SparkFun Forums

[Barx]

In relation to my 'monitoring temperatures > 100c' post. I have decided to get a current transformer to detect when the heater element is on (drawing current). It's a MI20 if that helps. Any tips on how to use it. It will either be hooked up to either a pic or arduino. More likely the latter if I can get 1. I been searching the world wide pages and there seems to be alot of mention of a burden resistor. Is this to limit the current of something like that. Any help o. The matter would be great. Thanks

[esklar81]

Barx,

Very briefly: You need to put a resistor in series with the secondary side of the current transformer. The current induced (by the current you are trying to sense) in the CT will then flow through that series resistor, developing a voltage across that resistor. It is that voltage that you measure with the ADC of a microcontroller.

Eric

[jksoft]

Another thing to consider is that the ADC is generally looking for a DC voltage and the current transformer will be producing AC. If you are just trying to detect if there is current flowing or not, this might work fine, I'm not sure, but if you are actually trying to measure the current via the voltage, then you will need to rectify the AC.

[Barx]

Not looking to measure it, Just trying to tell whether current is flowing or not.

So, the CT has 2 leads. black and grey and on top it says black is ground. Say I decided to use an arduino. I connect black to ground. Then put a resistor in series with the grey into either a recifier or straight into the input and do some time sampling to detect the ac. As I've proven before i'm rubbish at calculating resistors. The output voltage of the CT is 0.4 - 4 Vp so I would think this doesn't need changing and the resistor just needs to limit the current. The output impedance is stated as <200 ohm. I'm not sure of the arduino characteristics. How do I get rating and size (wattage) of resistor from this.

Thanks

[jksoft]

Two things:
Even though you aren't worried about measuring the current, you should still have a diode in series with the transformer to protect the ADC from the negative half of the AC voltage (half wave rectifier).

The burden resistor needs to be in parallel to the transformer and to the ADC. As for the value of the resistor, that is usually determined by the voltage range you are trying to achieve for the ADC. It is usually a fairly low value, but without seeing the specs of the current transformer, it's hard to say for sure. 100 ohms is a fairly common value.

j

[Barx]

Thanks jksoft. I've decided to use a small 220 ohm pot as the burden resistor to allow me to 'tweak' the output voltage (well thats my theory, probably got wrong end of stick as usual) and then run through a full rectifier made from 1n4002 diodes that I have laying around. I will leave room for a smoothing cap in there too in case I cant get it to read reliably without. I have some pcb crew terminals laying around too so will use a 2'er block for the connection from CT to rectifiying circuit. Going to knock it up on matrix board.

[jremington]

A rectifier diode will introduce a fairly constant voltage drop of about 0.6-0.7 volts and it will be difficult to calibrate your current transformer/ADC setup. The circuit won't even work for low currents, when the transformer peak output voltage doesn't exceed 0.6 V. To overcome this limitation you will need an op-amp active rectifier circuit. Here is a link to a PDF file that discusses the principles of active rectifiers and gives a simple example http://www.niu.edu/~mfortner/labelec/lect/p575_01b.pdf

Jim

[Barx]

I was kind of hoping for that effect and was going to try it without. The current it will be detecting will be >10 amps and the CT is 20 amp max. So was hoping it would kick out 2-3 volts. loose a bit though diodes still gives me 1.5 - 2 volts to detect with adc. Kind of hoping this will rule out false detections incase there are induced emf from nearby cables. The reason i'm doing this with a CT is it seemed simpler than using a optocoupler or transformer to detect the voltage. I just want to know when the heater is active or not. voltage or current will do.