Bench Testing DC Current Transducer - Page 1
Forum section: electronics / beginners

author: am1
author_rank: Contributor
timestamp: April 02, 2024, 02:58:34 pm
content: Hi all,I am new to instrumentation and want to test out a DC CT (DCT100-42-24-F). I have an ACT CT in my electrical cabinet that is working well, but I cannot get the DC one to work. I have tried 2 different units and both do not seem to be outputting a 4-20 mA output. I was to externally test the DC CT before putting into my cabinet, but I am not sure how to approach it. I am also limited with equipment to do this. I have a basic MM, a basic signal generator, and a variable PS (see below). Any advice is greatly appreciated!https://www.amazon.com/Adjustable-Generator-Simulator-Controller-Calibration/dp/B0991ZSBHJ/https://www.amazon.com/SHNITPWR-Universal-Adjustable-Switching-100V-240V/dp/B08N4R48LJ/

author: BeBuLamar
author_rank: Super Contributor
timestamp: April 02, 2024, 03:33:09 pm
content: Connect the 24VDC supply to terminal 1 and 2 of the transducer. Use the DMM in mA mode and connect the meter to terminals 3 and 4. You should see the current when there is current flow thru the center of the donut. You simulator simulate a source and thus can't be used for the read out.

author: Jeroen3
author_rank: Super Contributor
timestamp: April 03, 2024, 07:04:49 am
content: Your SHITPWR unit does not appear to have current limiting. That means you'd need a dummy resistor to create a known amount of amps. I suggest a 24 Ohm 50W resistor to get 1A, you'd need the 24V to power the thing anyway.Then with your DMM you can probe the 4-20mA output.The amount of passes through the hole determines how many times it should read the known current of 1A.Eg: 1 pass, 1A should be around 4.16 mA if the thing is 100A full range.Challenging to get to 100A full range, you'd have to pass the wire thourgh 100 times....If you have a lab power supply capable of 5 or 10 A it gets easier as you don't need the resistor since you use constant current (CC) mode.

author: am1
author_rank: Contributor
timestamp: April 11, 2024, 03:04:55 pm
content: I also have this 24V 6.5A PSU (https://www.meanwell-web.com/en-gb/ac-dc-single-output-enclosed-power-supply-output-hrp--150--24). I am assuming I will not need a resistor if I use this. I also have my jumper removed on my DCT for a 0-50A range. How would I set up to measure the mA output in this case? If I have 24V across terminals 1 and 2, and have the MM leads on 3 and 4 set to DC mA, would this cause a short of any kind or could damage the instrument?I have seen on another forum platform of someone using a very similar DCT unit (just a different A range) that said they only had power on terminal 1 and 2 and no load/wire through the core, and they read 4 mA. I am assuming I must have something wrong with my set up or measurement reading if I cant get a similar output?

author: Ian.M
author_rank: Super Contributor
timestamp: April 11, 2024, 03:30:26 pm
content: That's *NOT* how this works. If the power supply doesn't have an adjustable current limit, you need a load resistor for it to determine the output current. Unfortunately for a reasonable current of a couple of amps from a 24V PSU this will be a very high power resistor, which wont be cheap, and if metal cased, will also need a good heatsink.Most of us would test this sort of thing on a 0-30V, 0-3A or 0-5A CV/CC Lab Bench PSU, in CC mode with a multimeter on 10A range in series with its output, and however many turns of wire through the DC CT core hole it takes to get the desired effective maximum current as Jeroen already described. A 5A bench PSU could reach 100 effective amps with only 20 turns of wire through the hole.Its designed for use in a 4-20mA signalling loop - 4mA would be zero current through it and 20mA full scale current. As BeBuLamar has told you, when supplied with 24V power on terminals 1 & 2, a multimeter connected between2 & 33 & 4on a 200mA range should read 4mA with no wire (i.e no current) through the CT core hole. If you can manage to pass 50A DC through the hole, and you have removed the range setting jumper to select 50A (low) range, it should read 20mA. If it stays at 4mA the current flowing through the core hole is probably in the wrong direction.Edit: corrected output terminals

author: BeBuLamar
author_rank: Super Contributor
timestamp: April 11, 2024, 03:32:13 pm
content: No connecting the lead from the DMM directly to the output 3 and 4 doesn't cause the short. The output will output 4-20mA when terminated from 0 Ohm to about 250 Ohm. The DMM put near 0 Ohm but that isn't a problem for the current transformer as it would work down to 0 Ohm.

author: BeBuLamar
author_rank: Super Contributor
timestamp: April 11, 2024, 03:38:51 pm
content: If you don't have anything to test then you can simply trust it. I used current transformer from Audomation Direct before and they all good.

author: am1
author_rank: Contributor
timestamp: April 11, 2024, 04:30:26 pm
content: Quote from: Ian.M on April 11, 2024, 03:30:26 pmThis is my main issue/confusion. When I apply 24 V with the above mentioned PSU, and no wire in the core, I read 24V across terminals 1 and 2, and when I measure the DC mA output with my MM on 3 and 4, I get 1.2 mA output. I have checked the MM fuses as well. With JUST power, why dont I see the 4 mA reading for no current?You mentioned measuring mA output across 2 and 3, should I measure from 2 and 3 or 3 and 4?

author: Ian.M
author_rank: Super Contributor
timestamp: April 11, 2024, 05:02:00 pm
content: Aaghhh typo!!! "2&3" should be "3&4", its output terminals.It seems that either your multimeter is on the wrong range so isn't reading the 4mA accurately, or the CT is out of calibration.

author: am1
author_rank: Contributor
timestamp: April 11, 2024, 06:05:02 pm
content: I will get a hold of a lab bench PS so I can further test the device. But if I want to simply test that, with 24V applied, I get 4 mA output with no load/wire in the core, will the SHINTPWR supply or the the 24V PSU linked be suitable for this purpose?

author: am1
author_rank: Contributor
timestamp: April 11, 2024, 06:46:41 pm
content: Just an update... On a new unit, I used that SHNTPWR PS for 24V for pin 1 and 2, when I measured in mA on 3 and 4 I read the 4 mA like I expected finally. But I tried it a few minutes later, and I cannot get any output, I just get 0 mA, I checked the fuse and it looks good. Any thoughts on why I suddenly cant see an output??Update: rookie mistake. I needed to screw down the terminals for contact on 3 and 4. This sensor seems good, so Ill do some further testing with my other components. Thanks all!

author: BeBuLamar
author_rank: Super Contributor
timestamp: April 11, 2024, 08:21:21 pm
content: About Automation Direct, I found that their stuff are good and the prices are low. I like them a lot and use a lot of stuff from them, from PLC, terminal blocks, M12 connectors, pressure sensors, power supplies, current transformers, timers, etc... but if I remember correctly I never could get them to fix or replace any bad part. It's always for some reason they wouldn't fix it.

author: am1
author_rank: Contributor
timestamp: April 12, 2024, 04:16:23 pm
content: Quote from: BeBuLamar on April 11, 2024, 08:21:21 pmThat is good to note, thanks!I have 2 other questions:1. I have the sensor hooked up to my IO-Link convertor and my PLC fully integrated with the controls and motor now. The full load amps is 10.4 on the motor, which I do not believe we get close to. Right now, with the 0-50A range, my sensor values are ranging from 4-7 mA with the machine conditions and rpm. If I had a second winding, this would essentially half my sensing range, correct? Right now, its somewhat tough to strip my cable to get enough length to get 2 windings, but if i did, what would I expect? double what I read right now with a single wire in the core (e.g. single wire sensor output = 5.2 mA, 2 winding sensor output = 10.4 mA? Is my thinking wrong?2. if I use only one wire in the core and want to put some kind of 3d printed insert to hold and center my wire, will there be any significant affect on the sensing ability with the additional material between core and conductor?

author: BeBuLamar
author_rank: Super Contributor
timestamp: April 12, 2024, 05:27:47 pm
content: If you can get another loop in the core then you double the reading that would give PLC a higher reading. I have never used a DC current transformer but as I use the clamp meter if the wire is not in the center you could have somewhat different reading. I don't think the added material will make a different just as how thick the wire insulation wouldn't matter. I didn't check but don't they have a current transformer with lower range than 50A? I have AC current transformer (from automation direct) with 10A range.

author: Ian.M
author_rank: Super Contributor
timestamp: April 12, 2024, 05:47:42 pm
content: 4-20mA loop signalling has 4mA offset. For a non-inverted unipolar linear value like the DC CT output 4mA is 0% and 20mA 100% of full scale. This is a range of 16mATherefore:Ict= Ifull_scale* (Iloop_out- 4mA) / 16mAWith two turns through the CT core your 5.2mA single turn output would become 6.4mA. However the installation instructions say to "Select the range that is equal to or slightly higher than thenormal operating amperage" so you would want four or even five turns through the core for a full scale of 12.5A or 10A respectively.The turns should not be tight wound on the DC CT, but should be formed into a bundled coil of at least the same diameter as the CT, should be laced or cable tied so they don't flop about, should be centered in the core hole and should be straight where they pass through the core. Insulating materials in the core hole that don't contain any metal will have no effect on the reading.Probably the best option is to wind such a coil through the CT with a length of suitably rated insulated wire (<6mm dia) before installation leaving tails of a good length, lace it to hold its shape and fit your centring insert, working on the bench. Finally install the completed assembly and splice the coil wireto one of the wires of the existing cable.

author: am1
author_rank: Contributor
timestamp: April 17, 2024, 08:50:09 pm
content: Quote from: BeBuLamar on April 12, 2024, 05:27:47 pmYes the AC options seem to have much lower amp range options, the lowest DC one they offer is the unit I have for 0-50 A.Quote from: Ian.M on April 12, 2024, 05:47:42 pmIs there risks or downsides to me using the multi-turn method to lower the sensing range (accuracy, saturation, calibration, etc.)?My motor cable was really short and terminates on my motor drive with a female spade terminal. So I added a lengthy jumper wire (14 AWG like the motor cable wire) with a male spade terminal on one end and a female spade on the other as an extension. I think my coils are a bit tight at the moment; I will bundle them with zipties in looser loops and insert them into my housing like suggested.