Hi Everyone,
I need assistance in measuring current of a high current rated device which is around 187 Amperes AC. I am using a CT 200/5A for stepping down the current to 5A and using ACS712 module of 20A, just in case if the current goes above 200 Amperes, to read the current.
Is this a correct approach?
I am confused about this. Please Guide
I tried it without setting the CT on live Wire and only analog read is already giving an output of 549.
As far as I can tell from the ACS712 datasheet it does not rectify the sensed signal. It looks like for zero amps the output sits at about mid-rail or 2.5v with a 5v supply, and increases or decreases from this according to the current. What you are seeing is the mid-rail voltage. This means the device can happily read either AC or DC, but if being used for AC you will need to sample the sinusoidal output and determine its mean and peak values - in effect to "rectify" it.
Does the CT have a built-in burden resistor? As it is, connected to the ACS712 its output is short circuited and this might affect its calibration. Can you post a link to the datasheet?
Actually I am not sure why you are using the ACS at all if you have a CT. The "200/5A" just specifies its ratio, i.e. it has 40 turns on the secondary (the cable through the hole counts as 1 turn). A current of 200A rms in the primary would generate 5A rms in the secondary, a 1 ohm burden would then develop 5V rms or an 0.2 ohm burden 1v rms. For these current levels you might just connect the secondary (with burden) to a small bridge rectifier with an RC smoothing circuit to generate a positive dc voltage to ground and sense this through an analogue pin. If the CT would be damaged by a current higher than 200A then the ACS won't help.
The spec of the CT will tell you the size range of the burden resistor - which MUST be fitted .
You then have to convert the AC voltage to DC somehow ( the volt drop across a bridge rectifier is significant )
Buy an AC voltage transducer with , say a 0-5vdc output , much easier
Thank you so much for your guidance.
No there is no burden resistor in the CT. But I can add one.
challenge is to design it with this configuration because these parts are readity available in the market.
So what do you suggest about this?
I am basically trying to just check whether the machine is in working or in standby condition without contacting any electrical part of the machine.
not used a CT 200/5A but used three SCT-013 100amp current sensors to measure 3 phase power consumption
I sample the SCT-013 output voltage (offset to 2.5V) over 10 seconds and calculate the RMS voltage which gives me current
Thank you so much that is really helpful.
So accuracy of the current measurement isn't particularly critical?
What is the function of the processor? Will it just be driving an indicator? I think the CT + 1 ohm burden resistor + rectifier + smoothing would work fine For the current your machine takes you will get ~6.6v out of the smoothing circuit and you could just drive an LED. Or use a potential divider to drop it by a half say and monitor with the analog input of an Arduino.
Note that as hammy says you must fit a burden resistor - if you don't you will get dangerous voltages on the secondary as it tries to force 5 amps through air! Actually a burden is often fitted and the data on that product doesn't say anything about what value is needed or if it is included. If you do have to fit one note that it will get HOT so you need a power resistor.
You have to use a burden resistor ( google) and you are right it’s not given in the specification . It maybe intended that you calculate from some other parameter … think
I would ask .
Quote :
“ You need to watch a couple of things:
Seems correct to me.
You don't need a burden resistor if you're going to use an ACS712.
The ACS712 is the burden resistor (0.00x ohm).
Just make sure the wiring to the ACS is short and solid.
A bad connection there could start a fire.
Leo..
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