high sensitivity current sensing

[fridgedr]

Hi,
As part of an invention I’m working on, I’ve found myself in a technological realm I have no training in, and know little about.

I’m wondering if anyone here may know of a simple electronic method, circuit, or perhaps integrated circuit that would provide an indication that a small load, around one tenth of an amp, has been turned on. Contacts closing in a relay or optically isolated switch would be perfect as only the analog state of the load needs to be sensed by a typical PC. Measuring the actual amount of current is not important.

The sensing circuit or device can’t tamper with the load it is sensing so a current transformer such as the CR19 would have seemed to be the perfect solution. However, based on my experiments, before these transformers generate enough current to even turn on an LED, the load its sensing needs to be drawing at least one amp.

Therefore could the small amount of current generated by the current transformer be used to trigger a transistor or perhaps be amplified? It should be mentioned the load can draw as much as 20 amps so the device or circuit may require some type of over current protection as well.

Please be specific, as I can accurately build circuits from a diagram but little else.

Thanks for any help anyone may be able to offer

[frodo.rok]

I think the problem will pesists if you continues to have a current transformer as your current sensing device.
Because current transformers are made to have low current in load side or sensing side.And you need a relatively high current on sensing side.
Also building of enough magnetic flux in the circuit will take time which you cannot allow.

You can solve both of this problem if you chose a PERMANENT MAGNET type relay circuit to switch on the LED circuit which will be supplied power through battery.
Just try to remember the famous experiment where 'a permanent magnet placed parallel to a current carrying coil moves clockwise or anticlockwise depending upon the direction of current', and you will get the idea.

[frodo.rok]

It is difficult to get a commercial PM releay switch but you can make your own arrangement it will only need a seperate circuit for LED and a permanent magnet placed parallel to the load current carrying wire.
the relay switch will be normaly open and will close when the current reaches the value 0.1amp.This will turn on the LED.

[vissin]

As I understand it, you only want to sense the presence of current and not measure it. In this case you don't need a very complicated circuit.

As for the current limits, neither is a tenth of an Amp too small a current nor is 20A too large.

The simplest way to sense the presence of current without disturbing it much is to use a shunt. You only need to Ohm's law. Process the drop across the shunt with a comparator and use it's output to drive a relay.

[frodo.rok]

But Vissin he does not want to affect the original circuit anyhow.
He wants to sense the current without affecting the current through the original circuit and for that he needs to isolate its sensing circuit from the original one.
Practicaly it is though impossible to have no effect on original circuit but use of isolated circuit will have less effect than the use of shunt.

[reachrkata]

Just a small change in Vissin's logic. no need of another shunt. the comparator needs to be connected across the "load" through which the 0.1A current needs to be measured.

One thing is sure. No measurement technique whatsoever can do a measurement without affecting the load itself. So simple is to use the op-amp because they usually have pretty high impedances and therefore have very negligible effect on the load.

Karthik