An actual electronics/physics question borne from a topic in a forum where they discuss electronics, not have feuds.
An optocoupler uses a photo sensitive transistor. A transistor that is controlled by something other than base current. A Hall effect sensor is a transistor also not controlled by base current built by magnetism instead of light.
I know for a fact that a simple, analog opto can be used up to 4 MHz. Why not a Hall effect ? The fastest ones we can find are about 20 KHz bandwidth, maybe a hair more. But certainly not 4 MHz.
I don't know any detail of how they are built, so this is just hand- waving. But it is normal for anything to do with magnetism to be frequency-sensitive; why surprising?
ntrolled by something other than base current. A Hall effect sensor is a tr ansistor also not controlled by base current built by magnetism instead of light.
A Hall effect sensor isn't a transistor. It's a semiconductor where the cur rent flowing across the device gets bent up or down by a magnetic field.
The classic part had four terminals and you drove two of them to set up a c urrent across the device and measured the voltage across the device with th e other two orthogonal terminals to work out how big the magnetic field goi ng through the device was.
It's temperature dependent, but if you measure the resistive drop between t he current terinals you can work out how warm the Hall plate is, and correc t for it.
Presumably the transit time across the device puts an upper limit the frequ ency response, but the wikipedia article on the subject suggests that this is a lot higher than a few kHz. My guess would be that the voltages aren't large and that the amplifiers with enough gain to detect them and jack them up to useful levels in cheap parts aren't all that quick.
not a Hall effect ? The fastest ones we can find are about 20 KHz bandwidt h, maybe a hair more. But certainly not 4 MHz.
In phototransistors, incident light generates charge carriers in the base o f a transistor, one pair of charge carriers per photon absorbed.
controlled by something other than base current. A Hall effect sensor is a transistor also not controlled by base current built by magnetism instead o f light.
urrent flowing across the device gets bent up or down by a magnetic field.
it doesn't even have to be a semiconductor you can use copper, it just does n't give as high an output and is hard to make as thin
Probably noise. Hall sensors are predominantly used in motors and position sensors, which aren't that fast. The Hall effect is based on electron drift in a compound semiconductor, and doesn't depend on diffusion, but the voltage produced is pretty small.
Cheers
Phil hobbs
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Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
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Hall effect isn't a transistor thing -- it's a physics thing. (Well, transistors are physics too, so, nevermind...) Diagrams usually show a square of silicon with four leads (one each edge), bias on one (opposed) pair, sense on the other pair. The output signal is inversely proportional to carrier concentration, so silicon is nice -- you can control that with doping. Still, for reasonable doping, and magnetic fields from modest currents (regarding current transducers here), the signal is ~mV, so needs much gain and compensation (tempco and such) to be useful. The result is fairly poor bandwidth and SNR.
Most integrated sensors are in the 100kHz range, IME. Ballpark, that means
20kHz and 1MHz aren't unreasonable, depending on construction, and tweaks. But probably not 10MHz+.
It's still base current, just the base current is a photocurrent.
Nah. inside a 3 wire hall sensor is a hall-effect cell of semiconductor, a current source and an amplifier
The hall cell is big and flat, and the output voltage is proportinal to the (tiny) current through it, there will also be some capacitance effects slowing it down.
I guess nobody's asked for a fast one. or maybe noone wants to use one to receive AM radio broadcasts.
Fast magnetism sensors (like hard disk read heads) are magentostrictive SFAIK.
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