Forums

Why Is Hall Effect So Slow ?

Started by Unknown May 12, 2018
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. 

Any clues as to why ? 
In article <60480a12-24e8-4d05-8c89-a4baf792eed3@googlegroups.com>, 
jurb6006@gmail.com says...
> > 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. > > Any clues as to why ?
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? Mike.
jurb...@gmail.com wrote:
> > > An actual electronics/physics question borne from a topic in a > forum where they discuss electronics, not have feuds. >
** Yet they let people like you join ??
> 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. >
** How about 1MHz ?? http://www.eenewseurope.com/news/hall-effect-current-sensor-has-1mhz-bandwidth
> Any clues as to why ?
** Hall sensors are mostly used with mechanical devices like motors where bandwidth above 20KHz is not needed. Much greater bandwidths are possible, IIRC HP made a 10MHz Hall current probe for use with a scope. This paper discusses going much higher. https://acta.imeko.org/index.php/acta-imeko/article/viewFile/IMEKO-ACTA-06%20%282017%29-04-03/pdf .... Phil
On Saturday, May 12, 2018 at 5:40:57 PM UTC+10, jurb...@gmail.com wrote:
> 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.
A Hall effect sensor isn't a transistor. It's a semiconductor where the current 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 current across the device and measured the voltage across the device with the other two orthogonal terminals to work out how big the magnetic field going through the device was. It's temperature dependent, but if you measure the resistive drop between the current terinals you can work out how warm the Hall plate is, and correct for it. Presumably the transit time across the device puts an upper limit the frequency 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.
> 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.
In phototransistors, incident light generates charge carriers in the base of a transistor, one pair of charge carriers per photon absorbed. It's a very different mechanism. <snip> -- Bill Sloman, Sydney
On 12/05/2018 08:40, jurb6006@gmail.com wrote:
> 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. > > Any clues as to why ? >
No idea, but these are magneto resistive rather than Hall effect and claim 110Mbps. https://www.nve.com/il800.php Cheers -- Clive
l&oslash;rdag den 12. maj 2018 kl. 10.40.05 UTC+2 skrev bill....@ieee.org:
> On Saturday, May 12, 2018 at 5:40:57 PM UTC+10, jurb...@gmail.com wrote: > > 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. > > A Hall effect sensor isn't a transistor. It's a semiconductor where the current 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 doesn't give as high an output and is hard to make as thin
On 05/12/18 03:40, jurb6006@gmail.com wrote:
> 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. > > Any clues as to why ? >
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 -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
On 2018/05/12 9:30 AM, Phil Hobbs wrote:
> On 05/12/18 03:40, jurb6006@gmail.com wrote: >> 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. >> >> Any clues as to why ? >> > > Probably noise.&nbsp; Hall sensors are predominantly used in motors and > position sensors, which aren't that fast.&nbsp; 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 >
Hall effect switches were used in keyboards in the 70s as I recall. John
<jurb6006@gmail.com> wrote in message 
news:60480a12-24e8-4d05-8c89-a4baf792eed3@googlegroups.com...
> 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. > > Any clues as to why ?
Gain and noise. 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+. Tim -- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: https://www.seventransistorlabs.com/
On 2018-05-12, jurb6006@gmail.com <jurb6006@gmail.com> wrote:
> 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.
It's still base current, just the base current is a photocurrent.
> A Hall effect > sensor is a transistor also not controlled by base current built by > magnetism instead of light.
Nah. inside a 3 wire hall sensor is a hall-effect cell of semiconductor, a current source and an amplifier
> I know for a fact that a simple, analog opto can be used up to 4 MHz. Why not a Hall effect ?
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.
> The fastest ones we can find are about 20 KHz bandwidth, maybe a hair more. But certainly not 4 MHz. > Any clues as to why ?
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. -- &#1578;