quick electronics q: sensing current without taking it?

[dpopiz]

how can I "sense" whether or not there is current flowing on a wire without drawing any current from it?

 

[kaizersose]

technically, you cant. the heisenberg uncertainty principle (i could be wrong on the name) says that you cannot measure a system without affecting it. example: by measuring the temperature of water, any measuring device you insert you will introduce a temperature change.

in reality, if you want to measure current, you would use an ammeter which theoretically has infinite resistance (since its wired in parallel to the line you want to measure, no current shoud flow through it). however, a small amount of the current will be lost to the meter but unless you need superacurate results, its not that big of a deal. if you really need accuracy, you are going to have to spend spend spend.

if you just want to know whether current is simply present and not actually measure it, bring a compass close to the wire and see if the needle aligns itself suddenly (perpendicular to the current flow). if the current is too small, this may not work though. hope that helps!

 

[thorin]

Originally posted by: kaizersose
technically, you cant. the heisenberg uncertainty principle (i could be wrong on the name) says that you cannot measure a system without affecting it. example: by measuring the temperature of water, any measuring device you insert you will introduce a temperature change.

in reality, if you want to measure current, you would use an ammeter which theoretically has infinite resistance (since its wired in parallel to the line you want to measure, no current shoud flow through it). however, a small amount of the current will be lost to the meter but unless you need superacurate results, its not that big of a deal. if you really need accuracy, you are going to have to spend spend spend. hope that helps!

Hmmm I don't really agree with that statement do you think pointing a radar gun at a car causes it to slow down (even an unperceptable amount) ? Can't temperature be measused by raidiant energy (externally without affecting how the body is raidiating such energy). The wire in question would have to generate magnetic, thermal, photonic or other externally detectable elements in normal operation that can be observed without increasing or decreasing those elements. (Elements isn't the perfect word in there but I couldn't think of anything better while throwing this together).

Thorin

 

[oog]

the heisenburg uncertainty principle has to do with things at a quantum level, not every day macro-sized stuff. even so, the answer about measuring current with a high resistance device will do fine in most cases.

 

[dpopiz]

well, here's what I want to do:
I'll have an led wired up to a power source that can be either on or off. I want to have something that will electrically switch a seperate circuit on or off depending on what state the led circuit is in. however, it can't affect the brightness of the led or the amount of current it draws very much and I have to be able to see the led (can't just put a phototransistor in front of it)

 

[dullard]

1) What is the LED voltage?
2) What is the LED current?
3) Is the power supply current maxed?

I'm basically thinking of a relay. If the power source current is not maxed, then put the relay in parallel with your LED - and your LED will be unaffected. If your power source current is maxed already, a relay that uses little power could be put in series with the LED and yes it would affect it, but maybe not too much. I get all my relays here due to their great prices and wonderful fast service.

 

[dpopiz]

I don't know. it's a power led connected to a motherboard

 

[merlocka]

Originally posted by: kaizersose
technically, you cant. the heisenberg uncertainty principle (i could be wrong on the name) says that you cannot measure a system without affecting it. example: by measuring the temperature of water, any measuring device you insert you will introduce a temperature change.

in reality, if you want to measure current, you would use an ammeter which theoretically has infinite resistance (since its wired in parallel to the line you want to measure, no current shoud flow through it). however, a small amount of the current will be lost to the meter but unless you need superacurate results, its not that big of a deal. if you really need accuracy, you are going to have to spend spend spend.

if you just want to know whether current is simply present and not actually measure it, bring a compass close to the wire and see if the needle aligns itself suddenly (perpendicular to the current flow). if the current is too small, this may not work though. hope that helps!

Hmmm, I always thought an ammeter would have a very small resistance and would be wired in series... but what do I know. A voltmeter would probably do well to have a very high resistance an be wired in parallel.

 

[kaizersose]

just to clarify about a couple of things:

1. the heisenberg uncertainty principle was originally used for quantum applications, but it does apply for other circumstances. also, its purely theoretical--not a law.

2. i could be confusing ammeter application with voltmeter (i majored in mech eng, not EE), but the end result is the same: an imperfect measurement that works fine for 99% of all applications.

i know i am not the high priest of truth, just trying to offer some advice.

 

[LordMorpheus]

Originally posted by: kaizersose
technically, you cant. the heisenberg uncertainty principle (i could be wrong on the name) says that you cannot measure a system without affecting it. example: by measuring the temperature of water, any measuring device you insert you will introduce a temperature change.

in reality, if you want to measure current, you would use an ammeter which theoretically has infinite resistance (since its wired in parallel to the line you want to measure, no current shoud flow through it). however, a small amount of the current will be lost to the meter but unless you need superacurate results, its not that big of a deal. if you really need accuracy, you are going to have to spend spend spend.

if you just want to know whether current is simply present and not actually measure it, bring a compass close to the wire and see if the needle aligns itself suddenly (perpendicular to the current flow). if the current is too small, this may not work though. hope that helps!

You are wrong about the ammeter, Ammeter has near zero resistance and is placed in series, voltmeter has near infinite resistance and is placed in parallel.


But, you can measure current without all that stuff.

Moving charge creates a magnetic feild. If it is direct current, just hold a compass above (not next to) or below the wire and watch its deflection. With some physics calculations, you will be able to match an angle of deflection with a current through the wire, and there you have it.

 

[dpopiz]

I was hoping there would be some simple way I could splice a transistor into the led circuit. remember, I don't need to measure anything, I just need to tell whether the circuit is on/off.