AC Current Sensing without using a current shunt

[jbarnaby2000]

I was hoping that someone maybe able to point me in the right direction here. I would like to Measure AC current to determine the power usage of a circuit. The current I am looking at is from 0A to 5A, at 120V AC. I have found several current shunts that will do it, but I was hoping to do it without a shunt resistor. I have found current loops like the one attached. But this one also measures upto 50A, and I only need about 5A. I am looking at making this as small a possible as well.

Any Ideas or places to point me to research this??

 

[RadioRon]

I was hoping that someone maybe able to point me in the right direction here. I would like to Measure AC current to determine the power usage of a circuit. The current I am looking at is from 0A to 5A, at 120V AC. I have found several current shunts that will do it, but I was hoping to do it without a shunt resistor. I have found current loops like the one attached. But this one also measures upto 50A, and I only need about 5A. I am looking at making this as small a possible as well.

Any Ideas or places to point me to research this??

Try googling "current transformer". Here's one link that I found:
https://www.crmagnetics.com/8300n.pdf

They also point to this:
**broken link removed**

 

[jbarnaby2000]

Perfect...the first one is exactly what I was looking for....

thx so much...

 

[jbarnaby2000]

Ok...now that I found the part I would like to use...the CR8401... I sort of know I have to thread the Hot side of the AC power through it and it generates an AC signal right?? I would assume that signal is proportional to the current running in that wire that is going through its core right??

If that is the case, I would like to measure that with a PIC's Analog to digital converter, so I would assume that I have to run it through a diode bridge of some sort to convert it to DC right??

 

[ericgibbs]

Ok...now that I found the part I would like to use...the CR8401... I sort of know I have to thread the Hot side of the AC power through it and it generates an AC signal right?? I would assume that signal is proportional to the current running in that wire that is going through its core right??

If that is the case, I would like to measure that with a PIC's Analog to digital converter, so I would assume that I have to run it through a diode bridge of some sort to convert it to DC right??

hi,
Have a look at this forum link:
https://www.electro-tech-online.com/threads/ac-bridge.34189/?highlight=opa+rectifier

If you need to measure from 0 thru 5A, a standard bridge rectifier would have problems at the lower voltages, remember the diode forward drop of about 0.7v.

 

[Boncuk]

This is a cheap one.

**broken link removed**

 

[ericgibbs]

This is a cheap one.

I agree, I would wind my own current/ferrite transformer.

 

[jbarnaby2000]

This is a cheap one.

**broken link removed**

Where can I get this??

 

[ericgibbs]

Where can I get this??

hi,
You get a ferrite ring [toroid] from an old computer, etc and wind up it yourself.
The thick wire on the ring is one line of the mains wiring.

If you look thru old electronic equipment you will find allsorts of interesting bits.

 

[jbarnaby2000]

I agree, I would wind my own current/ferrite transformer.

I would wind my own but I want to do a bunch of them, and I would really like a reproducable outcome. I also would do this if I just wanted to detect some AC voltage but I would like to measure the AC current in able to determine the power used or being used rather.

In terms of the output of this transformer, I would assume that it is a mirror of the input wave correct?? Just a lot smaller right??

 

[crutschow]

I would wind my own but I want to do a bunch of them, and I would really like a reproducable outcome. I also would do this if I just wanted to detect some AC voltage but I would like to measure the AC current in able to determine the power used or being used rather.

In terms of the output of this transformer, I would assume that it is a mirror of the input wave correct?? Just a lot smaller right??

It is indeed a "Current Transformer" thus it generates a current which is a mirror of the current going through it's center. The transformers are designed to generate a voltage across a relatively small value load resistor which is determined by the transformer's turns ratio and design (too high a value will reduce accuracy). The AC voltage accross this resistor can be measured, or you can meaure the winding current with an AC current meter.

You can not directly rectify the transformer current to get DC but you could amplifiy and rectify the voltage across the load resistor. For best accuracy you would use an RMS IC which converts the AC to a DC voltage equal to the RMS value.

CAUTION: Current transformers are not designed to work into a high impedance and can generate dangerously high voltages if their outputs are open.

 

[jbarnaby2000]

Ok so the ultimate question is....

How do I hook this to a PIC's ADC to get a value for the AC current then??

 

[Nigel Goodwin]

Ok so the ultimate question is....

How do I hook this to a PIC's ADC to get a value for the AC current then??

Rectify it using a true RMS rectifier, and measure the voltage. You need true RMS as many items (switch mode units etc.) don't produce consistant sinewave currents in the supply.

 

[jbarnaby2000]

Rectify it using a true RMS rectifier, and measure the voltage. You need true RMS as many items (switch mode units etc.) don't produce consistant sinewave currents in the supply.

So you are saying, use a unit like this one: AD736 and then the output of this chip can connect directly to the PIC ADC port right??

So a higher RMS voltage would equate to a higher RMS current of the wire being measured right??

 

[Nigel Goodwin]

Yes, something like that IC - to measure the current you connect a load resistor across the output of the transformer, and then measure the voltage across it (simple ohms law). Higher the current the higher the voltage across the resistor.

 

[RODALCO]

I have successfully used an old 220 / 5 or 8 volt TX and wound a couple of turns of 1.5 mm² wire around the core.

The 8 volt or the 220 volt output can be used for the measuring circuit. In case of the 220 volt winding, put a resistor 100 or 470 ohm ½ watt across the terminals. A CT does not like open circuit voltages across it's terminals.

I used above system to check my outside security lights. lamp ON, LED ON. circuit working OK.
The LED is directly driven from the CT - TX. with an antiparrallel diode (1N914).

I made up 4 of these CT's to measure all 4 outside lights.

 

[jbarnaby2000]

I used above system to check my outside security lights. lamp ON, LED ON. circuit working OK.

Thats awesome, do these measure current or just an AC presense?? I really want to measure an AC current so I can determine how much power that circuit is using. I guess I am looking for something a little more consistant in the actual measurement. If I were just needing to detect an AC current(to just detect if something is on or not) I am sure your idea would be a wonderful one.

Thx again...

 

[Boncuk]

Where can I get this??

There are toroidal core inductors used in many dimmer circuits. You might want to look up those and make a primary winding using isolated electric cable. (of course the cross section fitting your needs)

The ratio should be about primary:secondary=1:10.

Hans

 

[RODALCO]

I use my circuit to detect a current flow in an AC circuit.

Current flow, lamp working, CT secondary drives LED on indicator panel in my hall.

In commercial metering proper CT's are used.
These are mounted at a link section in the main busbars of a switchboard.

Ratios may vary from 50/5 to 2000/5 Amps.
Generally these CT's have a seconday of 5 Ampères which will drive the kWh or kVArh metering via a testblock. The voltage potential is directly taken from the busbars via potential fuses.

On high tension sites also voltage transformers are used and the metering potentials generally run on 57½/100 or 63/110 Volts.
These values ascertain safe voltages on the metering equipment and create easy multipliers on 10 and 11 kV systems or multiples thereoff.
The yellow phase is usually earthed at the secondary of the VT.

 

[tyru]

JBaraby,

Did you ever get the circuit figured out? I'd be very interested.

I have a similar interest. Want to use a CT to measure current (0-5 A) on a motor to detect a jam condition using an a/d on a pic micro controller. I need a circuit to convert the AC volt to DC Volt (0- 5 VDC) for the pic input.

found this at the CR micro site.

https://www.crmagnetics.com/pdf/ancrct-4.pdf

Any help would be appreciated