Download Hall Effect Devices as Current Sensors

Hall Effect Devices as Current Sensors
We are probably all familiar with semiconductor devices that respond to light or heat.
Light (photons) and heat (phonons) increase minority current carriers in reverse biased
semiconductors and thus increase conduction. Light and heat after all are just
electromagnetic radiation at different frequencies. Hall Effect Devices are
semiconductors that respond to magnetic fields at DC to 100 KHz. There are Hall Effect
Switches that turn on and off at certain magnetic field strengths and there are Hall Effect
Sensors that output an analog voltage proportional to the magnitude and polarity of the
magnetic field. So how can we use these fantastic devices?
Monitoring current in a test fixture is typically accomplished by monitoring the voltage
dropped across a resistor in series with the load. In Figure 1 we see a typical circuit.
When the voltage across R5 reaches about 600 mV the transistor Q2 starts to conduct. As
Q2 conducts the voltage across R7 and R9 increases. As the voltage across R9 reaches
the point R11 is set to the output of the Voltage Comparator goes low, turning on the
LED (in this case). We can turn on an LED, turn off a voltage regulator or inform a
microprocessor of the event.
This circuit works better for lower current levels but at higher current levels the value of
the resistor becomes difficult. We also waste about 1 Volt across the resistor. When the
output voltage is in the neighborhood of 3.3 Volts to 5 Volts wasting almost a volt at this
point of the circuit seems like a bad idea. If we insert a ferrite core toroid choke coil in
place instead of the resistor we can monitor the current through the circuit by monitoring
the magnetic field in the ferrite core. Being steady DC in this part of the circuit the
inductor has almost no voltage drop across it. We modify the coil slightly by cutting a
slot in it big enough to insert a Hall Effect Sensor. An example of what the sensor itself
looks like and the modified coil is shown in a picture included here.
Referring to Figure 2, as current through the coil increases so does the magnetic flux in
the core of the coil. The Hall Effect Sensor responds appropriately and increases the
output voltage. We have an analog out voltage proportional to the current through the
coil. The rest of the circuit is much the same as described in Figure 1. We can use the
output of the Voltage Comparator to turn off the voltage regulator in the circuit and have
an electronic fuse we can adjust with a pot. There are many possibilities available to us.
If you have an interest in learning more about Hall Effect Devices than we can include in
a short magazine article I have put together a short course that demonstrates various Hall
Effect Switches and Sensors in easy to do exercises. It gives you hands-on experience
with the parts and shows examples of how to use the parts, including data sheets and
where to buy the devices. You may contact me directly through my e-mail
([email protected]) or through the Slot Tech or Bench Tech forums at Delphi
forums, or if you must use old technology, phone me at (541) 553-3079 (Warm Springs,
OR).