I know enough to be dangerous...
Archiver
Posts: 46,084
This is a good subject line that someone used once before.
I have what I hope is a simple problem. Perhaps because I can't remember
much from the EE classes I took in the 70's. (I wound up as an IE - and
probably for a good reason!)
I'm attempting to use a small current transformer on the line of an AC load,
and I want a voltage output to be proportionally linear so I can read it
with an ADC, in turn, into my controller. The dropping resistor idea is
great and simple, but whatever R value I try works up to a point, then, the
curve reverses and decreases as my load current continues to increase. What
am I missing?
Chris
I have what I hope is a simple problem. Perhaps because I can't remember
much from the EE classes I took in the 70's. (I wound up as an IE - and
probably for a good reason!)
I'm attempting to use a small current transformer on the line of an AC load,
and I want a voltage output to be proportionally linear so I can read it
with an ADC, in turn, into my controller. The dropping resistor idea is
great and simple, but whatever R value I try works up to a point, then, the
curve reverses and decreases as my load current continues to increase. What
am I missing?
Chris
Comments
If I understand your question, you are using a current transformer on your
AC line, and are experimenting with different value terminating sense
resistors. First, you must use a transformer rated to the max current being
measured. If it's too small, the core will saturate, and output will fall
off. Second, current transformers usually come with a recommended
terminating resistor. (Some units have it built-in). Using a larger value
than recommended can cause saturation at the high end. It is OK to use a
smaller resistor, but volts-out/amp will be proportionately less.
Ray McArthur
Original Message
From: Chris Loiacono (E-mail) <chris01@t...>
To: Basicstamps Egroup (E-mail) <basicstamps@yahoogroups.com>
Sent: Wednesday, March 21, 2001 1:40 PM
Subject: [noparse][[/noparse]basicstamps] I know enough to be dangerous...
> I'm attempting to use a small current transformer on the line of an AC
load,
> and I want a voltage output to be proportionally linear so I can read it
> with an ADC, in turn, into my controller. The dropping resistor idea is
> great and simple, but whatever R value I try works up to a point, then,
the
> curve reverses and decreases as my load current continues to increase.
What
> am I missing?
Yup, that's basically the question. I have the CT secondary into a FW
Bridge. The DC points on the bridge have the sense resistor and a filtering
cap across them. The - side goes to ground and the + is for my analog input.
The problem is that any value resistor that I try, down to 50 ohms has the
output reducing (seems like the high end saturation you mention) instead of
increasing once a certain level has been reached. My CT is a 1,000 turn
unit, and I'm only sensing a load that goes from zero up to about 1.6 Amps.
I am hoping to use this for larger loads, and I'm not getting any closer.
Perhaps I only need something like 5 or 10 ohms.....Hmmmm. Since the CT is a
constant current source, is there any danger in using such a small
resistance?
______________________ +
| /
CT 100uF = \ R?
| /
______________\_______ gnd
Chris
>
Original Message
> From: Ray McArthur [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=6KSvGDoLl3HQQXqiHtgTiHsK7xfp5XO-69-wCyOFCVXVPACRLG_xLMEXQXFVh2Rt9T11DTossByD3eYLnQ]rjmca@u...[/url
> Sent: Wednesday, March 21, 2001 3:56 PM
> To: basicstamps@yahoogroups.com
> Subject: Re: [noparse][[/noparse]basicstamps] I know enough to be dangerous...
>
>
> Chris:
> If I understand your question, you are using a current
> transformer on your
> AC line, and are experimenting with different value terminating sense
> resistors. First, you must use a transformer rated to the
> max current being
> measured. If it's too small, the core will saturate, and
> output will fall
> off. Second, current transformers usually come with a recommended
> terminating resistor. (Some units have it built-in). Using
> a larger value
> than recommended can cause saturation at the high end. It is
> OK to use a
> smaller resistor, but volts-out/amp will be proportionately less.
>
> Ray McArthur
>
>
Original Message
> From: Chris Loiacono (E-mail) <chris01@t...>
> To: Basicstamps Egroup (E-mail) <basicstamps@yahoogroups.com>
> Sent: Wednesday, March 21, 2001 1:40 PM
> Subject: [noparse][[/noparse]basicstamps] I know enough to be dangerous...
>
> > I'm attempting to use a small current transformer on the
> line of an AC
> load,
> > and I want a voltage output to be proportionally linear so
> I can read it
> > with an ADC, in turn, into my controller. The dropping
> resistor idea is
> > great and simple, but whatever R value I try works up to a
> point, then,
> the
> > curve reverses and decreases as my load current continues
> to increase.
> What
> > am I missing?
>
>
>
>
>
> Your use of Yahoo! Groups is subject to
http://docs.yahoo.com/info/terms/
Yes, the CT is a constant source and could even be shorted without damage.
If the smaller resistors don't solve the problem, the core is too small.
You need a conservatively rated unit because the FWB introduces an
additional 1.4 volts on the secondary. Digikey lists 50/60 Hz units on page
630 of their catalog. They start at 5 amps on up, and start at $7.80.
Ray Mc
> Yup, that's basically the question. I have the CT secondary into a FW
> Bridge. The DC points on the bridge have the sense resistor and a
filtering
> cap across them. The - side goes to ground and the + is for my analog
input.
> The problem is that any value resistor that I try, down to 50 ohms has the
> output reducing (seems like the high end saturation you mention) instead
of
> increasing once a certain level has been reached. My CT is a 1,000 turn
> unit, and I'm only sensing a load that goes from zero up to about 1.6
Amps.
> I am hoping to use this for larger loads, and I'm not getting any closer.
> Perhaps I only need something like 5 or 10 ohms.....Hmmmm. Since the CT is
a
> constant current source, is there any danger in using such a small
> resistance?
You may need to rectify it or use an RMS convertor like the AD-637.
On Wed, 21 Mar 2001 13:40:01 -0500 "Chris Loiacono (E-mail)"
<chris01@t...> writes:
> This is a good subject line that someone used once before.
> I have what I hope is a simple problem. Perhaps because I can't
> remember much from the EE classes I took in the 70's. (I wound up as an
IE -
> and probably for a good reason!)
> I'm attempting to use a small current transformer on the line of an
> AC load, and I want a voltage output to be proportionally linear so I
can
> read it with an ADC, in turn, into my controller. The dropping resistor
idea
> is great and simple, but whatever R value I try works up to a point,
> then, the curve reverses and decreases as my load current continues to
> increase. What am I missing?
>
> Chris
I just needed a different winding. Thanks to Ray McArthur, I found my CT to
be undersized, although it didn't exacly act as expected, a larger one
solved the problem. Now, I have a simple (4 component) circuit for reading
proportional values from AC lines. Thanks, Ray, and all who responded!
Chris
>
Original Message
> From: agarb@j... [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=ThZJHiG11TCO08AM6vJHwg0Fl4zoM2lKYO-568vOzXML0Uw32rGgqyo875mXr-XwSI12lNKJcGk]agarb@j...[/url
> Sent: Thursday, March 22, 2001 6:27 PM
> To: basicstamps@yahoogroups.com
> Subject: Re: [noparse][[/noparse]basicstamps] I know enough to be dangerous...
>
>
> Does this current transformer convert the AC into a DC
> representation?
> You may need to rectify it or use an RMS convertor like the AD-637.
>
> On Wed, 21 Mar 2001 13:40:01 -0500 "Chris Loiacono (E-mail)"
> <chris01@t...> writes:
> > This is a good subject line that someone used once before.
> > I have what I hope is a simple problem. Perhaps because I can't
> > remember much from the EE classes I took in the 70's. (I
> wound up as an
> IE -
> > and probably for a good reason!)
> > I'm attempting to use a small current transformer on the line of an
> > AC load, and I want a voltage output to be proportionally
> linear so I
> can
> > read it with an ADC, in turn, into my controller. The
> dropping resistor
> idea
> > is great and simple, but whatever R value I try works up to
> a point,
> > then, the curve reverses and decreases as my load current
> continues to
> > increase. What am I missing?
> >
> > Chris
>
>
>
> Your use of Yahoo! Groups is subject to
http://docs.yahoo.com/info/terms/