just use a standard analogue hall effect sensor with the split ferrite ring. It is possible to split ferrite torroids yourself, but you can buy them in halves e.g. clip-on EMI/EMC ferrites. e.g. https://www.ebay.com.au/sch/i.html?_nkw=ferrite+clip&_sacat=0 You can also use a standard C-I core, or break a leg off an E core, after some grinding.

These devices produce an analogue output and you'll need a comparator to convert this to an "ON" or "OFF" signal.

You can also get hall effect switches that include the comparator.

 

[crutschow]

You could use a hall sensor, such as one of these, and make a sort of clamp on sensor by adding a magnetic toroid, such as one of these with a gap, to concentrate the magnetic field around the wire. You may have to do some experimenting to get your desired sensitivity. You can wrap several turns through the core to increase the sensitivity.

Edit: Dougy83 beat me to the punch with a similar suggestion.

 

[lilimike]

Other than the piezo, thermistor and incandescent lamp I was aware of the other options but was looking for a more simple solution. What if i wind the wire around a reed switch? how many turns would it take to trigger the reed switch if the device under test is between 1.5A to 4A?

Mike

 

[alec_t]

You would probably need lots of turns (depending on the switch sensitivity)! Also you would have to wrap only one of the two wires making up the cable going to the appliance.

 

[lilimike]

Would you define "A lot of turns" as 10 to 20 or 50+ ?
I guess I can just try to find the most sensitive switch and test, the problem is it will most likely be different with wire sizes and current consumption...

Mike

 

[crutschow]

Reed switch sensitivity is defined in AT (ampere-turns). These, for example, have a sensitivity varying from 15-35AT depending upon the model. Thus for 1.5A sensitivity the minimum number of turns for the most sensitive device would be 10 turns.

 

[kubeek]

Are you sure that putting the turns around the body will create the correct direction of the magnetic field to activate that switch?

 

[crutschow]

Are you sure that putting the turns around the body will create the correct direction of the magnetic field to activate that switch?

Makes no difference. A reed relay is simply a pair of spring contacts made from a magnetic material. The contacts are attracted to each other by reluctance due to a magnetic field of either polarity directed along the axis of the relay.

 

[kubeek]

That is what I meant, whether the field needs to be along the axis or perpendicular.

 

[lilimike]

Reed switch sensitivity is defined in AT (ampere-turns). These, for example, have a sensitivity varying from 15-35AT depending upon the model. Thus for 1.5A sensitivity the minimum number of turns for the most sensitive device would be 10 turns.

10 turns may be hard to do on a 12mm x 2mm unless I can overlap the turns and also 1.5A is probably a minimum so the higher the Amps the less turns required. I think I will play with these figures and do some testing.

Thanks for the info.

Mike

 

[crutschow]

That is what I meant, whether the field needs to be along the axis or perpendicular.

OK. You wind to coil around the reed switch, like a solenoid, so the magnetic field inside the coil is parallel to the reed relay axis.

 

[crutschow]

10 turns may be hard to do on a 12mm x 2mm unless I can overlap the turns and also 1.5A is probably a minimum so the higher the Amps the less turns required. I think I will play with these figures and do some testing.
.................

Overlapping turns should be fine.