Question on Hall Magnets

[Rdean33422]

I have used hall sensors on several projects but have never seen which type or size magnet should be used.
The size of the magnet with its larger magnetic field would affect the distance at which the sensor is triggered but what about the type of magnet? Can the sensor be damaged by using the wrong type of magnet?

Ray

 

[aka9950202]

Ray, the short answer is No. The magnet just needs to be strong enough for the Hall sensor to detect it. I used 3mm magnets in my engine.

The magnet needs to have the right pole moving passed the sensor. Some sensors work only with the North pole while the other type only works with the South pole.

Cheer,

Andrew in Melbourne

 

[Rdean33422]

So an old style ceramic magnet and the newer rare earth magnets make no difference to the sensor except how soon it will see the field and operate.

Thanks
Ray

 

[Tim Wescott]

The magnet needs to have the right pole moving passed the sensor. Some sensors work only with the North pole while the other type only works with the South pole.

And some will see either, although I don't know if those have made it into model ignition systems. Your best bet is to try what you have & verify.

https://www.digikey.com/products/en?keywords=SS451A

So an old style ceramic magnet and the newer rare earth magnets make no difference to the sensor except how soon it will see the field and operate.

I've always suspected that there's some magnetic magic one can do to make the sensor more precise -- but I don't know what it is .

 

[dsage]

The Magnet magic is to keep the magic stationary and make a rotating steel plate with a hole an appropriate size located between the magnet and the sensor. Commonly referred to as a shutter (amongst other things). As long as the field that makes it through the hole is strong enough it can be very precise.

 

[Charles Lamont]

With rotating magnets and a fixed hall sensor, it is necessary to find the trigger point by testing the installation. Once found, it is not going to vary much, and the advance angle can be measured from that point. Using a fixed magnet and rotating shutter puts the magnet further from the sensor, so a bigger magnet might be needed, and the timing will still have to be found by experiment - to determine just how much of the face of the magnet needs to be exposed by the aperture in order to trigger the sensor.

 

[dsage]

All of the above experiments need to be made regardless of how you mount it. The point of the aperture is that you can EFFECTIVELY have a very small and strong magnet with little to no flux fanning out from the magnet. Your trigger points will be much sharper. You'd have a hard time making an 8 cylinder distributor small with 8 magnets. The flux might be so wide you have little off / on gap. One magnet with an eight hole shutter is much easier.
The shutter doesn't have to be thick. Just sheet metal will do. so the distance from magnet to sensor doesn't increase much if you can limit the runout in the rotating shutter. Super magnets are plenty strong to be spaced back from a hall sensor. I've found more than a quarter inch away they are still effective so 10 or twenty thou extra back from typical operating space wont make much difference. It has always worked well for me.

 

[ozzie46]

All of the above experiments need to be made regardless of how you mount it. The point of the aperture is that you can EFFECTIVELY have a very small and strong magnet with little to no flux fanning out from the magnet. Your trigger points will be much sharper. You'd have a hard time making an 8 cylinder distributor small with 8 magnets. The flux might be so wide you have little off / on gap. One magnet with an eight hole shutter is much easier.
The shutter doesn't have to be thick. Just sheet metal will do. so the distance from magnet to sensor doesn't increase much if you can limit the runout in the rotating shutter. Super magnets are plenty strong to be spaced back from a hall sensor. I've found more than a quarter inch away they are still effective so 10 or twenty thou extra back from typical operating space wont make much difference. It has always worked well for me.

What about Steve Hucks Lil Demon, it has 8 small magnets in a distributor that is a little less than 1 inch in dia. and seems to work fine. Magnets are about 1/8 dia.
Am I missing something here?
Ron

 

[dsage]

What about Steve Hucks Lil Demon, it has 8 small magnets in a distributor that is a little less than 1 inch in dia. and seems to work fine. Magnets are about 1/8 dia.
Am I missing something here?
Ron

What's he triggering? If it's a CDI then it needs almost no dwell. A CDI is usually edge triggered. If your replacing points where you need some dwell for the coil to charge then it would be a different story. Every engine is different. Just another one of the many things that have to be considered.

 

[tornitore45]

Yesterday at 5:23 AM
The Magnet magic is to keep the magic stationary and make a rotating steel plate with a hole an appropriate size located between the magnet and the sensor. Commonly referred to as a shutter (amongst other things). As long as the field that makes it through the hole is strong enough it can be very precise.

When I built my Lynx engine I had a hell of time phasing the spark. With a unipolar Hall sensor it gave two close triggers.
My theory is that when the magnet moves and there is conductive aluminum all around the induced eddy currents distort the magnetic field and makes different from the static magnetic pattern experienced at very low speed. The trigger point is affected by speed, it can act as a sort of advance or act in the opposite direction.

That is why a shutter type work best, the high permeability shutter shield the sensor until the hole passes by.

 

[gbritnell]

All my engines, 4-6 and 8 cylinder have 1 magnet and a shutter disc to trigger the Hall transistor. I've never had problems with this design and my 302 V-8 engine will spin 7800 rpm. I have a V-twin that Dave Sage has built and it has 1 magnet with a flat shutter disc and it also works fine even with a waste spark.
gbritnell

 

[bluejets]

One can always mount the magnet hard on the back side of the hall effect switch, reverse polarity from usual operating flux, and use a metal vane/vanes as a trigger.

 

[ozzie46]

What's he triggering? If it's a CDI then it needs almost no dwell. A CDI is usually edge triggered. If your replacing points where you need some dwell for the coil to charge then it would be a different story. Every engine is different. Just another one of the many things that have to be considered.

I believe he used cdi. I will be using your circuit.
Ron