# Batteryless current clamps?

Discussion in 'Electronic Equipment' started by Fester Bestertester, Nov 17, 2009.

1. ### Fester BestertesterGuest

I'm curious how the Fluke i200s current clamp probe can give mV output
without the use of batteries.

How is this done? If one is measuring 200A I can see how the magnetic field
could generate enough current in the probe to support some high-impedance,
low-draw circuitry.

But when measuring on the low scale, say, 2 or 3 amps, how could the probe
output a few hundred mV? (The clamp is spec'd to output 100mV / amp on the
20A low scale, 10mV on the 200A high scale.)

Can someone explain this to me? I'm fascinated to see it's possible & curious
to know how.

Thanks.

2. ### Jeroen BellemanGuest

You could have added a line for the OP, saying that a passive
current clamp is a transformer, or some such.

Jeroen Belleman

3. ### pimpomGuest

You seem to have a preconceived notion of what constitutes large,
small and insignificant currents levels in terms of the fields
they generate, but such categorisations are only relative. "2 or
3 amps" is quite huge in some contexts and generate an
appreciable flux in the magnetic core of the clamp. The
alternating magnetic field induces a voltage in the clamp's
pickup coil and this voltage can certainly reach "a few hundred
mV" if enough number of turns are used.

You can also think of the clamp as a current transformer. The
wire being measured for current is the primary and the pickup
coil of the DMM is the secondary.

If you're more familiar with voltage transformers, think of it
this way:
Suppose you have just 1 mV output from a microphone. Connect it
to the primary of a 1:10 transformer and you will get 10 mV at
the secondary terminals. Use a 1:100 transformer and you get 100
mV and so on, theoretically up to any voltage.

4. ### pimpomGuest

I don't think that's quite what John meant. Anyway, that reminds
me of a practice by some villagers in my area. They cannot
afford, or don't want to pay, the power connection charge and
monthly bills. They wire their homes for a few incandescent bulbs
and keep a pair of solid-cored wires with the ends stripped bare
and bent into a U shape, the other two ends feeding the house
wiring. When it gets dark, they use a dry bamboo pole to hook the
bare ends to the overhead power lines. Free power - until they
get caught. The power company - the government here - usually
does nothing more than reprimand the offenders, but the practice
is rare now.

5. ### Martin RiddleGuest

You need a loop to form an air core transformer, which this has been
done.

Cheers

6. ### Fester BestertesterGuest

---
OK.

A passive clamp-on ammeter is essentially the secondary of a transformer
wound on a core that can be opened or closed in order to get it around a
conductor so the current in that conductor can be measured without
cutting it and using a conventional ammeter.
[...]
JF[/QUOTE]

FINALLY an answer on-topic. Thank you.

After watching the 3 Stooges act that is aee / sed...

Sheesh!

I AM FBt

7. ### Pointless PostsGuest

Admitted that the S/N ratio on Usenet can be frustrating. But did
you stop to consider the possibility that a) you failed to grasp
other attempts to explain it to you; b) your question was so
elementary for *this* group that few people bothered; c) your
last post might be taken as a slap in the face by those who tried
to help.

8. ### Fester BestertesterGuest

Admitted that the S/N ratio on Usenet can be frustrating. But did
Right you are.

A big thank you to those responders who gave answers to my question. Much
appreciated.

My comment was addressed to the "noise".

FBt

9. ### John NagleGuest

Yes. Classic AC clamp-on ammeters are simply transformers. One
"turn" through the clamp, many turns in the fixed coil for output.
The output feeds into a voltmeter.

Those are AC-only devices. There are also Hall-effect clamp-on
ammeters, and those work for both AC and DC. These have been
available for a decade or so, and pricing is now down as low as
\$60. I used to have one that could read down to about 500mA DC,
and it only cost \$129. Very useful in robotics and controls work.

John Nagle

10. ### DaveCGuest

[...]
Which make & model would that \$129 model be? It's always useful to know
someone else's favorite tools...

Dave

11. ### Tzortzakakis DimitriosGuest

There's nothing fancy about that, the electricity meters of a medium-voltage
consumer (real and reactive energy) are powered from the two potential and
the two current transformers, without any other power supply (medium
voltage=15 kV in Crete).

12. ### Fester BestertesterGuest

So, for a millivolt output probe, this might be as simple as 2 windings (or a
tapped single winding) with a range switch to select the winding?

13. ### daestromGuest

If you make a 'tap' upstream of the revenue meter, even with just
transformer action, you're stealing. Revenue meters (kilowatt-hour
meters) have always had terminal voltage as one of their inputs. An
illegal tap upstream may affect the voltage at the service entrance some
small amount, but the metering will reduce the billed kWh accordingly.
So regardless of the exact voltage supplied by the utility (it often
varies slightly throughout the day), the amount of energy delivered at
the service entrance is what is billed for. Power drawn off before the
meter isn't measured and is 'stolen'.

Of course if you just 'wrap some turns around the power line' without
orienting the coil properly in relation to the line, you're not going to
get any power because transformer action won't work when your turns of
wire are parallel to the power line's magnetic field (i.e. 'wrapped
around' the power line). And I think that was John Field's point.

daestrom

14. ### Tzortzakakis DimitriosGuest

Anyway, current transformers must always be operated with the secondary
shorted. In the generating facilities in Kozani, West Macedonia, where 400
kV current transformers were involved, the operators of the plant had a
special indicator whether the secondary was shorted.

15. ### daestromGuest

Some old switchboard CT's I worked on in the Navy had very thin
insulator between two spring clips. Whenever we wanted to remove a
meter for cal, we slip the insulator out so the two clips would short
together, shorting the CT. Then we could open circuit the meter and
remove it from the panel. I don't remember exactly what the blade was
made of, but it's surface wasn't perfectly smooth like polished
material, more porous like unglazed ceramic (of course it wasn't any metal).

The reason they built the insulator so thin was that if one accidentally
open-circuited the CT without removing the wafer first, the high voltage
developed by the CT would just 'punch thru' the wafer and safely short
the CT. Then all you had to do to repair things was make sure you
closed the circuit and replace the wafer-thin insulator blade.

Was kind of surprised when I moved to commercial power systems that they
didn't use something similar. Just has to have a breakdown voltage that
is low enough to avoid damaging the CT.

daestrom

16. ### JosephKKGuest

1/3 either of those. No battery whatsoever and AC only.
Only active probes do that.
I can make any output ratio i want, i know how they work.

17. ### JosephKKGuest

There is no CT in residential metering, the energy meter is connected
directly. Even commercial / light industrial you do not see CTs in
the meter circuit until 600 A, and before that you are typically at
480 V 3-phase (in the US).

18. ### James SweetGuest

My uncle's house (in the US) has a 400A service with current
transformers. They're not common but they do exist. IIRC 200A is the
largest residential meter.