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current sensing in AC

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rjvh

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hi i need to sense a current in a AC aplication so that i have a indication that current is flowing (switched on)

alsio i need to give that signal to an imput of a plc (for monitoring)

i have this idea and it works but is not perfect for all situations (see schematic)

if the current is lower than 1 Amp i will use the 1N4007

if the current is lower than 3 amp i will use the 1N5408

but i have to monitor a 11 KW pump that consumes +/-24 Amp

diodes are getting a bit expensive in this case

is there an other solution to tackel this problem?

Thanks

Robert-Jan
 

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hi i need to sense a current in a AC aplication so that i have a indication that current is flowing (switched on)

alsio i need to give that signal to an imput of a plc (for monitoring)

i have this idea and it works but is not perfect for all situations (see schematic)

if the current is lower than 1 Amp i will use the 1N4007

if the current is lower than 3 amp i will use the 1N5408

but i have to monitor a 11 KW pump that consumes +/-24 Amp

diodes are getting a bit expensive in this case

is there an other solution to tackel this problem?

Thanks

Robert-Jan


Your circuit will not work..

1) the circuit as-is will blow up very quickly. There is nothing to current limit all those diodes in there. Dont do it!

2) when you say sense if current is flowing, I take that to mean you are sensing whether or not current is flowing elsewhere.. your circuit will force current to flow (it is a load in its own right) so it will *always* tell you there is current flowing - not very useful and fundamentally flawed.

3) If by L1 & N you are refering to mains line voltage.. these things are dangerous. do not tinker with it. Use a compeltely isolated solution like you tried to do with opto's or take the other posters suggestion and use a hall effect device. Galvanic isolation must be maintained. A current transformer is another option for you.

expensive? HA! if you think $0.1 diodes are expensive then you won't like the $3 hall effect device.
 
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If you are going to monitor the current with the diodes you should put a resistor in series with the LED or the input of the opto isolator to limit the current.

You should also put a resistor in parallel with the diodes to make sure that thee LED or opto isolator does not turn on at low currents.

A current transformer is a very good idea as it isolates for safety and it reduces the current which makes it easier to measure.
 
CT

A current transformer is the best way to measure that type of load.

Bear in mind that a CT is not allowed to be open circuited under loaded condtions because very dangerous voltages will appear across it's terminals.
A adequately rated resistor should be across its' terminals and a LED can be put in parrallel to provide a visual indication.
A CT will give AC so a diode may be put in antiparrallel with the LED.

I take it that the 11kW motor is 3 phase, so only one phase needs to be sensed with the CT.

Alternatively one or two windings of 6 mm² may be wound around a normal 220 / 12 Volts transformer, the 12 or 220 Volts winding can be used through a rectifier to drive a LED with parrallel resistor
 
Your circuit will not work..

it does it's on my work bench

1) the circuit as-is will blow up very quickly. There is nothing to current limit all those diodes in there. Dont do it!

could be duration test is not performed yet, and your right there is no current limiting component in this schematic the apliance conected in serie is that

2) when you say sense if current is flowing, I take that to mean you are sensing whether or not current is flowing elsewhere.. your circuit will force current to flow (it is a load in its own right) so it will *always* tell you there is current flowing - not very useful and fundamentally flawed.

not agreeing with you on this point if there is no load between the L1 and N than the circuit is not closed so where does the current go?
test on the bench with this circuit shows also that if the aplaince is switched off the indication is also swiched off


3) If by L1 & N you are refering to mains line voltage.. these things are dangerous. do not tinker with it. Use a compeltely isolated solution like you tried to do with opto's or take the other posters suggestion and use a hall effect device. Galvanic isolation must be maintained. A current transformer is another option for you.

Point taken but i do have expertise and experiance with these things (but your advise is the same as i tell people) you can't be warned enough about it

to be onest the hall effect device i didn't think of it as i never seen them sold here in Cambodia it's one of the problems where i walk in constantly (avalibility of for the rest of the world common components)


expensive? HA! if you think $0.1 diodes are expensive then you won't like the $3 hall effect device

i dont think $0.1 dollar diodes are expensive it gets different if you need diodes that have to handle 24 Amp (again not availible here so i have to go source them elsewhere).

Any way thanks for your reply and also the other members

i wil look for Hall sensors and current transformers to tackel this problem

Robert-Jan
 
Hiya Robert-Jan,
Eh mate one solution might be the Allegro AC75X series of hall effect sensor chips, I just checked the faq file I have here and yes they will sense AC current. I'm not sure if they would be suitable but it's always work a look.

Regards Bryan
 
hi Bryan
it looks like a good deal for my aplication :)

Thanks

I will find out tomorow how painfull it is to get it here and on what for conditions :D

Robert-jan
 
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which is why the first reply to your question was me pointing you to them

Hi ubergeek63

the confusing part on the first link whas for me that it whas directed to digikey and didn,t supply me any technical spesifications (and I had also not the time at that moment to dive into it right away to find out at the first time i saw)

But thanks for your reply and it's the component i do need for this job

Your help is highly appriciated

Robert-Jan
 
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sorry I like to supply the link to their product page so you can see how much it costs and can get the link to the data sheet from there.

The one I linked to is the less expensive "little brother" of the ACS75x family
 
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sorry I like to supply the link to their product page so you can see how much it costs and can get the link to the data sheet from there.

The one I linked to is the less expensive "little brother" of the ACS75x family


I also didnt click on the link when I suggested the AC75X but now I have, I still reckon the AC75X is the go as it isnt a smd chip, rather it has 2 big leg's on the back to sense the current.

Cheers Bryan :D
 
very true.... The ACS7xx also has a large internal lead frame, they just package it that way for convenience. I do understand it being a little harder for a hobbyist to handle though. I just got done hand soldering 7 ICs with 0.5mm lead pitch
 
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