Re: AC Current to Voltage Transducer

[Ant Skelton <ant@xxxxxxx>]

Christian Cundall wrote:


> However wiring help please. It has +15 volts, - 15 volts, output and 0
> volts.

> I am currently not getting anything out of it, can anyone help?
> 
> i.e. what do I wire to what? I have never understood -15 volts


-15 volts is like 15 volts, except in the other direction! Imagine two 
15v batteries like this:

         
---------.            
---------.
       A  -      + ]
B ----  C  -      + ] D

        
---------'            
---------'


If you connect B directly to C so that the two batteries are in series,
you effectively have a 30v battery, ie the voltage across the terminals
A and D is 30v.

Voltages are always measured "across" or "between" two
points, like 
distance. "How far is it to the shops?" can only accurately be
answered 
if you know where you're starting from. This is why engineers often 
refer to voltage as "potential difference" - it's the difference
in 
'potential' between two points in a circuit.

However, if we were chatting and I asked you "how far is it to the
shops?" you'd probably assume that I meant "from where we
are" and be 
able to provide an answer (it's about half a mile, FYI). That is, we 
have an implied reference point, so you're actually giving me a distance
>from
Similarly, the implied reference point for measuring voltages in a 
circuit is the 0 volt "rail", A in this example. So we can say
"D is 
30v" but what we really mean is that "D is +30v with respect to
A".

Now we could get really fancy, and decide to call movement from
the reference point positive polarity, and movement toward the reference
negative polarity. Thus it's +0.5 miles to the shop, yet -0.5 miles 
back, from the viewpoint of where we originally started.

Because electric current likes to move from a high potential to a lower
one, we label D positive and A negative, because we know that 
conventionally current moves from positive to negative*.

Thus measured from A, D is at +30 volts. If we were measuring from D, A
would appear to be at -30 volts.

Now, what happens if we decide to use the B-C connection as our 
reference point? That'd be like asking the distance from here to the 
shops as measured from the pub, which is exactly half way in-between.

The distance between here and the pub (the reference point) is +0.25 
miles, because it's a distance of 0.25 miles and we're moving pubwards.
After a swift half we want to press on to the shops though, which 
involves moving -0.25 miles, because it's a distance of 0.25 miles but 
it's anti-pubwards. We've still moved a total of 0.5 miles in absolute 
terms, but from the pub's point of view, we're as far away as when we 
started.

So with B-C as our reference, D is at +15 volts, and A is at -15 volts.

By "D is at +15 volts" we mean p.d. between them is 15 volts, and
current flows from D to B-C (ie towards 0v).

By "A is at -15 volts" we mean the p.d. is 15 volts, and current
moves 
>from
So in wiring up your sensor, you need to provide +15v to pin 1 

(with respect to pin 4, 0v) and -15v to pin 2 (with respect to pin 4,
0v).


If you were using the above two batteries, you'd connect pin 1 to D, pin
2 to A, and pin 4 to B-C.

The output you get from the sensor (on pin 3) alternates between +4v and
-4v, relative to the 0v line (pin 4 and B-C). The polarity of your 
reading indicates the direction of current flow in the wire you're
sensing.


Right, I'm off to the shops. Via the pub.


cheers


ant

[ * Actually it's electrons flowing the other way, but that's another 
story. ]

-- 

                            
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