Current type Voltage Transformer - Page 1
Forum section: electronics / beginners

author: Phaedo
author_rank: Contributor
timestamp: November 27, 2023, 11:17:27 am
content: Hello World ,I was just opening an chinese inverter just for the giggles.It has a transformer similar to the datasheet I have attached. I am curious about what it does. Its configured like a CT but I guess that's not what it does.Any suggestions ?TIA

author: theHWcave
author_rank: Regular Contributor
timestamp: November 27, 2023, 01:12:48 pm
content: I think its a tiny isolation transformer providing a galvanic separation between mains and the measuring circuit. The transformer is for voltage only. The text about current is just to explain that it needs a limiting resistor. If both resistors are the same, then the output voltage is the same as the input because the transformer ratio is 1:1The baffling thing is the 50 Ohm part. I don't think that is correct. 500KOhm would make sense to limit the current to 2mA.

author: Phaedo
author_rank: Contributor
timestamp: November 27, 2023, 03:37:32 pm
content: Hi,Thanks for replying.QuoteIn figure I they are feeding the output directly to an Opamp, I don't know it would work with a voltage of 230VAC.Cheers !

author: uer166
author_rank: Super Contributor
timestamp: November 27, 2023, 05:04:07 pm
content: This part is actually a CT. It's a neat trick to make the core smaller to handle less volt-seconds. First it converts voltage to current via that primary resistor, then current to voltage using a normal burden resistor.This is NOT a voltage transformer and it'll blow up if u use it as one.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 27, 2023, 07:02:11 pm
content: It is a small transformer that is designed to be used as a 1:1 current transformer for relatively low currents. 10 mA and 50 ohm gives 500 mV. So if used as a voltage transformer it would be for low voltages like 0.5 V with still good linearity. So it may also work with 1 V or so, but not much more at mains frequency.It is a neat tricky to get mains voltage sense with isolation, e.g. for a power meter. The current transformer may be more linear and smaller than a voltage transformer.The circuit version with the OP-amp keeps the voltage even lower and thus possibly slightly better linearity, though there is still the wire resistance of the turns that may not be that much lower than 50 ohms.For the intended use as voltage sense there is no need for higher current, more the question if lower current (e.g. 100 ohm shunt and 220 K to 230 V mains) may also work.

author: Phaedo
author_rank: Contributor
timestamp: November 28, 2023, 05:16:31 am
content: Thanks for the reply gentlemen.Just as a follow up, these would be made with CRGO cores ? 1000 x 2 turns at 230V seems to be quite a challenge.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 28, 2023, 08:09:40 am
content: The transformer itself would not see a higher voltage over the windings. The 230 V to most part are dropping on the series resistor in the 100 K range. The idea is to convert voltage to current with the resistor and than use a 1:1 current transformer. The current transformer can be more linear than a conventional transformer. Especially small conventional transformers below some 3 VA don't behave all that well because the magnetizing current can be quite relevant and distort the waveform / add a phase shift.I don't know what type of core is used in this small current transformer, but ideally it would be a high µ and low hysteresis material, like Mu metal or nanocrystalline. However for cost reasons it could also be a hih µ ferrite or classical FeSi electrical steel, likely as a torroid.

author: theHWcave
author_rank: Regular Contributor
timestamp: November 28, 2023, 09:32:35 am
content: After thinking about it, I agree its a (strange) current transformer for tiny currents. I'd like to see how they used it. Probably there is a big shunt somewhere that handles most of the current and only a fraction is passed through this thing for measurements.

author: SeanB
author_rank: Super Contributor
timestamp: November 28, 2023, 11:06:07 am
content: Seen them as a single heavy turn, then a secondary with a few hundred turns, and a load resistor to convert that current to a voltage. For the one referenced there likely is a current shunt, a small way away, and then 2 thin traces to the transformer, that them convert the high current to a small voltage drop, which is then applied to the 50R impedance of the transformer, and converted to a similar small current into the 50R load on the secondary.Other sensors are a single heavy turn, then a 100 turn secondary, and a hall sensor in the gap of the core, that drives a current through the coil, either unipolar or bipolar, depending on type, to drive the flux measured by the hall sensor to zero, so the current is proportional to primary current, but you have the advantage of also being able to measure DC current along with AC current.

author: MrAl
author_rank: Super Contributor
timestamp: November 28, 2023, 11:34:14 am
content: Hi,Current transformers are usually used to measure AC current. They are similar to voltage transformers, although the calculations are better done with regard to current than with voltage, because the primary voltage is so low while the current is usually substantial.For example, if you have 1 turn on the primary and 1000 turns on the secondary, and the input current is 1 amp, then the output current will be 1ma because 1/1000=0.001, and if you have 10 amps through the primary then you have 10ma through the secondary.Now the resistor that accompanies this setup is across the secondary so the current through the secondary can produce a voltage which can be measured and corelated to the primary current. If you have a 100 Ohm resistor across the secondary then that 1ma secondary current will produce 0.1 volts, and if you have 10ma through the secondary that resistor will produce 1 volt. That allows the circuitry to tell what the primary current is, and still provide galvanic isolation.Of course this doesn't work with DC voltages and currents, but it works pretty well with AC. It's not perfectly linear like that but it usually works out good enough over a certain range of currents and voltages.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 28, 2023, 11:42:45 am
content: Normally current transformers are used for high AC currents with one a single and in rare cases a few (mainly if different ranges are needed) primary turns.This special one is for a small current. The datasheet shows the typical use case with a large series resisor R' = 100 K to measure a voltage (especially mains voltage) with galvanic isolation. This could be useful for power meters to also get isolation on the voltage side. The current sense would than use a separate more classical current transformer.