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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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1st Dec 2018, 10:09 am | #1 |
Dekatron
Join Date: Jul 2009
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Current transformer.
Been meaning to put a volt and ammeter on my variac/iso tfmr for a while so dug out out two old 4" panel meter's to use. The voltmeter was pretty straight forward, two diodes, a big 2200u cap, with a fuse and 100k 10 turn preset in series to calibrate. The voltmeter tracks the actual RMS voltage pretty well.
The amp meter is presenting problems however, I've tried using sense resistors, I first tried 0.1r, not enough OP, 1r just about works, but needs about 6A before the needle starts to deflect, and tracking is bad, 10r works better, but obviously gets very hot and would need a stupid big wattage. I tried winding 1000 turns onto an old bobbin, OP is about 2uA for 2A. At first I tried the "current tfmr" with the wire to be sensed through the middle, so windings are at 90 deg to the wire, just "1 turn". I tried wrapping it around a few times, not much better. Wrapping the wire to be sensed around the outside of the bobbin, as in a traditional tfmr gives better results, but then I'm getting a voltage ratio, not a current one ( I think). What's the best way to go about this? I don't want to go down the extra circuitry route, say using an opamp to amplify the OP of the sense tfmr, just keep things simple. Ideally I'd like to be able to read about 0.5A to 5A, accuracy isn't that important, the meter is more there to give a visual indication of shorts/fault conditions when powering up any DUT's as well as a lamp limiter. Andy.
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1st Dec 2018, 11:03 am | #2 |
Dekatron
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Re: Current transformer.
Moving iron meters are best for Variacs. They can stand brief shorts without being damaged however the scales are not linear.
RS used to sell square ones in the 1980s. |
1st Dec 2018, 11:37 am | #3 |
Heptode
Join Date: Jan 2008
Location: Sheffield, South Yorkshire, UK.
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Re: Current transformer.
I used one of these https://www.ebay.co.uk/itm/AC-Ampere...72.m2749.l2649
Accuracy wasn't that important to me, more an indication of no current or too much current, cheap and cheerful! I also bought a matching voltmeter. No connection with seller.
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1st Dec 2018, 11:38 am | #4 |
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Re: Current transformer.
Use 5A mains transformer the wrong way round. This will step up the voltage and step down the current. Make sure there is a load resistor across the higher voltage winding to stop sparks and insulation breakdown.
The system should be set up to match the transformer to the meter. There will be a voltage drop across the transformer which may be important. |
1st Dec 2018, 4:17 pm | #5 |
Dekatron
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Re: Current transformer.
Thanks for the reply's. Will dig into this further.
Andy.
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1st Dec 2018, 5:40 pm | #6 |
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Re: Current transformer.
There are many "one chip" current to voltage converters such as https://uk.farnell.com/allegro-micro...egro%20current cheap and easy to use for the very sensitive versions you have to zero out the earths magnetic field, not of any consequence on AC though.
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1st Dec 2018, 8:48 pm | #7 |
Dekatron
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Re: Current transformer.
Hi Andy, current transformers are quite easy to make using a small transformer core and have the advantage of providing isolation. They MUST be used with a "burden" resistor across the output. The AC signal can then be rectified and used to feed a DC meter, it will however bit non-linear.
I did an article for Elektor on this, published about 5 years ago. Ed |
1st Dec 2018, 10:56 pm | #8 |
Heptode
Join Date: Oct 2009
Location: Melbourne Australia
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Re: Current transformer.
I just use a cheap ac mains plug in power meter, with digital lcd display, and a few functions incliding A, V, W, PF. It tells me if the load connected to the variac is pulling too much current. I could put another power meter on the variac output, but it would drop out at lowish 'line voltage' and doesn't really give much more info for how i use the variac.
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2nd Dec 2018, 12:40 pm | #9 |
Nonode
Join Date: Jun 2010
Location: Spalding, Peterborough, Cambridgeshire, UK.
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Re: Current transformer.
About 20 years ago I made a cased and metered 2A variac.I think the metering side was from ETI, Elektor or similar. No idea, so I took it apart just now.
I used a 3" Scale Sifam 50uA that I already had, scaled 0- 10 & 0--25. I had this meter connected to wipers of a dpdt switch, V OR I. Volts, just a 1N4007 diode in series with a 270k pot and 2x 1M resistors feeding the meter. Amps, a lowish voltage 5A bridge with the ac terminals connected in series with the variac output. The DC + & - side of the bridge fed either end of a home made shunt, approx 12-18" insulated hook up wire estimated by looking at it, as I don't want to dismantle it. This went to the I terminals of the switch. You would need to experiment with shunt and series resistor to suit your meter. I also have a 2A panel mount fuse on the input side and a neon. This has served me well all these years. Rob
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2nd Dec 2018, 3:06 pm | #10 |
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Re: Current transformer.
Isn't most of the non linearity of rectification on the output of a current transformer taken care of by having the burden resistor (which can be the ammeter and its shunt) after the bridge rectifier?
I reckon the reason that you need a burden in the first place would "take care of" the diode forward voltages, at least to a first order. Feel free to shoot this idea down (Ed?) if it's nonsense......
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2nd Dec 2018, 5:01 pm | #11 |
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Re: Current transformer.
Re: post #9 by Robinshack.
I have never seen a FW bridge used in that manner to provide a means of monitoring a.c. current. Intrigued, I traced out the cct. to analyze it - and yes, it works on paper - and it obviously works in practice, since you've proved that. To me, it's just another one of those clever, useful but simple ccts. (and I emphatically do not mean that condescendingly) which someone with an insightful & creative imagination has come up with. So, another little job for the bench one day: build it and investigate it. Thank you for sending it in. Al. |
2nd Dec 2018, 5:47 pm | #12 |
Nonode
Join Date: Jun 2010
Location: Spalding, Peterborough, Cambridgeshire, UK.
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Re: Current transformer.
OK Al, I wish I knew which magazine it was in. Wouldn't have been swm or pw. I occasionally took WW, ETI and most of the Elektor, including the 100 circuit specials.
It was built around 20 or less years ago. Rated 2.5A, not 2A that I said before. If I want an accurate o/p voltage I use my dvm, as the display meter is not calibrated to any great accuracy, but is within 5%. Current reading, I would have checked when I made it. UPDATE! I just checked again with a 100W bulb and varying voltage. At 200mA, 300mA and 400mA it is within the thickness of the needle pointer! Voltage. At 100, 200 and 230V on meter scale it reads 4-6V high on my fluke dvm, so the preset inside needs a slight tweak. Job for another time though. I have 2 leads with plugs to fit the small o/p socket. One is terminated in a 13A socket. The other in insulated croc clips. It certainly is useful when testing unknown items such as transformers. May give Andy some ideas in what to do for his variac metering. Rob
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2nd Dec 2018, 6:47 pm | #13 | |
Dekatron
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Re: Current transformer.
Quote:
A transformer is a transformer, it's only the use of it which makes a current or a voltage transformer. You don't need a burden resistor if you are loading the secondary with an ammeter. You do need it if you are using a voltmeter and sense resistor as an ammeter though. Herald's comment about putting rectification diodes before a burden/sense resistor is bang on! Apologies for brief reply... I'm just catching up on UKVRR after a hectic day. This looks a fun project! |
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2nd Dec 2018, 7:33 pm | #14 |
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Re: Current transformer.
The windings at 90 degrees bit confuses me.
If the current transformer is a toroid, then the single wire straight through the centre is a single turn winding from the viewpoint of the toroidal core because whatever you do with the wire away from the toroid completes that turn, or else you have no complete circuit for the current to flow around! The secondary winding is just an ordinary toroid winding. You can even do this at RF and for screening you can use a lump of coax for the primary - but just remember not to ground the outer at both ends. Leave one end floating so you don't create a shorted turn. For a normal E-I type core, a normal winding goes on for the secondary, and the primary goes through one hole and back through the other. Again, this counts as a one-turn winding. Going through one hole only is not good. The flux in the core is asymmetric and the limb around the other hole acts as a flux shunt. I did an RF directional power meter some years ago with the well-known toroidal current transformer and coax trick, the difference being in using two of them, so the whole machine scales based on the turns ratio of the two identical transformers. Look for Sprat 61 or google Stockton Power Meter (The G-QRP lot seem to like things named after towns or rivers. I just called it a dual directional power meter) David
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2nd Dec 2018, 9:19 pm | #15 |
Dekatron
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Location: Newcastle upon Tyne, Tyne & Wear, UK.
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Re: Current transformer.
Hi Gents, Kalee has covered the topic about the burden.
From memory my article about CT's was not long after Oct 2012 edition. The atrilce covered the design of the device and used a small EI 48 laminated core I think. Details were also given of using the CT to drive a normal milliameter in voltage mode. This was intended for the OP's type of application, on a variac. Ed |
2nd Dec 2018, 9:35 pm | #16 | |
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Re: Current transformer.
Quote:
The volt meter might seem ok with this method, for now or on a one off test. However, if it is a type of circuit that rectifies the voltage and is filtered with a capacitor to a peak value there might be an issue. I have found (at least in my locality) that the mains sine wave is quite distorted, especially to the extent that the peaks are flattened down. (Probably the many switchmode psu's loading the mains !). The worst of it though is that the peak flattening effect changes during the day and the peaks vary a lot more than the average value. Because of this on a mains voltmeter I made I didn't use the filter capacitor on the rectifier outputs and just drove the meter circuit with rectified(pulsed DC) so the meter responded to the average value of the wave and this correlated better with the true rms, and it saved a part. Most commercial moving coil AC voltmeters do not have the capacitor. Hugo. |
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3rd Dec 2018, 6:00 am | #17 |
Heptode
Join Date: Oct 2009
Location: Melbourne Australia
Posts: 898
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Re: Current transformer.
The CT followed by full bridge and then burden resistor is a common used technique for measuring high secondary side dc current by actually sensing primary side phase currents in 3ph rectifiers, with all phases having a CT and bridge, and using a common burden, and then a 0-1V mc meter. There is some non-linearity at lower current, and due to waveform (eg. in phase reg rectifiers).
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3rd Dec 2018, 6:46 am | #18 |
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Re: Current transformer.
The resistive load on a current transformer is effectively in series with the circuit being measured. By transformer action it sees an isolated and scaled version of the current, and if there is a bridge rectifier we can add rectified to that list. The metering measures the voltge drop across this resistor, or else shares a proportion of the current through it.
This makes the resistor technically a shunt, not a burden. The circuit under test thinks it sees a shunt of this resistance divided by the turns ratio squared. The voltage drop being metered is increased by the turns ratio. Add in isolation and that current transformer does several useful things at once. Tek and HP used to make current probes whose tip was a current transformer with a shunt on its secondary and a cable to a BNC connected to your scope. Tek also did some amazing ones with hall-effect sensors which worked down to DC. Brilliant for SMPS development or TV deflection circuits. Even the labs at HP bought these things from Tek. David
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3rd Dec 2018, 7:51 am | #19 |
Dekatron
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Re: Current transformer.
The thread's woken up since last I looked, will try and answer all questions.
Thanks for the link Merlin, but trying not to go down the Xtra circuitry route. Ed, thanks, PM sent. Trobbin's great idea but skint and also doing it this way for the craic and learning potential. Thanks Robin. I found if I used only diodes, the needle vibrated, after experiment a 2200u cap stops this. The problem I have with A meter is that it's 10mA FSD, any diode drop hurts. I tried a ge and schottky type diode, problem is meter needs a few volts to get the needle shifting, using a FWBR will waste precious V, more on that later. Why the need for a burden R? Is it because if reading with just a voltmeter a current tfmr is effectively shorted? The 90 deg...looking online most commercial current tfmr's sense I with a wire (primary) though the middle, so turns are at 90 deg and not parallel to the primary. I found V/I out increased if pri in parallel with sec. I also had it in my head that to sense I pri needed to be at 90 deg to sec. I think I conflated the memory aid thing where to explain a magnetic circuit you use your hand, index finger pointing, thumb at 90 deg to it. Lastly my problem is the large FSD of the meter I'm using and also the I tfmr I wound was just a plastic bobbin, no core. Wondered about using one of those big ferrite's you get in SMPSU leads as the core, not sure what it's flux properties are. According to this article - https://www.electronics-tutorials.ws...ansformer.html I need a low loss core. Will experiment more. Thanks for all your IP. Andy.
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3rd Dec 2018, 8:36 am | #20 |
Nonode
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Location: Spalding, Peterborough, Cambridgeshire, UK.
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Re: Current transformer.
Andy, try the bridge circuit that I used, no capacitor, just a shunt to suit the meter. Mine is extremely accurate at the less than 20% fsd current I tested it at. Ie, just a few mV on the meter.
Rob
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