CSA-1V Hall Effect Current Sensor
First published in The BC Society of Model Engineers "The Whistle" December 2004
A Little Physics ...
If you know all this stuff, skip ahead. Otherwise, read on ...
When a charged particle moves in a magnetic field, it experiences a force which is at right angles to the field and also at right angles to the particle's direction of motion. This is called the Lorentz force, and is used, for example, to make conventional TV sets work.
If you place a block of
current-carrying material in a magnetic field, the Lorentz force displaces the
charge carriers so they bunch up on one side of the conductor. They will
continue to bunch up until the electric field so generated just exactly balances
the Lorentz force. The voltage you can measure at right angles to the magnetic
field and at right angles to the current flow is called the Hall voltage
(discovered by Edwin Herbert Hall in 1879).
Unfortunately the Hall voltage is very small for metallic conductors, and it required the development of the semiconductor industry to make practical Hall devices (semiconductors have many times fewer charge carriers, so for the same conditions of current and field, the carriers move faster and experience a stronger Lorentz force).
Semiconductor Hall devices have their own problems too. The Hall voltage must be compensated for temperature, because the number of charge carriers present in a semiconductor changes with temperature. (Do you remember germanium power transistors, which used to experience thermal runaway ?)
A practical Hall device has a block of semiconductor as the sensing element, supplied by a constant current source, and a programmable amplifier to raise the millivolt output to a reasonable value, in addition to the temperature compensation circuit. Older devices used laser trimmed thick film resistors to adjust the programmable amplifier to give a standard output voltage under standard conditions of magnetic field. Newer devices use a flash memory to hold the amplifier gain setting.
At this level of sophistication, Hall devices were useful for specialist instruments (such as DC clamp ammeters) and for sensing permanent magnets (car ignition systems), but weren't really sensitive enough to detect currents in the tens to hundreds of amps.
My employer, Universal Dynamics, has supplied Hall effect current sensing systems for use in electrochemical plants, where the typical currents are from 50,000 to 200,000 Amps !
So what's new ?
A DC clamp ammeter uses a hinged set of transformer-type iron laminations to intensify the magnetic field to the point where is can be sensed with a Hall device.
The designers at Sentron (www.sentron.ch) have put the ferromagnetic material inside the integrated circuit, giving it about 10 times the sensitivity of previous devices, and incidentally, changing the direction of the magnetic field sensitivity (the IC used to have to be mounted at right angles to the current-carrying conductor - now it can be mounted flat against the conductor).
The CSA-1V device is powered from 5 volts, and its useful sensing range is +/- 5 milliTesla. It is supplied in a plastic SO-8 package approximately 4 mm x 5 mm by 1.5 mm thick.
How do I use it #1 - Individual motor ammeter
Let's suppose you use #8 wire to power your 24 volt 1 HP DC locomotive traction motor. 1 HP of mechanical power out requires about 950 watts of electrical power in, say 39 Amps at 24 Volts. Your #8 wire is approximately 6 mm OD, which means that the distance from the centre of the wire to the current sensor is approximately 4 mm.
The magnetic field in free air is (μ I) / (2 π r) Tesla, where
μ is defined as (4 π 10-7) Henries per meter
I is the current in Amps
and r is the distance in meters.
In our example, the magnetic field is 1.95 milliTesla, right in the middle of the sensing range of the CSA-1V.
How do I use it #2 - Motor fault detection
If a CSA-1V is placed exactly in the middle of 2 wires carrying current in the same direction, it will sense the difference in current between these two wires.
Let's look at a DC electric locomotive with 4 motors, two at the front connected to one wire, and two at the rear connected to another wire. These wires are connected to a motor controller. Let's examine what happens to a train moving at less than full speed on a level grade. Assume that the controller output to balance the train's rolling resistance is 36 Amps, and that each motor is drawing an equal share of 9 amps. The current in the wire to the front truck is 18 Amps, as is the current in the wire to the rear truck. The current imbalance is 0 Amps, and the CSA-1V output is zero.
Let's suppose that #1 motor lead becomes disconnected. The train still requires 36 Amps to balance the rolling resistance, but the current is now shared between three motors, 12 Amps each. The current to the front truck is 12 Amps, the current to the rear truck is 24 Amps, and the difference is 12 Amps. The output from the CSA-1V would be about 0.2 Volts, which is where we might set our current imbalance warning buzzer.
The reason I specified "less than full speed" is that when the controller output is the same as the battery voltage, the motor speeds must drop a little to draw more current. However if the controller output is less than battery voltage, the train's engineer becomes part of a feedback loop which compensates for the reduced performance of the locomotive by turning up the speed control a little. This is particularly noticeable when starting a train - loss of one motor appears to make the locomotive "more slippery" as the engineer applies power faster to compensate.
In practice our motors are unlikely to be damaged by this level of current imbalance. It's much more serious when we are climbing a grade drawing our maximum current of 4 x 39 Amps = 156 Amps, and this becomes 3 x 52 Amps (still 156 Amps). The CSA-1V output would be approximately 0.85 Volts, so we might set our alarm threshold at 0.5 Volts from the CSA-1V.
Remember that motor heating is proportional to current squared. Suppose our motors are designed for 75 degrees C rise at their rated current of 39 Amps. At 52 Amps we would expect their temperature to rise 75 x (52/39) x (52/39) degrees, that is 133 degrees, or an additional 58 degrees. This is how motors get "cooked", and why it is useful to install a motor current imbalance sensor.
Owners of 30 Volt locomotives with 24 Volt motors should note that, under fault conditions, the extra voltage available can mask the loss of performance due to a failed motor, leading to a cascading failure.
How do I use it #3 - non-contact speed control
The magnetic field being sensed can equally come from a permanent magnet. Throw away your carbon film speed control pot with its finite number of operations, and replace it with a bar magnet moving in front of a CSA-1V. Unfortunately you are limited to 180 degrees of rotation (because the magnetic field is symmetrical). But you can now be completely waterproof.
Where can I get one ?
North American sales are handled by GMW Associates in California (www.gmw.com). The 10-off price is approximately $10 Canadian per sensor. They are currently in stock.
How do I handle such a tiny IC ?
The CSA-1V is about 4 mm x 5 mm (half the height of the sails on the schooner on a Canadian dime).
I cut a 5 mm x 5 mm hole in a 0.4 inch x 0.6 inch piece of Veroboard (see photos). The integrated circuit has 8 pins : 2 are used for the output, 3 are connected to the 5 Volt supply, and 2 are connected to ground (the 8th pin is not connected inside the integrated circuit, and it's convenient to connect to + 5 Volts).
A 0.1 microfarad capacitor is recommended across the 5 Volt supply.
The whole assembly can then be protected with glue-lined heatshrink - you can make your own with regular heatshrink and a couple of blobs of low-temperature hot melt glue.Bruce Wilson (AScT)