Hall effect sensor questions

Thread Starter

l0vot

Joined Apr 29, 2013
107
Basically I'm trying to amplify the output of a hall effect sensor with a BJT, but I have no experience with hall effect sensors, or BJTs, but there is quite a bit more information on BJTs.
this is the BJT I'm looking at:
https://www.digikey.com/en/products/detail/stmicroelectronics/BDX34C/2827181

these are the two hall effect sensors that look like they can drive it:
https://www.digikey.com/en/products/detail/infineon-technologies/TLE4905LE6433HAXA1/1650234
https://www.digikey.com/en/products...sing-and-productivity-solutions/SS466A/701359

And there's going to be a 500 ohm resistor between the base of the BJT and the output of the sensor to prevent the BJT from overloading the sensor, trouble it, I don't know how to set up the sensor properly.

The objective is to drive an ignition coil with a transistor and hall effect sensor instead of mechanical contacts, so a nice sharp on/off is what i'm trying to get. The coil has a resistance of ~6 ohms, nominal operating voltage is 13.5V-14.5V, so the current draw should be ~2.3A, going for dedicated coils to eliminate normal spark distribution system, so the duty cycle should be less than 25%, and the max frequency should be 50HZ.
 

drc_567

Joined Dec 29, 2008
1,114
How is the Hall effect sensor to be activated? There would have to be a moving magnetic field in order to switch the Hall effect sensor on and off. A permanent magnet glued to a flywheel should work. ... Something like that.
That transistor in your post is of the PNP type, which means that current must be drawn out, rather than put in to the transistor base, in order to switch it on. Its datasheet shows that it may be necessary to boost the Hall sensor current sinking range somehow, in order to allow it to turn the transistor on to a sufficient degree. That is, 20 ma sinking current is allowed by the Hall sensor, however the transistor requires up to 250 ma (sinking) to switch on completely.
 

bwilliams60

Joined Nov 18, 2012
1,398
A couple of things pop up here. The first being the point mentioned above about how the hall effect sensor is being actuated. The second for me is the resistance value for the ignition coil? What is this coil off of? Most ignition coils have a primarymresistance of 0.5 - 1.5 ohms. A resistance of 6 ohms seems very high. Why not just drivea small distributor on your bench or are you trying to get jus a good clean square wave electronically? I guess what I am trying to get at, is what are you trying to do? Sounds like an automotive test bench of sorts?
 

Thread Starter

l0vot

Joined Apr 29, 2013
107
A couple of things pop up here. The first being the point mentioned above about how the hall effect sensor is being actuated. The second for me is the resistance value for the ignition coil? What is this coil off of? Most ignition coils have a primarymresistance of 0.5 - 1.5 ohms. A resistance of 6 ohms seems very high. Why not just drivea small distributor on your bench or are you trying to get jus a good clean square wave electronically? I guess what I am trying to get at, is what are you trying to do? Sounds like an automotive test bench of sorts?
The objective is to make an abomination with mechanical timing, but solid state switching and individual coils to use on actual engines to escape from the points system, and the necessity of distributing the spark without the ignition system actually having any brains inside of it.

It's a standard ignition coil, apparently I remembered the wrong value because it's closer to 1.5 ohms, like you suggested, thanks for that correction, the BJT should still be able to drive it, but drc_567 is right, it takes a quarter of an amp to turn on (a gain of only 40!), I'm going to have to get more BJTs to drive it, or use some FETs i already have, or find a BJT with enough gain to be dirven directly by the sensor. but in any case I still don't know how to wire up a hall effect sensor correctly.
 

Thread Starter

l0vot

Joined Apr 29, 2013
107
How is the Hall effect sensor to be activated? There would have to be a moving magnetic field in order to switch the Hall effect sensor on and off. A permanent magnet glued to a flywheel should work. ... Something like that.
That transistor in your post is of the PNP type, which means that current must be drawn out, rather than put in to the transistor base, in order to switch it on. Its datasheet shows that it may be necessary to boost the Hall sensor current sinking range somehow, in order to allow it to turn the transistor on to a sufficient degree. That is, 20 ma sinking current is allowed by the Hall sensor, however the transistor requires up to 250 ma (sinking) to switch on completely.
I understand that the hall effect sensor is going to be activated by magnetic fields, what I don't know is how to wire it up so that it can drive a transistor without blowing up.

The coil is designed to be driven off the negative rail, a PNP transistor was selected specifically because of that, it loked like the transistor sarurated at 6mA on the listing, i didn't realize the full switching current is only found in the datasheet, which is going to make it harder to find the right BJT for the job, since I can't sort by base current, but now that I know, I can hunt one down.
 

Thread Starter

l0vot

Joined Apr 29, 2013
107
Ok, this BJT looks like it turns on almost completely at 20mA base current with a 10A load, according to the datasheet graphs:
https://www.digikey.com/en/products/detail/on-semiconductor/MJH6287G/13468485
It can also handle 20A, instead of 10A, looks like ~14W heat dissapation when there's 20mA base current, and a 10A load

One of the hall effect sensor packages I listed can put out 100mA:
https://www.digikey.com/en/products/detail/infineon-technologies/TLE4905LE6433HAXA1/1650234

It looks like a 250 ohm resistor would prevent the transistor from blowing the sensor, and still let it operate down to 6V so it still sparks when cranking.

Does that look like it would work?
 

drc_567

Joined Dec 29, 2008
1,114
... From the MJH6287 transistor datasheet graph, the very high hfe gain is what allows the high collector current to be generated with such a relatively low base current. The hfe value appears to peak at a collector current of about 3 amps, so maybe try limiting the transistor current to that range before trying for 10 amps or greater. The transistor is necessarily operating in the saturation region, so the transistor Vce would only be something like 1 to 3 volts if everything is working properly.
... An approximate calculation for a resistor in series with the ignition coil that would limit the transistor current to 3 amps is a value of 2.0 Ω, and having a power rating of 20 watts. This will allow verification of the transistor Vce saturation voltage, as well as the current sinking capability of the TLE4905 sensor.
... There are two versions of the TLE4905 sensor. The TLE4905 is for use with a one-way magnetic flux arrangement, and the TLE4905L version is for bi-directional flux, where the magnet poles would be alternating with respect to the sensor.This would depend on the operational set-up.
... The TLE4905 sensor may be able to accommodate enough sinking current to turn the transistor collector current on to the required level without any series limiting resistor. The sensor datasheet suggests a maximum of 100 ma.
... It may be useful to obtain several of the sensors and transistors for initial trials, if costs permit.
 
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Thread Starter

l0vot

Joined Apr 29, 2013
107
... From the MJH6287 transistor datasheet graph, the very high hfe gain is what allows the high collector current to be generated with such a relatively low base current. The hfe value appears to peak at a collector current of about 3 amps, so maybe try limiting the transistor current to that range before trying for 10 amps or greater. The transistor is necessarily operating in the saturation region, so the transistor Vce would only be something like 1 to 3 volts if everything is working properly.
... An approximate calculation for a resistor in series with the ignition coil that would limit the transistor current to 3 amps is a value of 2.0 Ω, and having a power rating of 20 watts. This will allow verification of the transistor Vce saturation voltage, as well as the current sinking capability of the TLE4905 sensor.
... There are two versions of the TLE4905 sensor. The TLE4905 is for use with a one-way magnetic flux arrangement, and the TLE4905L version is for bi-directional flux, where the magnet poles would be alternating with respect to the sensor.This would depend on the operational set-up.
... The TLE4905 sensor may be able to accommodate enough sinking current to turn the transistor collector current on to the required level without any series limiting resistor. The sensor datasheet suggests a maximum of 100 ma.
... It may be useful to obtain several of the sensors and transistors for initial trials, if costs permit.
I will give it a go, and see what happens. It will take a while for parts to come in, but I will post results. Thank you.
 

Thread Starter

l0vot

Joined Apr 29, 2013
107
Aren't breakerless ignitions already available? Are you trying to do it a new way or just avoid the cost?

https://www.summitracing.com/parts/pnx-1281
https://www.hagerty.com/media/maintenance-and-tech/ignition-debate-points-vs-pertronix/
I'm trying to go for individual coils, which is a step further. There is a distributor available that will do what those conversion systems do right off, it takes some modification in order to make it fit in the hole, but It does work once it's made to fit.
 

jpanhalt

Joined Jan 18, 2008
11,088
Aren't you describing coil on plug? Are you using a micro to control the spark or trying to use the Hall sensor directly as the distributor? Maybe a sketch would help.
 

drc_567

Joined Dec 29, 2008
1,114
I will give it a go, and see what happens. It will take a while for parts to come in, but I will post results. Thank you.
It may turn out that it will be necessary to bias the transistor base terminal with some intermediate voltage. A voltage divider would accomplish this and establish a known voltage at the transistor base terminal. That base voltage should enable the calculation of a resistor placed between the sensor signal terminal and the transistor base. Then the sensor sink current could be adjusted to the 20 ma corresponding to the collector current specification. ... This approach may make more sense than using no resistor at all between the two parts.
 

Thread Starter

l0vot

Joined Apr 29, 2013
107
Aren't you describing coil on plug? Are you using a micro to control the spark or trying to use the Hall sensor directly as the distributor? Maybe a sketch would help.
my sketching skills kinda suck, but basically i'm modding a mechanical dizzy to fire coils on the plugs, the spinny bit gets magnets glued to it to run the sensors, the sensors are positioned to fire one cylinder each, then the sensors go to transistors to amplify the output so it can actually fire the coil, there's probably a more efficient way to do this since the dizzy i got the idea from didn't need to dissipate very much heat, and it used magnetic pickups to drive the coil, but i'm a solid state n00b, so if it works at all that's a huge step in the right direction.
 

drc_567

Joined Dec 29, 2008
1,114
A possible test circuit:
Capture.PNG
This is the configuration shown in the TLE4905 application circuit. If the transistor base current is excessive, then a resistor can be used to limit it.
... Don't actually know how much transistor collector current this will produce. If the sensor output is pulsed, then it may not be too much. However, ... another guess, the transistor must be in saturation mode for the 10 to 20 amp collector currents to be generated. This means large base currents, up to 200 ma. Again, sensor pulsing will have some unpredictable effect.
... Another idea to try maybe.:
If a single TLE4905 does not sink sufficient current to put the MHJ6287 transistor into saturation mode, with 10 amps or so collector current, then arrange a second TLE4905 in parallel so that both sensors are able to sink the base current from the transistor. The physical placement of the sensors would just be one stacked on top of the other, so that the magnet flux would intersect both sensors at the same time. The assumption is that the flux stream would go through both sensors and turn the transistor on simultaneously. The combined base current would not exceed each individual sensor current limit, but would put the transistor into the high collector current saturation mode.
 
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Thread Starter

l0vot

Joined Apr 29, 2013
107
A possible test circuit:
View attachment 223527
This is the configuration shown in the TLE4905 application circuit. If the transistor base current is excessive, then a resistor can be used to limit it.
... Don't actually know how much transistor collector current this will produce. If the sensor output is pulsed, then it may not be too much. However, ... another guess, the transistor must be in saturation mode for the 10 to 20 amp collector currents to be generated. This means large base currents, up to 200 ma. Again, sensor pulsing will have some unpredictable effect.
... Another idea to try maybe.:
If a single TLE4905 does not sink sufficient current to put the MHJ6287 transistor into saturation mode, with 10 amps or so collector current, then arrange a second TLE4905 in parallel so that both sensors are able to sink the base current from the transistor. The physical placement of the sensors would just be one stacked on top of the other, so that the magnet flux would intersect both sensors at the same time. The assumption is that the flux stream would go through both sensors and turn the transistor on simultaneously. The combined base current would not exceed each individual sensor current limit, but would put the transistor into the high collector current saturation mode.
I could also add a transistor to boost the sensor output, I'll try that test circuit once the parts come in. My caps aren't the right value tho, I have 1nf caps, then they jump to 100nf, is the cap value critical it looks like filtering to me?
 

drc_567

Joined Dec 29, 2008
1,114
I could also add a transistor to boost the sensor output, I'll try that test circuit once the parts come in. My caps aren't the right value tho, I have 1nf caps, then they jump to 100nf, is the cap value critical it looks like filtering to me?
Try the 1nF capacitors. There is usually some tolerance. Also it should be the same to put the capacitors in parallel ... they add up that way.
... A transistor might work to increase the sensor drain capability. The details would have to be worked out. ... Suggest trying a simple sensor - transistor configuration to start with, once you obtain the parts. That should provide an indication of how the circuit is actually performing, and what is required to get the collector/emitter current up to 10 amps or more.
... and also if this is on the right track or not.
 

Thread Starter

l0vot

Joined Apr 29, 2013
107
Try the 1nF capacitors. There is usually some tolerance. Also it should be the same to put the capacitors in parallel ... they add up that way.
... A transistor might work to increase the sensor drain capability. The details would have to be worked out. ... Suggest trying a simple sensor - transistor configuration to start with, once you obtain the parts. That should provide an indication of how the circuit is actually performing, and what is required to get the collector/emitter current up to 10 amps or more.
... and also if this is on the right track or not.
will do
 

Wolframore

Joined Jan 21, 2019
2,283
The issue with driving the transistor directly by the hall sensor is that although you can set timing based on sensor placement, there will be no advance curve. Not sure what this is for but it will limit the powerband. Have you thought about the advance curve?
 

Thread Starter

l0vot

Joined Apr 29, 2013
107
all right, some of the stuff came in, the sensors, and the original 10A transistors I was going to use, originally I had the sensor wired up with the resistor, and caps, and it worked, but due to the small breadboard size, and the fact that the caps have plated iron leads, some of the components shorted out because they were attracted to the magnet, lost 2 sensors that way, i removed the resistor, and caps, and it still works, the sensor drives the transistor well enough to light a LED.

I have the output routed through a resistor to measure the current, there's 1.3mV across a 10 ohm resistor, or 72mV across the 500 ohm resistor, both come out to ~0.14mA. The base current seems to self limit based on the current the transistor controls. The LED is drawing 70mA, the base is pulling 0.14mA, which is a gain of around 500, the base current of 250mA seems to be for a 15A load, from the graphs I have seen of other transistors, the gain gets really low when the transistor is switching a load larger than it's rated to continuously, and it tends to peak where they measure the gain (it's rated for 10A, gain is measured at 3A), unfourtunately, there's no graph on that datasheet, but it does say that it takes around 6mA base to switch a 3A load.

Could I use BJTs in parallel to stay close to the peak of the gain curve?
 
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