How to "sense" 1.5 Amp's of current?

[Krumlink]

I have been looking at how to sense current draws, but cannot find any good links, etc.

Anybody have any good links for a device that can sense current, around 1.5 Amps?

 

[dknguyen]

**broken link removed**
**broken link removed**

 

[Mikebits]

Simple, touch bare wire, if your hand jerks away, there is current. J/K
The way I have seen it done is to use a real low ohmic value resistor. When current flows through your line you can detect the voltage drop across the resistor. @1.5A using a .1 ohm you would have a .15v drop. This would be a 1/4w resistor. If you want more sense volts, you have to increase R, but then your VR drop goes up as well as power through the resistor.

DK beat me to the post. The IC method sounds cool, but old fashion methods work too.

 

[Krumlink]

Would there be any way to measure it? as in I would be able to distinguish 1.5Amps from say .75 Amps?

 

[dknguyen]

For the current sense resistor method:

You measure the voltage drop across the resistor. It is proportional to the current flowing through it. Lower R = less losses, less sensitivity. Higher R = higher losses, more sensitivity. It's a trade off and doesn't work in high power applications since R would need to be much too low.

 

[Krumlink]

Well it may help If I tell you what Im up to:

I am trying to sense when a outlet is using 1.5 Amps of current, it will turn on a relay .When the 1.5 Amps of current drops down to 0.75 Amps of current, it will turn off the relay. Perhaps that will help.

 

[Mikebits]

E=IxR so 1.5Ax1ohm =1.5v .75Ax1ohm = .75v scale resistor as needed. If you plan on using a pic, the A/D could do the math.

 

[Krumlink]

So a 1 ohm resistor then? How/where would I measure the voltage drop?

 

[Krumlink]

OH forgot to mention, 120VAC. Sorry.

 

[Mikebits]

Oh boy, that it different, and scary. Mains can be a hazard. I think this may be better suited for someone else to answer, as I am too poor for a law suit

 

[Mikebits]

Maybe you should have a look at current sense transformers for AC.

 

[Krumlink]

What if I stepped down the voltage, say to 12VDC?

 

[Mikebits]

That's a cool idea, but then you have to make sure your transformer primary is rated for the current of your AC breaker. A current sense transformer is non-invasive to your main circuit.
One example, but many more companies make these things so just search.
**broken link removed**

 

[Krumlink]

That would definately work, how would I use it though

 

[Mikebits]

Well, I am not an expert in this sort of thing, but I believe they work like any ordinary transformer. That is to say the mains line is the primary, the sense xformer is the secondary. There is a ratio of current exchanged between the primary and secondary as I am sure you already know. So you can figure 1A through primary X amount of amps will flow through secondary. This info is given in the current sense xformer data sheets.

So with x current flowing through secondary rectify AC, run voltage across a resistor, and using E=IxR you know voltage. Of course you must take into account E drop through rectifier.

This may not be the best way, but it gives you a general idea. I have a feeling someone will smack me, as I probably made it over complicated.

 

[ericgibbs]

hi Krumlink,

Use a 1R resisitor is series with the 0V line.

Connect a LM3914 input across the 1R, drive 10 leds with the outputs from the LM3914.

You can calibrate the LM3914 to give say 0.1A or 0.2A steps on the leds.

 

[kenjj1]

Hi. Roll your own relay. Put enough heavy wire of suitable guage on some soft iron, like a heavy nail. Secure this coil so it can't move. Get a reed switch and, while pumping 1.5 Amps through the coil you just built, slowly move the switch towards the coil. When the switch finally engages, you've found the 'sweet spot'. Secure the switch in place. Now you use a processor pin to detect the switch's closure, or an ohmeter with a continuity buzzer to signal closure, or whatever you envisioned. Maybe you can even find a relay built to do just this.

END TWO CENTS...
kenjj

 

[ericgibbs]

hi Krumlink,

Use a 1R resisitor is series with the 0V line.

Connect a LM3914 input across the 1R, drive 10 leds with the outputs from the LM3914.

You can calibrate the LM3914 to give say 0.1A or 0.2A steps on the leds.

hi,
I based this reply on your opening post, I see that the 'specification' changed as the thread developed.
So its now 120Vac mains, detecting a current window, as I cannot edit my previous post,
the solution I suggested [LM3914] will not apply.

 

[kchriste]

Use the current sensing toroid transformer. If you get a split core clamp on ferrite core and clamp it over one lead of a zip line cord, you can do this fairly safely.
You can buy commercial ones for this purpose:
**broken link removed**
Or you can roll your own transformer from an empty core:
**broken link removed**
which might be easier and cheaper to find. You'll need quite a few turns of wire on the secondary to get a decent sensitivity (500-5000 turns) so the premade one may be better for you.

 

[qsiguy]

Use a hall effect sensor, similar to what kchriste suggested, and a relatively simple comparator circuit. The hall effect sensor will output a voltage based on the current passing though it. Adjust the vRef on the comparator to match the voltage from the hall effect sensor when it reads 1.5 amps. You will, of course, need an ammeter to calibrate the circuit. Use the comparator output to drive a transistor which will drive a relay or whatever you need.