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Tech Forum

December 2017

AMP Clamp

Could someone explain in simple terms how an AMP clamp works? Does it have a transformer in it or Hall-effect sensors or similar?

Kevin Champion
Cleveland, OH


There are two major types of clamp on current meters. The AC type is a transformer; the wire in the clamp is the primary, the secondary is internal and is normally 1000 turns. If you are measuring 100 amps the secondary output will be 100 mA. The other type measures DC and could be calibrated for AC. The DC type uses a hall-effect or similar sensor. Amazon lists an AC/DC meter for $39.

Russell Kincaid
Milford, NH

Most clamp-on ammeters use a transformer located inside the body of the meter to step-up the measured current to a value which the unit can use.

Jeffrey Massey

I am no expert. But you are correct in your assumptions. I found this great article at Wikipedia on the subject:

Fernando Cordero

The common AC current meters use a transformer to measure the current. The “clamp” is the core for that transformer, and it is opened to go around the AC wire as it becomes the one-turn primary. There are simple components to scale the multi-turn secondary current down for the correct reading, either analog or digital. The digital part uses its own circuit to convert the measurement to its display.

A DC current meter cannot use the same transformer method, so it uses the Hall device that converts a magnetic field to a resistance, and the internal components convert that resistance to a reading, analog or digital.

Raymond Ramirez
Bayamon, PR

You are on the right track. The ones that measure AC use a pickup coil, and the ones that measure DC use a Hall effect sensor.

Chip Veres
Miami, FL

An AMP clamp is a transformer having a one-turn primary (the conductor carrying the current to be measured) and a multi-turn secondary (to feed the measuring instrument).

Being a transformer, it can only be used to measure alternating current. Be aware of its measurement rating, because too much current can saturate the clamp, resulting in inaccurate readings.

The clamp is rated for sinusoidal currents: measurements of non-sinusoidal current is best done with a true-RMS instrument connected to the clamp secondary winding.

Currents small relative to the capability of the clamp can be measured by passing the conductor through the clamp multiple times and dividing the measured value by the number of turns.

Peter A. Goodwin
Rockport, MA

AC amp clamps use a current transformer. The clamp is a laminated iron ring with a coil of wire wound around it. The alternating current in a powered AC line induces a magnetic flux in the clamp’s ring which the attached coil picks up via electromagnetic induction. That induced signal is fed to an amplifier who’s output is rectified and filtered and the resulting averaged (i.e. RMS) DC voltage is fed to the meter’s measuring circuit. The typical conversion is 1A AC = 1V DC on the meter.

DC clamps use Hall Effect sensors. As I understand them, the construction is similar to AC clamps, except an exciter coil is attached to the iron ring to pre-energize the ring with a specific frequency/voltage, which the Hall Effect circuitry sees as a zero level. DC current flowing through the clamp creates a magnetic field which alters that excite signal, which in turn changes the signal the Hall Effect sensor sees, which causes the Hall Effect detector circuit to output a voltage (positive or negative) proportionate to the current flowing in the measured conductor. Like the AC clamp, that detected signal is fed to an amplifier which outputs a DC level which is fed to the meter’s measuring circuit with a similar conversion scale (i.e. 1A DC = 1V DC on the meter).

As you’re aware, amp clamps are an EXTREMELY SAFE method of measuring high current (i.e. 10’s and 100’s of amps) in powered circuits because you don’t have to break any wires for making the measurement. HOWEVER, because their resolution is typically between 100 mA (0.1A) and 1A (due to the limitations of the magnetic detection circuitry), amp clamp’s use is limited to measuring power lines connected to large devices (i.e. pump motors) that naturally consume large amounts of current in operation.

Ken Simmons
Auburn, WA