Building an adapter for the current transformer SCT013-100 to make your own clamp meter for 100 amps 

Gracias y éxito con tu proyecto

Many thanks for the tip. I read the ACS712 datasheet and these are interesting sensors, no doubt, but not really what I was looking for in this project. The current has to flow through the chip which means the circuit has to be interrupted and while the resistance is small it causes again a limit on how long you can use them at high current before the chip overheats or at least changes results because of thermal effects (30A at 1.2 milliOhm = 1W loss). They also need their own power supply. The big advantage of these sensors as I see it is that that they work for AC or DC currents but translating the output voltage into a display of the measured current is not trivial either. I will definitely consider Hall-effect sensors, next time if I need to measure large AC/DC current with a micro-controller / Arduino / Raspberry Pi

It is true that current transformers must not be used unloaded. If you want to switch load resistances, using an MBB switch is a good idea, alternatively, as explained in my video, you can use a normal switch by always having a 200 Ohm resistor connected parallel to the transformer and the switch simply adds another resistor in parallel to the first one to get a lower overall value. In this case the switch does not need to be MBB since the 200 Ohm resistor stays connected all the time. The drawback of this scheme is that the 2nd resistor value is generally so unusual in that it can't be bought and must instead be approximated by combining other resistors with standard values (or replaced by a trim-pot)

The SCT013-10 has the resistor already built-in. At 10A it delivers 1V. There is no need to do anything else and you don't need any transient protection diodes unless you plan to send hundreds of amps through that thing which would destroy it anyway.

I noticed a part number issue with the video as well. The one pictured is the STC 013-000 which has an output of 50mA @ 100A but the spec's he's referring to are for model STC 0013-100 which has an output of 1V @ 100A so apparently the -100 model has an internal load resistor. Great informative video, just need to clarify the model numbers referenced.

Sadly no. Totally outside. At 100A that would produce 100V but the internal diode would try to limit that to 5V (and probably die in the process. Even 200 Ohm should only be used for currents of up to 10A

@@TheHWcave Ahh... I missed that detail. Good catch.

A tall order since I don't know what test gear is available. Anyway, my two SCT013-100 measure about 102.5 Ohm between sleeve and tip of the connector regardless of polarity. If you connect sleeve and tip to an AC multimeter with a 100 Ohm resistor bridging tip and sleeve, a 1A AC current in a single wire going through the hole in the transformer should produce 50mV AC on the multimeter. You can calculate the values yourself depending on what meter and resistors you have or test currents you can produce. 1A /2000 = 0.5mA , then 0.5mA * 100 Ohm = 50mV. You should stay below 200 Ohm, at least for high currents to avoid getting into the non-linear part of the transformer.

@@TheHWcave My pair arrived. One measures 179 Ohms and the other reads 181 Ohms. This is significantly different from your two, so additional testing is required. Just for clarity, the K connection is on the tip of the plug. The L connection is on the long sleeve which abuts the plastic base of the plug. The wire shield is on the middle ring. IIRC, this is a different arrangement from the typical audio use of this style of plug, where the long sleeves is usually connected to the wire shield/GND. Meaning, the data sheet must be consulted, before making assumptions about the connections.

It looks genuine to me. Mine came from Banggood though but I can't complain (in this case)

Its not quite as easy. These a current transformers so they produce an output current, not a voltage. Of course, once you terminate them with a load resistor of your choice, the current produces a voltage across the load resistor. Current transformer must always be loaded, never run them without a load as they can get destroyed (it's the mirror image of a voltage transformer which you should never short-circuit). The SCT013 produces 50mA when measuring a single wire of 100A current . That is because it has 2000 turns (100A/2000 = 0.05A). At 15A you only get 7.5mA. If you want this to be 1V you need a load resistor of 1V/7.5mA = 133 Ohm. However you are not entirely free in that resistor value because if you look at the SCT013 (and similar) sensors, you'll see that for resistors >50 Ohm the relationship between current being sensed and output current increasingly starts to become non-linear at the high end. Lastly nothing stops you to wind the wire with the "to-be-measured" current twice or 5 times through the transformer. This changes the winding ratio from 2000 to 1000 (x2) or just 400 (x5). If you do it 5 times, your 15A current now produces 37.5 mA (15A/400) and only 26 Ohms are need to produce 1V, or if still using the 133 Ohm, you now get 4V for 15A. So there is a lot of flexibility just with one model of transformer. If you choose another model, you would have to do similar calculations based on its datasheet and decide which one suits you more.

@@TheHWcave Hey, thanks a lot. I ordered the 20A version and was going to try and sense 10 - 20 amps in order to drive an opto isolator for an npn output. I may need to use an op amp but sounds like I may not have to. Optos can be very sensitive. We'll see. Thanks for the help. 2000 turns is sure worth the 10 bucks they cost.

Found it... We need TRS = Tip - Ring - Sleeve

3.5mm TRS is what you need (Tip-Ring-Sleeve) which is extremely common for stereo earphones. (TRRS having 2 rings and are only used if there is an additional microphone connection)

The output of a CT is current, not voltage. It only becomes a voltage if you send it through a resistor following Ohm's law U = R * I. The current (or the converted voltage) follows the input current instantaneously (within some frequency limitations, usually CTs are optimized for mains frequency). You can read the output as RMS if you measure the current (or voltage over the resistor) using an RMS meter. Or you can see the instantaneous wave form if you use a scope. It depends on what you use to measure, not the CT itself.

I had a quick look for a datasheet and this one appears to have an already built-in sampling resistor so it produces an output voltage (0..1V) and not a current. At 1A you get just 33.3mV which may be tricky to read. Because of the fixed resistor, you can't use the trick of trading less max current for a higher output voltage. With a -100 and a 200 Ohm resistor you would get 100mV for 1A. There are models like the SCT-013-005 which is a 5A sensor but also with fixed resistor to 0..1V. With this sensor you would get 200mV for 1A. My suggestion is therefore to not use the -030. Either get the -005 model or use the one I used for 100A and provide your own 200 Ohm resistor making the 100A sensor effectively a 10A sensor.

So if i buy the sct013-100A and connect it with 200ohm resistor. The ct will have max read to 10A max sensor ? And it is better for below 1A read use.

I am not at home for some time to check. What schematic do you need?