vw crank hall sensor

[o.g.johnson]

Hi folks, just wondering if anyone could confirm something for me. I'm using a vw hall crank sensor, with the 60-2 trigger wheel, as a tach input into microsquirt. I'm not getting an RPM input into megatune when I crank the engine over, so I tested the crank sensor.

Basically I connected the multimeter between sensor output (opto+) and sensor -ive (opto-/ground), and turned the engine by hand with a spanner. The output voltage remained the same at 4.15V (Vref powering the hall +ive). Am I correct in thinking I should be getting a voltage switching between this value and ground if it's working correctly? Is my crank sensor therefore suspect?

Ta.

[mxrob]

Yes, you should be getting a voltage pulse out of your hall sensor with each tooth/magnet it passes. Might need to run 12v to the sensor depending on if there is any internal circuitry in the sensor. I don't know anything about your sensor but I'm going through a learning curve right now with µs and hall sensors. Opto will work with a hall sensor but you may have an issue with the 60/2 wheel and using opto. VR1 would likely be a better choice (from what I've learned so far) but it does require the "hall offsetter circuit" designed by Bruce to work. Take a look here for my saga... http://www.microsquirt.com/viewtopic.php?f=87&t=23545

[o.g.johnson]

Ok, thanks Rob. I'll have a try with 12V as the supply for the sensor and see what I get. If it doesn't work out I'm going to try the hall sensor on the distributor instead, and go from there, as the crank sensor on the golf is a proper arse to get to (and I'm skint). Cheers.

[mxrob]

Ok, thanks Rob. I'll have a try with 12V as the supply for the sensor and see what I get. If it doesn't work out I'm going to try the hall sensor on the distributor instead, and go from there, as the crank sensor on the golf is a proper arse to get to (and I'm skint). Cheers.

If you end up using the hall sensor on the distributor then opto will likely work fine.... once you get a signal to it....

[o.g.johnson]

Bah! Now running from hall sensor on dizzy, and still no rpm input showing in megatune. Vref to hall +ive, optoin-/sensor ground to hall -ive, and optoin+ to hall output. This is on the dizzy the car runs on so is fine. Maybe I should be looking at supplying it with 12V then. Will try again tomorrow.

[o.g.johnson]

Wiring in a pullup resistor between Vref and optoin+ to give myself a better square wave, as I tested and wasn't seeing anything. General consensus is that I should use upwards of 5kohm for this purpose, to minimise the current drain on Vref, but does anyone know roughyl how much current the Hall sensor itself will be taking from Vref?

[mxrob]

Wiring in a pullup resistor between Vref and optoin+ to give myself a better square wave, as I tested and wasn't seeing anything. General consensus is that I should use upwards of 5kohm for this purpose, to minimise the current drain on Vref, but does anyone know roughyl how much current the Hall sensor itself will be taking from Vref?

Why use vref at all? Unless your sensor can only run on 5v... most have a pretty wide operating range. I'm using straight 12v with no issues. Since vref is supposed to only have a 20ma capacity it might be better to leave it to it's intended duties. I put a copper heat sink on my vref thinking, at first, I would use it for my sensors but the more reading I did the less it sounded like a good idea. The resistor size is going to vary depending on the signal your sensor puts out. As an example my Hamlin sensor required a 10k ohm resistor to get the maximum RPM I could get out of opto with my 36 tooth wheel..... 5800 clean RPM, which wasn't good enough for my application so I went with VR1 and Bruce's hall offsetter circuit. I now get clean signal all the way to my 8k redline. For a distributor set up on opto with an appropriately sized pull up resistor it should work fine I'd think.

[o.g.johnson]

So how I understand it so far, is that the pullup resistor holds the opto+ input high, and the ground holds the opto- low. When the hall sensor passes the window in the dizzy ring, opto+ is pulled down to ground (or near enough). When the window in the dizzy ring has passed the hall sensor, and there is metal in front of it again, opto+ is no longer grounded and gets pulled up to Vref again by the pullup resistor.

When opto+ isn't being pulled down, no current is being drawn from Vref (maybe a very negligable amount going into the opto circuit), but while opto+ is being grounded by the hall sensor, current is flowing from Vref, through the pullup resistor, to ground.

If the pullup resistance is too low, too much current will flow from Vref, and if this, together with the other sensors' demands, exceeds the total available of 20 mA or so, the Vref circuit can burn out. If the pullup resistance is too high, however, the pulling up of the opto+ will take so long that it doesn't give a decent signal above a certain RPM.

So there is an optimum size for the pullup resistance for each hall unit, and if this gives it a rev limit beneath that of the engine it's trying to control, the VR offsetter circuit is the next option. Forgive my long-windedness, just thinking out loud, but does that sound about right?

I thought it would be a good idea to use the Vref because it's clean and regulated, but this is starting to look like more hassle than it's worth. Next time I get a chance to play with it I'll try using 12v to power the Hall and see what it does, this should at least allow me to use a lower pullup value withoutt burning anything out.

[mxrob]

Snip.....
So there is an optimum size for the pullup resistance for each hall unit, and if this gives it a rev limit beneath that of the engine it's trying to control, the VR offsetter circuit is the next option. Forgive my long-windedness, just thinking out loud, but does that sound about right?

I thought it would be a good idea to use the Vref because it's clean and regulated, but this is starting to look like more hassle than it's worth. Next time I get a chance to play with it I'll try using 12v to power the Hall and see what it does, this should at least allow me to use a lower pullup value withoutt burning anything out.

Yes on the first part..... and I thought the same thing about vref. Reading about all the noise issues I've seen posted I was a bit too paranoid I guess.