Current Transformers

[RogerN]

In the winter I use oil filled electric radiators in the house to help even
out the heat. The problem is that I always have to remember to shut off the
heater to use other appliances on that circuit, such as the microwave oven.
If I could use a current sensor, and shut off the heater whenever the line
current is above 15A, and not turn back on until current is less than 5A,
that would save many circuit breaker resets.

Some of the current transformers I have seen give an output of 1mV per amp.
I'm guessing that this 1mV is AC? If so, how would you rectify it to use as
a DC reference voltage with comparators? (to turn a relay on or off at a
certain level) A rectifier would loose some 700mV and a 700A error is not
acceptable for a 15A circuit . Should I use an op-amp with feedback and
rectifier feeding a capacitor to charge to peak voltage? Just curious how
you normally use instrumentation with a signal 1mV AC.

Thanks!

RogerN

 

[Tim Wescott]

On Wed, 23 Sep 2009 19:59:10 -0500, RogerN wrote:

In the winter I use oil filled electric radiators in the house to help
even out the heat. The problem is that I always have to remember to
shut off the heater to use other appliances on that circuit, such as the
microwave oven. If I could use a current sensor, and shut off the heater
whenever the line current is above 15A, and not turn back on until
current is less than 5A, that would save many circuit breaker resets.

Some of the current transformers I have seen give an output of 1mV per
amp. I'm guessing that this 1mV is AC? If so, how would you rectify it
to use as a DC reference voltage with comparators? (to turn a relay on
or off at a certain level) A rectifier would loose some 700mV and a
700A error is not acceptable for a 15A circuit . Should I use an
op-amp with feedback and rectifier feeding a capacitor to charge to peak
voltage? Just curious how you normally use instrumentation with a
signal 1mV AC.

Thanks!

RogerN

Or find a more sensitive current transformer...

--
www.wescottdesign.com

 

[Phil Allison]

"RogerN"

In the winter I use oil filled electric radiators in the house to help
even out the heat. The problem is that I always have to remember to shut
off the heater to use other appliances on that circuit, such as the
microwave oven. If I could use a current sensor, and shut off the heater
whenever the line current is above 15A, and not turn back on until current
is less than 5A, that would save many circuit breaker resets.

Some of the current transformers I have seen give an output of 1mV per
amp. I'm guessing that this 1mV is AC? If so, how would you rectify it to
use as a DC reference voltage with comparators? (to turn a relay on or
off at a certain level)

** A simple over current alarm might be a better solution.

A few turns of mains rated wire around large size reed relay can be the
sensor - adjust the position of the reed to get the trip level you want.
The relay contacts can then trigger a 555 monostable to drive a 12 volt
piezo beeper.

There may be a short beep when the microwave comes on due to inrush surge.



..... Phil

 

[DJ Delorie]

A current transformer is just that - a *current* transformer. In
normal operation, it creates a fixed current regardless of the voltage
drop required, up to a given maximum voltage. In theory, if you put a
bridge rectifier on the signal, you still get the same *current* so
you can drop it across a load resistor and still get reasonable
outputs.

For example, a 1000:1 current transformer on a 15 amp line gives 15
milliamp output, not millivolts. You need to run that current through
a load resistor to generate a voltage.

However, most uses of current transformers use some form of sensitive
ADC to measure the signal, so they wouldn't rectify it anyway.

You could, however, use a comparator on an AC signal if you use it to
drive a 555 one-shot with a long enough period to stay "on" between
cycles. Look up "missing pulse detector".

 

[Rich Grise]

On Wed, 23 Sep 2009 19:59:10 -0500, RogerN wrote:

In the winter I use oil filled electric radiators in the house to help even
out the heat. The problem is that I always have to remember to shut off the
heater to use other appliances on that circuit, such as the microwave oven.
If I could use a current sensor, and shut off the heater whenever the line
current is above 15A, and not turn back on until current is less than 5A,
that would save many circuit breaker resets.

If you do this, you'll find that you've created a huge oscillator.

Put it on another circuit, (i.e., from a different breaker in the panel)
and you'll have no problem. If you'd need to rewire the house to do this,
then find another outlet that _is_ on a different circuit, and get a heavy-
duty extension cord, preferably one that's the same color as the
baseboards.

Good Luck!
Rich

 

[default]

On Wed, 23 Sep 2009 19:59:10 -0500, "RogerN" <regor@midwest.net>
wrote:

In the winter I use oil filled electric radiators in the house to help even
out the heat. The problem is that I always have to remember to shut off the
heater to use other appliances on that circuit, such as the microwave oven.
If I could use a current sensor, and shut off the heater whenever the line
current is above 15A, and not turn back on until current is less than 5A,
that would save many circuit breaker resets.

Some of the current transformers I have seen give an output of 1mV per amp.
I'm guessing that this 1mV is AC? If so, how would you rectify it to use as
a DC reference voltage with comparators? (to turn a relay on or off at a
certain level) A rectifier would loose some 700mV and a 700A error is not
acceptable for a 15A circuit . Should I use an op-amp with feedback and
rectifier feeding a capacitor to charge to peak voltage? Just curious how
you normally use instrumentation with a signal 1mV AC.

Thanks!

RogerN

I wanted to tell when my water heater was actually pulling current

(heating water) since I installed a switch on it to save power and
still wanted to take a hot shower now and then.

I had a small power transformer with side by side windings on the
split bobbin. I cut the secondary off and wound a coil tapped at 5
turns, 10,15,20,25 turns on it and ran one side of the power line from
my 40 watt soldering iron through the taps and rigged two switches
with 12 positions - one with 47 ohm resistors and one with 470 ohm
resistors in series - giving me 50 load (called "burden" in current
sensing speak) resistors from Zero to 2303 ohms and open.

What was the 120 VAC primary, on the transformer is the secondary for
my current sensing transformer.

Then I wired a voltmeter and AC ammeter across and in series with the
load (respectively) and collected data. The data went into a spread
sheet and I found my peak power at ~1250 ohms - but anywhere from 1000
to 2000 ohms was close enough.

I got ~7 volts @ 6 milliamps into 1175 ohms, with the 25 turn winding
and my 40 watt load. The 25 turn winding is the highest peak power
(naturally) and ~42 milliwatts.

On the water heater it's pulling much more current through a single
primary turn and gives about the same voltage out. I use the current
it generates to sound a buzzer when the heater turns on or off, and
flashes an LED while drawing power. A neon lamp tells me if I have
the water heater switched on.

--

 

[RogerN]

"Rich Grise" <richgrise@example.net> wrote in message
newsan.2009.09.24.17.22.37.356060@example.net...

On Wed, 23 Sep 2009 19:59:10 -0500, RogerN wrote:
snip
If I could use a current sensor, and shut off the heater whenever the
line
current is above 15A, and not turn back on until current is less than 5A,
that would save many circuit breaker resets.

If you do this, you'll find that you've created a huge oscillator.

Put it on another circuit, (i.e., from a different breaker in the panel)
and you'll have no problem. If you'd need to rewire the house to do this,
then find another outlet that _is_ on a different circuit, and get a
heavy-
duty extension cord, preferably one that's the same color as the
baseboards.

Good Luck!
Rich

I usually set the heater to the 900 Watt setting, that should give me less
than a 10A draw. I can put a time delay in the circuit incase the inrush
current is too high. I already use heaters on most of the other area
circuits. I use the main heat to get the house close to temperature and the
heaters warm the individual rooms a few degrees for even heat and comfort.
I guess this is more of a hobbyist way of having a thermostat in each room,
except my thermostat is a PID digital temperature controller (thermocouple,
soon to be RTD) firing a solid state relay. I know this is overkill but I'm
just having fun and learning too, in the summer the temperature control is
put to work on my Brinkman electric smoker so I don't have to rely on the
water pan to hold temperature.

RogerN

 

[RogerN]

Thanks for the excellent replies from all. The current transformers I saw
the spec of 1mV per Amp was the clamp on kind you use with your meter to
read higher currents. With a current output and enough voltage available to
get through a rectifier, I should be able to get a reasonable voltage output
to use for current indication.

As many mentioned, there are better and more practicle ways to solve the
problem but one of my goals is to learn about current transformers and I may
use this with a comparator circuit and/or I may connect it to the analog
card in my PLC. After I learn to use the current transformer I may end up
monitoring the current draw for my house or something.

RogerN

 

[John Fields]

On Sat, 26 Sep 2009 14:56:23 -0500, "RogerN" <regor@midwest.net> wrote:

Thanks for the excellent replies from all. The current transformers I saw
the spec of 1mV per Amp was the clamp on kind you use with your meter to
read higher currents. With a current output and enough voltage available to
get through a rectifier, I should be able to get a reasonable voltage output
to use for current indication.

As many mentioned, there are better and more practicle ways to solve the
problem but one of my goals is to learn about current transformers and I may
use this with a comparator circuit and/or I may connect it to the analog
card in my PLC. After I learn to use the current transformer I may end up
monitoring the current draw for my house or something.

---
If you're not familiar with current transformers, you may not be aware
that they can be _very_ dangerous if operated without a proper burden
across the secondary.

http://en.wikipedia.org/wiki/Current_transformer#Safety_precautions

Be careful!