MAX9924 configuration and use cases

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TonyS
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Re: MAX9924 configuration and use cases

Post by TonyS »

Fred wrote:View a VR wave form, and think about what the output of a VR conditioner is, and look at the two possible patterns and you will soon see why :-)
Sorry Fred, but I have not found a definitive diagram that correlates the position of the VR sensor (relative to the gear "tooth") and the resultant waveform. I have read (data sheet for the MAX9924) that the "zero voltage level in the VR sensor signal corresponds to the center of the gear-tooth...".
Well there are two zero crossings (positive slope / negative slope). Can you elaborate on your reply (I really don't know the answer).

Jared -
I found this VR sensor document that might be of interest to you.
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Re: MAX9924 configuration and use cases

Post by Costa »

Hi TonyS

see http://www.msextra.com/doc/ms1extra/MS_ ... l.htm#vrin

then scroll down a bit.
previously: ca7
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Re: MAX9924 configuration and use cases

Post by TonyS »

Hi ca7,
Thanks for the link as it had quite a bit of information, unfortunately I am still having a hard time correlating the time based waveform with the actual sensor / tooth position. Plus, the Mega information suggests that the correct zero crossing reference is the one on the negative slope, and looking at the data sheet for the MAX9924, it looks like it goes active on the positive going slope. I know I will figure it out sometime, but this is one of those "yeah I know how it works, wait a minute..." issues for me.
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Re: MAX9924 configuration and use cases

Post by jbelanger »

Tony,

You need to have a better look at the datasheet. It uses the down going zero crossing as the trigger. Look at figure 1.

Jean
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Re: MAX9924 configuration and use cases

Post by TonyS »

Hi Jean,
I have looked at the data sheet and that is / was part of what was confusing me. The MAX9924 goes active / low (it does have an open collector output) on the rising edge of the input signal. BUT, I think I am starting to understand how this all comes together.
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Re: MAX9924 configuration and use cases

Post by Fred »

The crux of it is that you get your "gap" split in half if you use the wrong edge, also, the wrong edge is typically not stable in position with RPM. For the record, I had to think about this a bit too, it was a long time ago, though, and probably there are posts on this forum showing me as the noob ;-)

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Re: MAX9924 configuration and use cases

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I think this explains the 100V issue. I know I'm blocked and all, but the blocking implementation has been as good as the MS implementation. R51 on the V3.0 schematic is 1m ohm. That's just bad design, and will produce very high voltages. I see the LM1815 datasheet notes a 18K with a max allowed voltage of 108vp-p, so actual power would be at least half of that 108, probably closer to 1/4 of that voltage. Also this is the max the chip can handle, not the max expected VR signal. I'm going to round that to 100v for the heck of it.

The MAX chip uses two 10k's that are effectively 20k-ish. This nearly matches the suggested impedance for the LM1815.

If we assume the VR signal is a sine wave that runs at the max allowed noted 100vp-p, the RMS voltage would be .707 of 100, so 70.7v, not 100. It's not a sine, and by eyeballing the curves I've seen, I think it's much closer to .5 then it is to .7. The power at 70.7v would be (70.7^2)/20,000 = .249 watts.

I believe the max expected rms power could be as high as 1/2 of 100 so 50v not 100v, then we calc (50^2)20,000 = .125 watts. That 1/2 is an estimated voltage. I would want to check and see several wave forms before I can consider that voltage to be correct. A fairly small change in that voltage will result in a large change in power. Also we will likely run at an elevated temp, and the 1/8 watt spec is based on ideal room temp conditions, so a 1/4 watt probably isn't a bad idea, but it's also designed on maintaining that %1 or %5 ect error bracket, not that it will burn up, or cause the solder to melt, or any thing significant like that. If the elevated temp cause the 20k to become 22K I don't think that's a problem, the issue is we would want to keep it 11k and 11k. If we heat sink one and not the other there is a chance it could become 10k and 12k, which I think is still not an issue.

Can anyone with a VR and 20k resistor confirm the RMS voltage? I'd like to know how much heat we are actually looking at. I'm curios if we will want to heat sink these to help keep the case cool. As far as I can tell, the resistors on spin1 puma are fine, but it would be nice to verify that we don't need to bump them up to a 1/4 watt. Then when we do bump it up, we'll know it's just excessive.

I'd also like to verify the max voltage on the max leads. How high can we go before it arcs. Perhaps these will need conformal coating to increase that voltage barrier. Or perhaps we can assume normal voltages will be present, and use a lower RMS voltage.
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Re: MAX9924 configuration and use cases

Post by Fred »

jharvey wrote:I know I'm blocked and all, but the blocking implementation has been as good as the MS implementation.
ROFL, I just about woke people up! :-) So true.
If we assume the VR signal is a sine wave that runs at the max allowed noted 100vp-p, the RMS voltage would be .707 of 100, so 70.7v, not 100. It's not a sine, and by eyeballing the curves I've seen, I think it's much closer to .5 then it is to .7. The power at 70.7v would be (70.7^2)/20,000 = .249 watts.
Keep in mind that such voltages (and power levels) are only found at the highest RPM the engine can turn! At any lower RPM the V is also lower, SO, unless you expect to hold your engine at 8k all day, you can probably derate the resistors by quite a bit before you get any issues.
Can anyone with a VR and 20k resistor confirm the RMS voltage?
Sorry, mine fell off and rolled away... lol.
I'd also like to verify the max voltage on the max leads. How high can we go before it arcs. Perhaps these will need conformal coating to increase that voltage barrier. Or perhaps we can assume normal voltages will be present, and use a lower RMS voltage.
Considering that this chip is DESIGNED for VR use, and the reference design shows 10k, I doubt you'll have arcing issues. Once again, you have a fixed voltage at the max chip itself, the voltage is lost across the resistors, hence their power dissipation, so by definition and law of physics (V=IR) the voltage is high at one end, low at the other, and power dissipated in the middle, thus the exposed voltage at the max pins is three fifths of fcuk all ;-)

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EssEss
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Re: MAX9924 configuration and use cases

Post by EssEss »

jharvey wrote:Can anyone with a VR and 20k resistor confirm the RMS voltage?
It's not as easy as this .. not all VR pickups are the same. The characteristics of the pickup are matched to the expected bandwidth. The pickup for a single tooth @ 10krpm is going to be drastically different from a pickup that can handle 60 edges @ 10krpm.

A classic indicator of 'insufficient bandwidth' is demonstrated in that turbomiata forum posting earlier where the null throws the next following tooth off. It never goes sufficiently negative to trigger the conditioner. In that case, it's usally easier to go to hall - the bandwidth on those things are huge (and you get free conditioning :) ).

I'll try to go lookup that picture I'm talking about.
here: http://img401.imageshack.us/img401/9299 ... 66rws8.gif
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EssEss
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Re: MAX9924 configuration and use cases

Post by EssEss »

A sensor of matched b/w should look more like this:
http://img37.imagefra.me/img/img37/6/4/ ... 7f8d08.gif

except it looks like he's got the pickup input backwards.
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