Maker Pro
Maker Pro

Non-contact AC current sense



Jan 1, 1970
I am looking for an inexpensive (<$15 ea in 100qty) method to detect the
presence of line AC in a single, large gauge wire and close a small
signal switch (fet, bipolar, or) when current is present. The following
criteria apply:

No measurement or linearity requirement - only sensing presence.
Min threshold is 200mA, max is 200A or more.
Non-contact only, and the wire cannot be disconnected for installation
(so no toroids).
Wire dia varies from 1/4" up to 1".

The only methods I have considered so far are two-piece CTs and Hall
sensors. The Hall sensors do not seem sensitive enough for the low end
without an (expensive?) flux concentrating split core. They also need
more power than I want to provide, but that's ok if the price is right.
Other methods (GMR, Rogowski?) are possible but seem to have higher costs.

I have built a small circuit using a clamp-on CT that operates off the
small secondary current. It meets all the criteria except the larger
wire dia., so I know what I am asking for is reasonable. I really want
to buy a module like this if they are available, but all I see are
expensive measurement solutions.

Does anyone sell this type of thing, or know of a superior method? A
souce for low accuracy, inexpensive clamp-on CTs would be good too.


Jan 1, 1970
HAVE you thought of a reed relay attached to the one of the wires.,
To answere me address mail to
[email protected]

Sounds interesting, but I dont see how this would be non-contact. The
reed relays I have seen require around 10mA coil current (or a magnetic
field) to activate. A CT can produce maybe 1mA AC on the secondary.


Jan 1, 1970
Hobdbcgv said:
go to your local appliance parts store and ask for a live wire detector,
about the size of a pen, runs about $10.

see what they have.

I got mine a few years back for $15 - but mine does a lot more than just check
for a field - also checks fuses, hi voltage, and a couple other tricks.

or wait for christmas and get a few of those bulb checkers for $5 a piece

I like the reverse engineering idea, but I need to sense current. These
cheap pens are probably not doing this but I'll check it out. My wires
are live all the time. I am trying to detect when a load is present. I
did see that Extech has a new non-contact current probe for around $40.
I am wondering how this works, since it does not use a clamp-on
transducer. My application is in an electrical cabinet chock full of
60hz EMI. Any method that cannot isolate the magnetic field around the
wire I want is going to fail.


Jan 1, 1970
I did pick up one of these pens for $7 . What is interesting is that
when I use it on some 110 zip cord, it lights up only for the hot wire
regardless of whether there is a load present or not. If I add a triac
style dimmer, then it lights up for both hot and neutral wires. I'd like
to see an explanation for how these work.