### SCT-013-030 ac voltage output

Hello, I'm new to the forum. Before I proceed to ask a question which has probably already been asked, a quick (sincere) thanks to all helpful members. I've learned quite a lot from forums posts but unfortunately haven't been able to find a post which discusses the issue I face.

I'm using an SCT-013-030 to measure current. I also have a clamp meter which I'm using to compare the reading from a 60W lamp the +ive lead of which I loop 8 times around either the clamp meter or the SCT-013-030. What's puzzling me (even before I get to the Arduino adc conversion/calibration) is that the device outputs an AC voltage as soon as I plug the lamp in with the switch off. The clamp meter as expected measures no current in the wire when the switch is off and 2A when the switch is on. But the SCT outputs an ac voltage when the lamp is plugged in and this voltage doesn't change when the switch is off/on.

I'm confused by this. I would've assumed (as shown by the clamp meter) that there's no current in the conductor when the switch is off but the SCT is obtaining energy from somewhere to output a potential difference - so there must be current flowing through the conductor. Can somebody please explain what/how the SCT is sensing when the circuit is open ?

Thanks for you attention.

### Re: SCT-013-030 ac voltage output

I'm confused by this. Me too!  What resistance do you measure for your -030 c.t? The only sensible reason I can think of is you're not actually connecting to the c.t. and your voltmeter is reading the mains voltage picked up by capacitance to your body or something like that.
[Edit: Proof would be if you read nothing or very little if you put your c.t. on the neutral.)

You should read about 37 Ω - the internal burden resistor is apparently tweaked in manufacture to calibrate the output and is around 75 Ω, the internal resistance of the secondary is similar and of course is in parallel.

What do you read on the mA scale? I'd expect close to 1 mA when the lamp is on, and zero off. (Don't worry, it is safe to short-circuit a c.t.)

### Re: SCT-013-030 ac voltage output

Robert, thanks very much for your reply. I do measure resistance of the -030 c.t. as ~38ohm. While measuring the resistance, I've realized that I'd made an incorrect assumption about the connection between the plug of the -030 and the pins on the 3.5mm jack. I'm now able to measure zero ac voltage when the switch is off and non-zero when the switch is on. So, a big thank you for suggesting I measure the resistance.

I'm still foxed why there's any voltage at all (up to 2V ac) on any of the parts of the plug when there's no current. I'm going to ignore this 'free' source of energy for now.

### Re: SCT-013-030 ac voltage output

I'm going to ignore this 'free' source of energy for now.  Ah, but I'm prepared to bet there's next to no energy there, as there will be almost zero current. As I sit here with a laptop on my knee, put one meter prod to earth and hold the other in my hand, I read 43 V.  If I put the meter on the µA range, I get about 50 µA. According to me, the maximum power I could get is about 1 mW into a 1 MΩ resistor – if I'm lucky.

You could try the same thing. I expect even less current in your case.

As I said, there's a capacitor connecting your c.t. to the mains. You can't see it, one 'plate' is the mains conductor, the other is the secondary winding in the c.t.. The dielectric is the insulation and the air separating them. The capacitance value is very small, a few tens of picofarads.

### Re: SCT-013-030 ac voltage output

Robert, thanks for another helpful post. I'll make a measurement similar to the one you made.

Did you really mean 43V ? I can believe (without doing the experiment myself) 43mV or perhaps even 4.3V but will have to see 43V to believe.

### Re: SCT-013-030 ac voltage output

Yes, I really did mean forty-three!  As I said, no current - I certainly can't feel anything. The reason is the laptop PSU is double-insulated and not earthed to the mains, and I suspect it has suppression capacitors across the mains input designed for the peculiar 'centre-tapped to earth' American system with the output 0 V connected to the centre tap of the capacitors, so the whole PSU and laptop tends to float towards half-supply - 120 V in my case. Laptop headphone socket to earth: 96 V, or 275 µA on the current range. I think pure coincidence that voltage is double what I measured yesterday, that is 52 V today. Maybe today's lower humidity has pushed it up?

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