AC current detection

trennonix · Dec 17, 2009

Hello,

(please look at the picture attached before reading)

When the relay ticks, i want to know if there's an attached load or if that load is actually working.
In order to do so, i want to detect the presence of a current.
I don't want to use a current transformer because i have little experience with it and it will be connected to a digital pin on a micro.
So i thought of a "device in question" that will:

*hold a voltage on its pins between 2 to 5 V (something that could be easily amplified with a transistor to reach 5V for the micro to handle)
*without causing a great limit in the current flow
(like a diode in DC circuit, it will hold 0.7V on its pins)

This is were I need your help: how do i adapt the diode concept to an AC circuit? (AC @ 220V)

Thanks

ronsimpson · Dec 17, 2009

I think you should rethink the current transformer idea.
R2 is optional if the transistor is too sensitive. (100 ohms)
R1 is a pull up resistor of 10k, or about any value.
T1 is any 60hz C.T.
The output is low if there is current flow.

KMoffett · Dec 17, 2009

Another option.

Ken

Boncuk · Dec 17, 2009

Hi trennonix,

why afraid using a current transformer?

It can easily be made using a toroidal coil as used in dimmers. Add four windings of flexible (insulated) wire for a secondary winding. (see attachment).

The power loss for the AC load depends on the maximum current the inductor can handle.

The circuit connected to the secondary winding delivers an output directly to the MCU.

The overall amplification factor is aproximately 1,000. If that doesn't suffice increase the value of R4. R7 is added to minimize pulse shaped charging peaks.

Note: Supply pins of ICs not included in schematic.

Boncuk

MrAl · Dec 17, 2009

Hi,

Current transformers are not hard to use, and you can make one by using
an old cheap wall wart.
If you still dont want that, then look for one of the Hall Effect current sensors
that are sold to sense current. They provide an isolated input for the ADC
too, and can sense both plus and minus currents for use with AC.
I think they come in 5 amp to 30 amp devices.
They run about 5 dollars each USD.

Boncuk · Dec 17, 2009

Here is the PCB design of the circuit I suggested.

The board is purely single sided and measures 1.8X1.4".

If you're interested leave a PM. I'll send you the Eagle files.

Boncuk

MrAl · Dec 17, 2009

Boncuk said:
Here is the PCB design of the circuit I suggested.

The board is purely single sided and measures 1.8X1.4".

If you're interested leave a PM. I'll send you the Eagle files.

Boncuk

Hi there,

Surely you can design a circuit simpler than that right?

Boncuk · Dec 17, 2009

MrAl said:
Hi there,

Surely you can design a circuit simpler than that right?

If there are "unknowns", e.g. the AC load current it's better to use amplifiers. Last not least the circuit output can be connected directly to an MCU without having to worry about insulation.

Boncuk

trennonix · Dec 17, 2009

Thanks alot for all your replies
guess that a current transformer is the way to go,

Boncuk, what's the difference between your circuit and ronsimpsons?
as a newbie, it's very tempting for me to go with the other circuit because of the low parts count.
I know that low parts count isn't always a good thing so this is why i'm asking.

Boncuk · Dec 18, 2009

Ron Simpson's circuit will work well if the signal from the current transformer is strong enough. (turns ratio)

My circuit already reacts if only 0.1mV are present from the current transformer.

Boncuk

smanches · Dec 18, 2009

Here is another option that I've used before. They work great for both AC and DC. Isolated, 0-5V outputs, easy.

**broken link removed**

Boncuk · Dec 18, 2009

smanches said:
Here is another option that I've used before. They work great for both AC and DC. Isolated, 0-5V outputs, easy.

The best idea yet!

Accurate and low component count.

Here are schematic and PCB layout. PCB measures 0.66X0.69"

Boncuk

Rajaneesh · Dec 18, 2009

trennonix said:
Thanks alot for all your replies
guess that a current transformer is the way to go,

Boncuk, what's the difference between your circuit and ronsimpsons?
as a newbie, it's very tempting for me to go with the other circuit because of the low parts count.
I know that low parts count isn't always a good thing so this is why i'm asking.

You simply need to know , whether the connected load is working or not..... or if the relay is damaged (load will not get the supply and hence no current).... here you go....

Use a Hall effect current sensor (3 pin) which gives 0 to 5V DC output depend on the load current... This is very precision also...You can even know whether the load is under idle or over load (high current) or dead short in load side... ( you can immediately stop the supply to the load)

This output is perfectly isolated from the load line (no direct contact), feed this directly to controller ADC....n do what ever you wish to do .... rest is firmware gimmicks....

You can find such sensor in ALLEGRO....Hall effect sensor...

Pin configuration..: Vcc, Grnd, O/p......... it will cost you less than 1 $

Hope this will help you...

Boncuk · Dec 19, 2009

Rajaneesh said:
You simply need to know , whether the connected load is working or not..... or if the relay is damaged (load will not get the supply and hence no current).... here you go....

Use a Hall effect current sensor (3 pin) which gives 0 to 5V DC output depend on the load current... This is very precision also...You can even know whether the load is under idle or over load (high current) or dead short in load side... ( you can immediately stop the supply to the load)

This output is perfectly isolated from the load line (no direct contact), feed this directly to controller ADC....n do what ever you wish to do .... rest is firmware gimmicks....

You can find such sensor in ALLEGRO....Hall effect sensor...

Pin configuration..: Vcc, Grnd, O/p......... it will cost you less than 1 $

Hope this will help you...

All that has already been mentioned.

Don't echo what has been said.

Boncuk

trennonix · Dec 19, 2009

Apparently this chip does wonders :O

However i don't know if it is available where i live (i'll just have to ask)
It just came to my attention that we didn't comment on KMoffet's sugestion; seems nice if the hall-effect chip is not available.
What do you think?

Rajaneesh · Dec 19, 2009

trennonix said:
Apparently this chip does wonders :O

However i don't know if it is available where i live (i'll just have to ask)
It just came to my attention that we didn't comment on KMoffet's sugestion; seems nice if the hall-effect chip is not available.
What do you think?

You can find the same in FARNELL.... they have allegro parts. There are different packages available. For DC application, PCB mountable packages are available... For AC application, i recomend you to use the sensor which can be mounted in a metal ring. You can feed through the load cable through this metal ring. If current flows through the cable, there will be a magnetic flux around the cable which will get coupled with the metal ring...and the flux get closed through the sensor.... this is how it works..

**broken link removed**
Farnell / Electronic Component Distributors / Suppliers / Electronics, Electrical Parts, Electrical Components and Wholesale Electronics.

You can check the part availability , price per each, volume price... and available models from different mfr....

This is one of the company offer 1 MOQ...

Hope this help u....

ronsimpson · Dec 19, 2009

There are several good ideas presented here.
Two of the ideas require cutting the household wiring and soldering to the wires. I don’t see you soldering #12 wire to an 8-soic. (see picture of ACS712)
I use the CTs because I have many, and I don’t have to touch 220VAC. I pull the wire from a breaker or switch, run it through the CT and put the wire back.
While I use real current transformers, I found a box full of ¾ inch toroid coils with many turns of very fine wire. (thank you used electronics store) It looks about like the last picture but with very fine wire. (500 turns)
As to the sensitive of the circuit, it has to do with the turn ration 100:1, the secondary inductance, and the load on the secondary of the CT. I just built one and tested it. What is your load current? I am test if a pump is on or off. If you want to measure 0.1 amps then you need some gain. (op-amp)

Rajaneesh · Dec 19, 2009

ronsimpson said:
There are several good ideas presented here.
Two of the ideas require cutting the household wiring and soldering to the wires. I don’t see you soldering #12 wire to an 8-soic. (see picture of ACS712)
I use the CTs because I have many, and I don’t have to touch 220VAC. I pull the wire from a breaker or switch, run it through the CT and put the wire back.
While I use real current transformers, I found a box full of ¾ inch toroid coils with many turns of very fine wire. (thank you used electronics store) It looks about like the last picture but with very fine wire. (500 turns)
As to the sensitive of the circuit, it has to do with the turn ration 100:1, the secondary inductance, and the load on the secondary of the CT. I just built one and tested it. What is your load current? I am test if a pump is on or off. If you want to measure 0.1 amps then you need some gain. (op-amp)

That is the reason i have suggested allegro....It is very precision...please go through the spec...

And regarding soldering, dont he have a 15W ordinary soldering iron?
A1321 series dont need any hard work to get it soldered, its same like a BC547 transistor...smaller than it...

You will get the metal ring also in the component shop.... it will have a small gap (3mm).. insert the sensor into the gap and put single drop of anabond to fasten it. Thats it over...

rest of the things are in firmware....You can define what ever you want...

Set Under load current..
Set Over load current
Set Load ON/OFF...status..

what else you want from this tiny component...

trennonix · Dec 19, 2009

OK, CTs and the Hall-effect/Allegro chips are nice, but what about KMoffet's circuit (first page) ?
How good is it? Do you recommend it?

Rajaneesh · Dec 19, 2009

trennonix said:
OK, CTs and the Hall-effect/Allegro chips are nice, but what about KMoffet's circuit (first page) ?
How good is it? Do you recommend it?

What kind of load you are going to drive? KMoffet circuit has diode in series to the load....

AC current detection

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