Could a CT secondary be connected to a Hall Effect sensor instead of a burden? - Page 1
Forum section: electronics / projects

author: unscripted
author_rank: Contributor
timestamp: August 09, 2024, 02:56:13 pm
content: Current transformer secondary is usually connected to a burden resistor. Would it be a good practice to connect it to a Hall effect sensor IC instead? Basically the CT secondary would see a shorted path through the primary path of the Hall effect.

author: coppice
author_rank: Super Contributor
timestamp: August 09, 2024, 03:11:43 pm
content: Quote from: unscripted on August 09, 2024, 02:56:13 pmWhat is the goal? It sounds like you are trying to achieve a multiplier effect. What's wrong with using a more sensitive sense amp?The resistance of a hall device isn't very stable. However, unless it sometimes gets high enough that the CT starts to loose linearity you should be OK for accuracy. As with any form of sensing from a CT, watch out for surge currents hitting the hall device too hard. Fault current surges can be pretty enormous.

author: ejeffrey
author_rank: Super Contributor
timestamp: August 09, 2024, 04:15:56 pm
content: You could do it, but what is the goal? I don't think it would do anything useful, at least in most cases.The reason you use a hall sensor for current measurement is that you want galvanic isolation and common mode rejection or that you have a large current and you want to avoid dissipation in a sense resistor. A current transformer provides all of these benefits -- isolation and common mode rejection by being a transformer and impedance transformation by having a large turns ratio. The main disadvantage of current transformers is that they don't work at DC, while hall sensors do, but if you cascade them you loose that benefit.If the current dissipation in the CT load resistor is too high, that's a sign that the turns ratio is too low.

author: Zero999
author_rank: Super Contributor
timestamp: August 09, 2024, 06:22:18 pm
content: Yes, there's no reason not to.As to why? I can think of a reason why. For example if the board is already made with Hall effect sensor, say to sense 5A and you want to use it to measure a much higher current, the easiest option would be to connect it to a current transformer.

author: Xena E
author_rank: Frequent Contributor
timestamp: August 11, 2024, 07:02:41 am
content: I don't understand the goal either, but a CT without a correctly defined load is a destructive event waiting to happen.It's the achilles heel of GFI/RCD's.A short circuit situation on the load side of those, often blows them to pieces because incoming overcurrent protection can't operate fast enough.Quote from: Zero999 on August 09, 2024, 06:22:18 pmIf, OTOH you wish to measure lower currents, just pass the primary through the CT more turns to decrease the primary/secondary ratio.X

author: inse
author_rank: Super Contributor
timestamp: August 11, 2024, 07:26:15 am
content: Zero999: Do CT‘s with such a high secondary current exist?I only came across types with sensor like secondary ratings and high transfer ratios…

author: Kleinstein
author_rank: Super Contributor
timestamp: August 11, 2024, 09:09:52 am
content: The common CT come with a rather large truns ratio, like 1:100 or 1:1000. However there is no problem making a CT with a smaller turns ratio.There is rather little sense in using a hall sensor behind a CT. If one really cases about the burnden to get good linearity one could use a transimpedance amplifier and one step further even with a serate sense and feedback winding to also compensate for much of the winding resistance.For protection a pair of anti-parallel diodes in parallel to the shunt is quite effective.

author: Zero999
author_rank: Super Contributor
timestamp: August 11, 2024, 09:35:53 am
content: Quote from: inse on August 11, 2024, 07:26:15 am5A appears to be a very common secondary current rating for a CT.https://www.rsisolsec.com/en/technical-support/select-a-current-transformer/

author: coppice
author_rank: Super Contributor
timestamp: August 11, 2024, 11:47:04 am
content: Quote from: Zero999 on August 11, 2024, 09:35:53 amYes, 5A and 6A are common CT output ratings, and are used as secondary multiplier CTs in light industry. A lot of 5A or 6A max 3 phases utility meters are manufactured to go with these. Three chunky CTs go around the thick cables carrying hundreds or thousands of amps, and three nice formable 5A/6A circuits go to the energy meter.A lot of people hate these things, as if anything happens to the 5A/6A circuit, and a CT loses its burden, bad bad things may happen.