Hall effect sensor questions

Thread Starter

l0vot

Joined Apr 29, 2013
107
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?
It's a mechanical distributor mod, the mechanical dizzy comes with mechanical timing, and vacuum advance, which I'm going to make use of. If I were to use this to forcibly convert an EFI engine with no way to hack a mechanical dizzy onto it, then i would need to worry about timing advance, but that's a different project for a different day.
 

Thread Starter

l0vot

Joined Apr 29, 2013
107
Oh, and the Honeywell sensor works as well, but it's a latching sensor, have to use a magnet of the opposite polarity to turn it off, both can be used here, but I think I prefer the Infineon.
 

Thread Starter

l0vot

Joined Apr 29, 2013
107
The ignition coil is an inductor, the back EMF it produces when the BJT turns off is a problem because the hall sensor can only take 30V, the BJT can take a lot more abuse, but still, I'm thinking of adding a ballast resistor (which it's supposed to have, anyway), and a small cap across the primary so the primary can "ring" without producing huge negative voltage spikes upstream.
 

drc_567

Joined Dec 29, 2008
1,135
The ignition coil is an inductor, the back EMF it produces when the BJT turns off is a problem because the hall sensor can only take 30V, the BJT can take a lot more abuse, but still, I'm thinking of adding a ballast resistor (which it's supposed to have, anyway), and a small cap across the primary so the primary can "ring" without producing huge negative voltage spikes upstream.
... The usual method of dealing with the inductor shutoff voltage is to place a reverse biased diode in para!lel with the inductor, the coil primary here.. This allows the current in the coil, at shut-off, to
re-circulate by going through the diode. ... The operation at high frequency has to be tested, but the reverse diode may be effective to some extent.The coil current is going to be 3 to 6 amps, so for a safe margin, the reverse diode should handle 10 amps or so.
 

drc_567

Joined Dec 29, 2008
1,135
... One other concern is the voltage that is developed by the current going through the coil primary. That voltage may be too great, and cause problems with the transistor Vce voltage. If the coil current is restricted to the normal 3 to 6 amp range, that would be a better choice. Without actual operating data, it is difficult to come up with any transistor current number.
... In order to see how the transistor is working, a low value resistor can be placed between the transistor base and the sensor signal terminal. The voltage developed across the resistor would be an indication of the base current drawn from the transistor. Without some type of oscilloscope, the best way to test the coil operation would be to put a spark plug on the coil secondary and see if a good spark is developed.
 
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shortbus

Joined Sep 30, 2009
8,719
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 mechanical distributor mod, the mechanical dizzy comes with mechanical timing, and vacuum advance, which I'm going to make use of.
How are you going to do those things? By that I mean how is the mechanical distributors advances going to work with what is basically called a crank trigger and individual coils?
 

Thread Starter

l0vot

Joined Apr 29, 2013
107
... One other concern is the voltage that is developed by the current going through the coil primary. That voltage may be too great, and cause problems with the transistor Vce voltage. If the coil current is restricted to the normal 3 to 6 amp range, that would be a better choice. Without actual operating data, it is difficult to come up with any transistor current number.
... In order to see how the transistor is working, a low value resistor can be placed between the transistor base and the sensor signal terminal. The voltage developed across the resistor would be an indication of the base current drawn from the transistor. Without some type of oscilloscope, the best way to test the coil operation would be to put a spark plug on the coil secondary and see if a good spark is developed.
I have to solder up a test rig so the transistor can get enough current to do it's job, I have 20A schottkey diodes I can put across the primary, it did actually spark with the breadboard test rig in the beginning (off a 5V USB charger no less!), but it seems like the resistance of various contacts has gone up enough that it can't deliver enough current to operate the coil, only 1A right now (but it will fire a neon power indicator, which also shows the polarity of the pulses), I'll find my mini scope, a 6V battery, and a heat sink so the BJTs don't end up cooked. 6V should limit the coil to 4A, and maybe the beefier transistors will show up, if not, the other transistors should still take less than 10mA to switch. I'll see about that ballast resistor, if only to be able to measure the current the coil pulls on the scope.
 

Thread Starter

l0vot

Joined Apr 29, 2013
107
How are you going to do those things? By that I mean how is the mechanical distributors advances going to work with what is basically called a crank trigger and individual coils?
The sensors are going inside the dizzy, it's a distributor mod.
 

shortbus

Joined Sep 30, 2009
8,719
The sensors are going inside the dizzy, it's a distributor mod.
So you mean your not going to use the distributor cap? If not why use a hall sensor any way? Why not opt-interupters(one for each coil) and a ring or disk(depending on how the interrupters are positioned) on the distributor shaft with only one slot on it. That way the width of the slot could equal the dwell angle and the single slot would do the same thing as the magnet for the halls. Cutting the correct size slot would be a lot easier than finding the correct size magnet.

https://www.rohm.com/electronics-basics/photointerrupters/what-is-a-photointerrupter
 

BobaMosfet

Joined Jul 1, 2009
1,823
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.
Put your thinking hats on:

Title: Understanding Basic Electronics, 1st Ed.
Publisher: The American Radio Relay League
ISBN: 0-87259-398-3
 

Thread Starter

l0vot

Joined Apr 29, 2013
107
So you mean your not going to use the distributor cap? If not why use a hall sensor any way? Why not opt-interupters(one for each coil) and a ring or disk(depending on how the interrupters are positioned) on the distributor shaft with only one slot on it. That way the width of the slot could equal the dwell angle and the single slot would do the same thing as the magnet for the halls. Cutting the correct size slot would be a lot easier than finding the correct size magnet.

https://www.rohm.com/electronics-basics/photointerrupters/what-is-a-photointerrupter
light based sensors are significantly easier to foul up than magnetic sensors, plus if the dwell needs to be longer than my 1/2" disk magnets provide, the latching Honeywells allow one small magnet to turn the coil on, and one to turn it off, a small steel disk and some little neodymium magnets allow the dwell to be set easily, then some epoxy can be added to make the positions more permanent.
 

Thread Starter

l0vot

Joined Apr 29, 2013
107
The diode shorts out the back EMF, this protects the circuit, but it also makes the coil not spark, the energy from the field collapsing ends up in the diode instead of the spark gap, a small cap in parallel with the primary results in a better spark, and the 1.1 ohm ballast resistor prevents the cap from hammering the transistor when it turns on. I think the lead-acid battery is also helping, they clamp any voltage spikes that hit them. This test is running off a 6V SLA battery to emulate worst case scenario when cranking. The starter wouldn't do it's job at 6V, so if the coil can spark at all it's going to be fine during actual use.

The ballast resistor was hooked up to the scope so it could read the voltage across it, the leading edge of the pulse is a little bit rounded, but I think that's because the coil is an inductor, and opposes sudden current spikes, the top is flat, the trailing edge is sharp, like it should be. Current appears to be 1.8A, base current appears to be 0.4mA, much lower than I expected, so the gain at this point is 4500.


With the ballast resistor, the BJT is now driving a 2.6 ohm load, with the ~1.4V drop across the BJT, that brings the voltage available to the coil when the engine is running down to around 12-12.5V, so the max current should be ~4.85A. The next thing to do is hook this rig up to a car battery just to make absolutely sure nothing is going to fry, then i can finish the circuit board in the distributor and see if it works for real.
 
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drc_567

Joined Dec 29, 2008
1,135
The diode shorts out the back EMF, this protects the circuit, but it also makes the coil not spark, the energy from the field collapsing ends up in the diode instead of the spark gap, a small cap in parallel with the primary results in a better spark, and the 1.1 ohm ballast resistor prevents the cap from hammering the transistor when it turns on. I think the lead-acid battery is also helping, they clamp any voltage spikes that hit them. This test is running off a 6V SLA battery to emulate worst case scenario when cranking. The starter wouldn't do it's job at 6V, so if the coil can spark at all it's going to be fine during actual use.

The ballast resistor was hooked up to the scope so it could read the voltage across it, the leading edge of the pulse is a little bit rounded, but I think that's because the coil is an inductor, and opposes sudden current spikes, the top is flat, the trailing edge is sharp, like it should be. Current appears to be 1.8A, base current appears to be 0.4mA, much lower than I expected, so the gain at this point is 4500.


With the ballast resistor, the BJT is now driving a 2.6 ohm load, with the ~1.4V drop across the BJT, that brings the voltage available to the coil when the engine is running down to around 12-12.5V, so the max current should be ~4.85A. The next thing to do is hook this rig up to a car battery just to make absolutely sure nothing is going to fry, then i can finish the circuit board in the distributor and see if it works for real.
... Try running it on 12 volts. The transistor collector current normally is 3 to 6 amps, from what I have read. That 0.4 base current is a good sign that the transistor is operating in saturation mode with associated low Vce. The primary current pulse seems to be on track. The reference shape is a ramp followed by a leveling off. ... The diode was a bad idea.
... more reading ... Don't perform a test with the secondary open circuited or the inductor energy will backfire into the transistor causing damage. So keep a spark plug on the coil secondary.
1606908009497.png
 
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shortbus

Joined Sep 30, 2009
8,719
plus if the dwell needs to be longer than my 1/2" disk magnets provide,
What type engine are you working with, how many cylinders? Have you ever looked inside an electronic ignition distributor? At 1/2" diameter the disc your putting it on would need to be very large to get the correct dwell. If you look at like say a GM distributor the magnetic reluctor has teeth of only ~3/32" wide, to give the needed dwell, on a reluctor diameter of ~1". If you want this to actually work, you should do some math and figure out the dwell and other things.

ballast resistor
You do know that the coil at plug individual coils run off of 12V and don't use a ballast resistor?

light based sensors are significantly easier to foul up than magnetic sensors,
You say that and yet there are many different makers out there that are doing it, many of them for high performance engines. Guess they should have asked you before wasting their time and money on them.
 

Thread Starter

l0vot

Joined Apr 29, 2013
107
OK, i hooked the test rig up to a running vehicle to see how it runs at full power, it sparked pretty well, i stuck a magnet to a socket being spun by a battery powered impact to see how the circuit would run close to redline speeds, to make sure it still worked. The Infineon sensor did well in the high speed test the spark was basically continuous, the Honeywell didn't do nearly as well, coil did not spark as reliably, which is odd, since the dwell time should have been ~50% instead of ~20%, and it was at low speed, but at high speed the dwell time was shorter than the Infineon.

the coil was pulling ~5A
base current was ~3.7mA
gain is ~1350 at this point, much higher than I expected based on the datasheet
voltage drop across the transistor was 1.8V, a bit higher than before, so about 9W TDP for all the transistors put together since they aren't going to be on at the same point with this setup.

There's a 0.25 micro farad cap across the primary to give the back EMF somewhere to go while still allowing the coil to spark, there will still be a voltage spike, and there should be high frequency AC from the coil ringing, but I purposfully picked transistors with a much higher voltage rating than appeared necessary for that reason, there is a 1 micro farad cap across the sensor supply rails for stability.
 

Thread Starter

l0vot

Joined Apr 29, 2013
107
What type engine are you working with, how many cylinders? Have you ever looked inside an electronic ignition distributor? At 1/2" diameter the disc your putting it on would need to be very large to get the correct dwell. If you look at like say a GM distributor the magnetic reluctor has teeth of only ~3/32" wide, to give the needed dwell, on a reluctor diameter of ~1". If you want this to actually work, you should do some math and figure out the dwell and other things.



You do know that the coil at plug individual coils run off of 12V and don't use a ballast resistor?



You say that and yet there are many different makers out there that are doing it, many of them for high performance engines. Guess they should have asked you before wasting their time and money on them.
If the exact dwell were that important the coils wouldn't work at different speeds, since the dwell changes based on RPM, on top of that, the dwell depends on things that are easier to test than attempt to calculate using ideal numbers, like how long it takes for the sensor to react to the trigger signal it's supposed to be receiving, and how much dwell the specific coil actually needs in order to function.

Why do you assume I'm using coil packs? Coil packs aren't reliable due to making the ignition transformer too small, they are also more expensive than a standard ignition coil, being more compact has consequences, i'll stick with a standard ignition coil every auto parts store stocks, and is less likely to fail to begin with.

Magnetic sensors are more rugged than optical sensors, and that is a fact, if they want to use optics instead, they can, that's their preference for their project, and it might work better for their specific application, but for my purposes I want something that can get schmoo on it and continue to function correctly, something I could potentially pot in epoxy, and then bolt to a defenseless engine that's either too old to find parts for, or it's too new and the brain shat itself, or it's an Onan with their craptacular version of the ignition system I'm trying to make that's under the flywheel and you gotta take half the unit apart to get at it, you get the point, or maybe you don't.
 

drc_567

Joined Dec 29, 2008
1,135
Is the vacuum advance mechanism on the distributor able to adjust the ignition timing in relation to the engine rpm?
 
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Thread Starter

l0vot

Joined Apr 29, 2013
107
Is the vacuum advance mechanism on the distributor able to adjust the ignition timing in relation to the engine rpm?
The vacuum advance is run off the carb, the centrifugal advance is based on engine RPM, the advance stacks, the vacuum advance rotates the turn table the switch contacts were mounted on, the centrifugal advance rotates the top section of the distributor shaft, where the cam lobes that operate the switch are. The switch mechanism is simply bolted to the turn table, and can easily be removed, the same threaded holes can be used to mount a circuit board.
 

Thread Starter

l0vot

Joined Apr 29, 2013
107
anyhow, during capacitor charging, which occurs very, very quickly, the cap is going to pull about 10A, I'm going to add a 250 ohm resistor between the base, and sensor to keep the sensor load from spiking, probably unnessary, but there's no kill like overkill.
 

drc_567

Joined Dec 29, 2008
1,135
Can you explain how each individual cylinder is receiving a spark pulse? It seems like the sensor would have to be turned on by a magnet signal for each cylinder. ... Something like the black disk in the picture here with multiple pole pieces.
1607002914280.png
 
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