YHDC connection challenge (50A=1V, not 50A=50mA model)

Dear Forumites

I have a YHDC SCT-013-050 CT (50A-1V), which I’m trying to connect to a current monitor using an Arduino.

When I measure the output of the CT loaded with 4.344A on my Voltmeter, I get 0V on the DC setting, and 0.087V on the AC setting. So far so good.

Since the CT has an internal shut resistor, I’m connecting it directly into the circuit shown at Learn | OpenEnergyMonitor, with 1k Ohm resistors for the voltage divider. I’m still trying to find a 10uF capacitor, right now I’m trying to make do with 1uF.

When I measure the voltage over “Arduino input” and ground, I get 2.642V if connected to the 5V Arduino power, or 1.714V when connected to the 3.3V supply. Both measurements on the DC scale, 0 on AC. If I load the wire through the CT with 8A, these measurements don’t change, at least not to the last mV.

What am I doing wrong? I`m not comfortable connecting the setup to the Arduino analog input before I’m sure that the circuit is behaving properly.

Best regards

  1. The resistor values are a little low, but not harmful - they just cost you a little current.
  2. The capacitor value is not critical, any value around 10μF will do, but 1μF may be a little low - but then your 2 × 1 kΩ in parallel provide a low-ish impedance source anyway, which is what the capacitor does.

It doesn’t sound as if you’re doing anything wrong - except what and where you’re measuring.

You should get 2.5 V on the d.c. range at each wire from your c.t., and that shouldn’t change with the load. The fact that it isn’t quite that means your two resistors are not exactly equal. It doesn’t matter to you, it only reduces the maximum current that you can measure, which is well in excess of 50 A anyway.

You should see 8 50ths of 1 V (160 mV) on the a.c. range when you measure across the c.t. output, i.e. from the junction of the two 1 kΩ resistors to the Arduino input. What you measure from the Arduino input to Gnd depends completely on how your meter measures a signal like that (a.c. with a large d.c. offset). The instruction manual might help. The diagram on the page you refer to tries to explain where the voltages are.

Thank you. So it means that my multimeter, in the absence of both an user’s manual and electronic expertise, won’t really tell me where I’m going with this? The voltages are safe for the Arduino, and I can commence to connect and test.

Is there any way I can change the circuit to change the maximum range of 1V of the CT to something more useable like 2.5V? Best would be to change it back to a 50A->50mA version. This could be done by removing the resistors as indicated.

Then, to get it rigged for an 2.5V output, I’d add a 68 ohm resistor over the output terminals, assuming that the number of windings for all YHDC CTs is 2000?

Your burden as manufactured is 37.1 Ω, so that tells me that your c.t. is 50 A : 26.95 mA. (We’ll assume the 1 V is accurate, as the turns ratio will be adjusted from batch to batch, depending on the properties of the ferrite core, so probably about 1855 secondary turns.)

Your 68 Ω burden will therefore give you 1.83 V rms. You should be aiming for nearer 1.6 V, so 56 Ω is closest value below.

What your multimeter will tell you depends (quite a bit) on what you paid for it.
On the d.c. range, it will give you the smoothed d.c. value, which will be correct. It’s all change on the a.c. ranges. If it’s a ‘budget’ meter, it will read the rectified average and display the equivalent rms value assuming a pure sine wave. If it’s a true rms meter, it will display the true rms value of the wave, which isn’t the same as the rms value of just the a.c. part, which is what you wanted. Or it might allow you to offset (remove) the d.c. part, and then it would give you the answer you were looking for.

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Time to dust off the old scope pic that shows all the possibilities. This signal is nominally a 1V rms sine wave swinging about a 2.5V rail, so something like what you might see at the ADC input of a 5V energy monitor:

I also measured it with a Fluke 177 TrueRMS multimeter and a $50 no-name multimeter (https://www.jaycar.com.au/cat-iii-multimeter-with-temperature/p/QM1323) on AC and DC.

Fluke: AC 994mV DC 2.491V
no-name: AC 992mV DC 2.493V

In this case it is a pure sine wave so any pure sine wave assumptions made by the no-name work out fine.

The truly truly truly true RMS value is actually 2.67V, displayed by the scope as “DC RMS”, but both of the above meters filter out the DC component and show the RMS value of what’s left, displayed by the scope as “AC RMS”. As best I can tell, “TrueRMS” on a meter just means it takes no shortcuts with regards assumptions about the shape of the AC component. It doesn’t mean it will necessarily include the DC component in the calculation. The specs on my Fluke say its AC measurements are good down to 45Hz, below that, the filter they’ve included to remove the DC component starts to eat into the reading.

As far as I know even the most budget of meters filters out the DC component when on the AC setting, since that’s cheap to do. They both did a good job of measuring the DC value, which the scope calls Avg (in fact they may well have done better than the scope on that one).

By splitting out the AC and DC components, the multimeters leave you to calculate the truly truly true RMS value of the combined signal simply by taking the RMS of the two values. So using the Fluke numbers as an example:

sqrt(0.994^2 + 2.491^2) = 2.68V.

So in this case both the Fluke and the no-name performed well. When I get a chance I’ll replace the sine wave with something else, and that should demonstrate the Fluke’s TrueRMS advantages.

Here’s a 1V rms Gaussian Pulse sitting on top of a 1V DC offset:

Fluke: AC 999mV DC 1.642V
no-name: AC 900mV DC 1.645V

You can see the no-name’s sine wave assumptions really let it down in that case.

But for the lack of an alternate negative swing, that signal isn’t a million miles from current signals you’ll find around the home. Compare it to the picture of a CT around the feed to my washing machine in this thread: Phase measurement and correction in IoTaWatt - #17 by dBC.

That’s why when calibrating Irms on your energy monitor (which measures True RMS current) you need to use a load that’s as close as possible to sinusoidal (typically a big heating element).