Help interfacing Current Transformers to ADC

[Joerg]

Hello John,

Nice, but it's still a lot of current to slam into the summing point
of an opamp. 20 amps RMS divided by 1000 is still 28 mA peak, and
metering systems should have a heap of headroom too.

I wonder what phase shift and linearity are like on these parts;
they're fairly small.

Usually there is a load (burden) resistor. The voltage drop is then fed
via a high impedance to an ADC or whatever active stage follows. The
high impedance being there just in case that burden resistor gives up
the ghost.

 

[Joerg]

Hello Yuriy,

Low-pass filer does not have to be active.
I did not think that I should state such trivial things...

Sorry, mea culpa. I meant to write active rectifier. If you really need
linearity down to zero one can use an inverted transistor as a rectifier.

 

[Jim Thompson]

Hello Yuriy,


Sorry, mea culpa. I meant to write active rectifier. If you really need
linearity down to zero one can use an inverted transistor as a rectifier.

If the "burden" resistor is after the rectifier you don't need
"active" rectification.

...Jim Thompson

 

[Jim Thompson]

Hello John,


Usually there is a load (burden) resistor. The voltage drop is then fed
via a high impedance to an ADC or whatever active stage follows. The
high impedance being there just in case that burden resistor gives up
the ghost.

If you study....

http://analog-innovations.com/SED/CurrentTransformer.pdf

The effective burden resistance can be made quite low, yet get 5V FS
performance.

If you take 5V as "burden", linearity will be bad.

...Jim Thompson

 

[Yuriy K.]

Fair enough. What is the turns ratio and burden limit for these?

Google does wonders.

http://www.alliedelec.com/Images/Products/Datasheets/967-1100.pdf

Jameco P/N 627591
Mfg TRIAD MAGNETICS
Mfg # CSE187L
RoHS? No
In Stock Yes

10+ 1.81 it will be $36.20 per 20 pcs. Who can beat that?

 

[beananimal]

Jameco P/N 627591

Mfg TRIAD MAGNETICS
Mfg # CSE187L
RoHS? No
In Stock Yes

10+ 1.81 it will be $36.20 per 20 pcs. Who can beat that?

Ok so these or the Avemco (or whatever they were).

Now, the past 6 or 7 exchanges have been interesting and I somewhat
follow, but am not sure where it leaves me. The gurus are starting to
talk among themselves (not a problem) but I am not sure what settled
upon consensus is and what I shoudl do with the information

I still understand that we are talking about active vc passive
rectification and/or filtration. The fact that you guys have narrowed
this down to two CTs and 2 basic topologies is certainly good news.

With regards to "my" need for a sampling rate for the data logging. 1
sec intervals would be fine. The software has a LOT to do otherwise and
I certainly don't want to spend CPU time to update the curernt
measurements if I don't need to. If the ony way to get reasonably
accurate readings is to sample the rectifier output at higher speeds
and average the data, then it will HAVE to be a uC that does this work
nonstop and then passes the data to the PC only when asked.

My need is mostly to keep a gui updated with somewhat realtime
information as to the status of my connected devices. I will be able
to set alarms in my software to trigger on abnormal trends. The actual
data logging to disk will be only several times an hour and when the
current draw exceeds a preset alarm. Long story short, I need to be
able to see how well the pumps and heaters are operating. As the pumps
build calcium deposits due to the heat, they begin to draw more
current. The current draw is a good indication of the pumps health. I
figured If I was going to monitor 10 pumps, I might as well monitor the
heaters, ballasts and fans.

This has been a very intersting learning experience so far, and I have
not even let the smoke out of anything yet!

 

[John Popelish]

Google does wonders.

http://www.alliedelec.com/Images/Products/Datasheets/967-1100.pdf

Jameco P/N 627591
Mfg TRIAD MAGNETICS
Mfg # CSE187L
RoHS? No
In Stock Yes

10+ 1.81 it will be $36.20 per 20 pcs. Who can beat that?

%00to 1 ratio is quite practical for up to 30 amps (60 mA out). With
a 60 ohm half watt burden, that gives 3.6 volts RMS. I would want to
either use a Schottky bridge with something like a 50 ohm burden or a
silicon bridge with about a 39 ohm burden. Even though the output
does not take up the full 5 volt A/D range, it allows for a bit of
surge current without producing a ridiculous voltage.

Very nice, if 30 amps load current is large enough.

 

[John Popelish]

With regards to "my" need for a sampling rate for the data logging. 1
sec intervals would be fine. The software has a LOT to do otherwise and
I certainly don't want to spend CPU time to update the curernt
measurements if I don't need to. If the ony way to get reasonably
accurate readings is to sample the rectifier output at higher speeds
and average the data, then it will HAVE to be a uC that does this work
nonstop and then passes the data to the PC only when asked.

If you intend to take a data point every second or so, you need to
average the signal over that amount of time (a 1 second or more time
constant) to get a representative value with each of those samples.

 

[beananimal]

%00to 1 ratio is quite practical for up to 30 amps (60 mA out). With
a 60 ohm half watt burden, that gives 3.6 volts RMS. I would want to
either use a Schottky bridge with something like a 50 ohm burden or a
silicon bridge with about a 39 ohm burden. Even though the output
does not take up the full 5 volt A/D range, it allows for a bit of
surge current without producing a ridiculous voltage.

Very nice, if 30 amps load current is large enough.

The largest loads will be

1) dual 150W magnetic MH ballast, so figure 450W draw during operation
and somewhat more than that due to the inrush when It powers on.
2) All of the pumps are less than 200W motors, and no more than 2 will
be on any given monitored outlet.
3) 500 watt heaters, 1 per monitored outlet.

So most of the stuff will be well less than 10A during operation,
inrush is a different story.

 

[Joerg]

Hello Jim,

If the "burden" resistor is after the rectifier you don't need
"active" rectification.

True. But it adds a bit of risk in that the rectifier might croak.
Probably not a huge risk if the diodes are nice and big. The inverted
transistor wouldn't be a very "active" rectifier except that it needs a
wee current into the base.

One issue to keep in mind with the "burden after diodes" approach is
harmonics. Might need little ceramics across the diodes so no hum is
modulated onto your favorite AM talk radio station

BTW, did you get that power-on detect installed for the kitchen TV? An
installation in our house is only considered "done" when the wife has
approved...

 

[Jim Thompson]

Hello Jim,


True. But it adds a bit of risk in that the rectifier might croak.

Come on, Joerg! You're picking at straws. Add zeners if you must ;-)

Probably not a huge risk if the diodes are nice and big. The inverted
transistor wouldn't be a very "active" rectifier except that it needs a
wee current into the base.

Who needs it?

One issue to keep in mind with the "burden after diodes" approach is
harmonics. Might need little ceramics across the diodes so no hum is
modulated onto your favorite AM talk radio station

Just like happens from your power supplies in the lab ?

BTW, did you get that power-on detect installed for the kitchen TV? An
installation in our house is only considered "done" when the wife has
approved...

Yes. But she-who-must-be-obeyed doesn't like the fan noise. So I'll
make a form-fitting shroud and use muffin fans.

Right now I'm sawing out pieces of ABS to make a wall-holder/charger
for the VTech ;-)

...Jim Thompson

 

[Michael A. Terrell]

Yes. But she-who-must-be-obeyed doesn't like the fan noise. So I'll
make a form-fitting shroud and use muffin fans.

Muffin fans, are you crazy? You aren't in the latte swilling North
East! You should use Biscuit fans like any good Southern boy. ;-)


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[Joerg]

Hello Jim,

Come on, Joerg! You're picking at straws. Add zeners if you must ;-)

I've seen the aftermath of a failed burden, on an RF board. It was not a
pretty sight. AFAIR even the fire alarm had gone off. And yeah, it was
after I was told "Joerg, you are too paranoid about that, it'll never
happen".

Who needs it?

Might be a few cents less ;-)

Just like happens from your power supplies in the lab ?

From the cheap computer age supplies, yes. The old Russky linear bench
supplies are very quiet.

Yes. But she-who-must-be-obeyed doesn't like the fan noise. So I'll
make a form-fitting shroud and use muffin fans.

If you could get a skinny squirrel-cage fan in there those can be very
quiet. Then whip up a VF supply and ask her "Can you hear it now?"

Right now I'm sawing out pieces of ABS to make a wall-holder/charger
for the VTech ;-)

Aw, c'mon. Didn't you do wood working once? How about a nice polished
teak wood Thompson-Design base.

 

[Jim Thompson]

Muffin fans, are you crazy? You aren't in the latte swilling North
East! You should use Biscuit fans like any good Southern boy. ;-)

I haven't bought a fan since all there were was muffin fans.

Right now I'm just using an 8" household fan.

How quiet is a biscuit fan?

...Jim Thompson

 

[Michael A. Terrell]

I haven't bought a fan since all there were was muffin fans.

Right now I'm just using an 8" household fan.

How quiet is a biscuit fan?

The ones I've used were very quiet. It's a slightly different style of
fan, but it has its uses.

<http://www.comairrotron.com/acblower_family.asp?FamilyID=2> is the
original source of these types.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[Joerg]

Hello Michael,

The ones I've used were very quiet. It's a slightly different style of
fan, but it has its uses.

<http://www.comairrotron.com/acblower_family.asp?FamilyID=2> is the
original source of these types.

Isn't that a really flat squirrel cage fan? Now I don't know how
"Southern" squirrels would be

 

[Michael A. Terrell]

Hello Michael,


Isn't that a really flat squirrel cage fan? Now I don't know how
"Southern" squirrels would be

Its biscuits that are a southern thing. Squirrels are all over the
place, including some newsgroups. (But they pretend to be bears and
stuffed donkeys when online)


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[John Woodgate]

In message <[email protected]>, dated Thu, 14

How quiet is a biscuit fan?

Very noisy, if you snatch his biscuit away.

 

[John Woodgate]

Isn't that a really flat squirrel cage fan? Now I don't know how
"Southern" squirrels would be

Sciurus roadkillensis. Very flat.

 

[Jim Thompson]

In message <[email protected]>, dated Thu, 14


Very noisy, if you snatch his biscuit away.

I forgot. To Brits and Aussies, biscuits are cookies ;-)

To me, a biscuit is what you pour sausage gravy over ;-)

...Jim Thompson