6-0 Magnet Meters and Hall-Effect Switches
The proper term for a sensor that measures magnetic field strength
and direction is a Gauss Meter and the value is called Gauss like Volts is a measure of voltage . The sensor used here
is a Hall -Effect sensor.
The discussion below starts with a brief history and how a Hall-Effect
sensor works. This is an important foundation for the applications which follow. The first application is to a linear Hall-Effect
sensor used to make the Gauss meter and is followed by two applications of a Hall- Effect switch for the ignition system of
an engine. I am indebted to Allegro Micro Systems who’s web site is the source of most of what I have learned and share
here, www.allegromicro.com and to Forrest M. Mims III little book Magnet and Magnet Sensor
Projects , Radio Shack 62-5020, 1998.
6-1 History and How a Hall-Effect Sensor Works
6-1-2 Physicist Edward Hall discovered the Hall-Effect
in 1879. He had a thin film of gold with a current flowing thru it and found that a voltage appeared across the film when
a magnetic field was applied. The voltage is called the Hall voltage and is proportional the magnetic field and applied
current. Today, the gold leaf has been replaced by semiconductor material and it is part of an integrated circuit on a tiny
chip. If you buy a new car, there may be as many as 100 Hall-Effect devices in it including the wheels, measuring the rotation
for ABS breaks and the speedometer. They are also under the capstans of VCRs. Thus production quantities are high and the
cost is down to that of a postage stamp. A UGN 3503UA is shown below:
6-1-2 Figure 12-1-1 illustrates the operation
and defines the terms we will be using. Imagine a cube with a voltage applied across the top and bottom faces. Vbat
is the supply voltage and must be regulated as any variation will show up at the output. R is a current limiting resistor
which, in conjunction with the regulated voltage, provides a constant current for the sensor. G is the magnetic field in Gauss
applied across two sides of the cube. Vh is the Hall voltage measured across the remaining sides of the cube.
6-1-3 It might seem logical that with no magnetic field
applied, the output voltage, Vh would be zero, but this is not the case. If there is no magnetic field applied, (G = 0), then
the output voltage, Vh, is typically ½ the supply voltage, Vcc, not zero. We call this the quiescent condition and Vh = Vq.
I visualize it this way. If Vcc at the + side is 5 volts and the opposite side is zero volts, then the voltage is dropping
as the current goes thru the device. If the taps for Vh are half way thru the voltage drop, then Vh would be 2.5 Volts with
no magnetic field applied.
6-1-4 If B is a SOUTH pole, Vh will increase
above Vq with an increase in the magnetic field. Thus I let Vh = Vq + Vs, where Vs is the voltage out put above Vq. (When
we calculate the magnetic field strength for a SOUTH pole, we need Vs. ) As the SOUTH magnetic field increases, it will saturate
the sensor and the output voltage will iccrease up the the supply voltage, Vcc then stay there.
6-1-5 When a North field is applied the output , Vh,
decreases below Vq. The stronger the NORTH magnetic field, the closer the output is to zero, saturation for the NORTH magnetic
6-1-6 Each Hall-Effect sensor has a sensitivity value which, for
the South field, is the increase in out put voltage divided by the associated increase in magnet field. For a NORTH field,
it would be the decrease in out put voltage with the associated increase in magnetic field.
6-1-7 In summary, for a Gauss meter, what we are really
trying to measure is the output voltage above the quiescent value for a South field and the output below quiescent for the
NORTH field. With these voltage measurements, we can either go to a graph to convert the measurement to Gauss or we can calculate
the magnetic field strength. Figure 12-1-7-1, Gauss4sm.jpg. Shows a conversion chart for a Hall-Effect sensor with a 2.5 milivolt
/ Gauss sensitivity.
|Figure T6-3, Gauss4sm.jpg
6-2 How to make a Gauss Meter
6-3 How to make a Hall-Effect Ignition