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Sensing current upto 5A from a 45VAC source using SenseFEt

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Mar 15, 2010
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Hi all,
i need to build a circuit to sense the AC current flowing to a load. The source can supply 5A at 45V ac.
I need some simple and not bulky kind of solution.

I think what i am in search of is SenseFET usage with virtual earth current sensing.
Following are my questions.

1. Is there any other apt solution for me?
2. Should i use any SenseFET? If not then what parameters should be considered?
3. Should i use SenseFET with CAT2300 or not? why not. i an unable to understand the line "CAT2300 is the single chip alternative to discrete circuits for monitoring and controlling 0.9 V - 1.5 V power busses". Does it mean i cannot use it for my 45VAC?
4. Should i go for Virtual earth current sensing for SenseFET or is there any other alternative where i need not any -ve supply?


I assume that the AC is mains frequency current? If yes, there are many possibilities. One is a current transformer, which gives total isolation of the measuring circuit from the monitored current. It is easy to implement. I have seen, that there are even cheap small current transformers available in eBay (for example: **broken link removed**). That way your 0 to 5 A current is converted for example, with the module in eBay, to 0 to 2.5 mA current. If you place a resistor of 100 ohms on the secondary side, the AC current would be converted further to an AC voltage of 0 to 0.25V (RMS). Smaller resistor would give proportionally smaller voltage, and higher resistor a higher voltage.

You may read a bit more about current transformers from for example:

Of course, a totally semiconductor-based circuitry is also possible, but you would like a simple solution, and it would be also easily more costly. The question is if the size of such miniature transformer is considered "bulky" - but likely it will not be enormously larger than a more complex circuitry. At least a transformer is a pretty accurate and easy way of measuring AC currents with isolation between measured circuit and the measuring circuit.


I had success with the following method, when I wanted to monitor my refrigerator's turning on and off.

It uses inductive coupling. I got sufficient power to light up a red led dimly.

1) Start with an inductor (either a choke, or one side of a transformer). It is probably most effective if it is designed to operate at mains frequency.

2) Isolate a few inches of a SINGLE mains wire, which carries load current.

3) Wind the single mains wire around the inductor, a few turns.

4) Attach your voltmeter to the wire ends of the inductor.

That BradtheRad's construction is essentially a home-made current transformer. There is no magic in making them self, indeed. Still the physics apply: Assuming, that the transformer is made "right enough" to have good magnetic coupling, the current is transformed by ratio of the windings. To use such transformer as a current transformer, one has to load it heavily, by having a relatively low resistance load on it. "Relative" means, that depending on current to be measured and the transformer windings ratio (secondary/primary), the resistance should be heavy load/virtual short-circuit for the transformer. Then the voltage over that resistor presents the current, being U=(n1/n2)*I*R where n1 is the primary number of windings, and n2 the secondary number of windings. In a pass-through-the-wire -type current transformer n1=1, and n2 is whatever is wound around the core for secondary (the core is often a toroid for good magnetic coupling).
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