Microsquirt and 60-2 VR Sensor - What value shunt resistor?

[Tippy]

Originally Posted by flightlead404

boy I get a massive backfire when I loose rpm signal, made worse as I don't have the exhaust on right now. First time it happened my gf came running in, thought there was an explosion. The cops came over and said they had reports of shots fired

BTDT, when I had the Ford EDIS coilpack wired wrong....

[Reanimotion]

The MicroSquirt v3 circuit is based on a max9926


full document is here microsquirt_v3_0_1.pdf

[Reanimotion]

the internal 5k shunt on the MicroSquirt is inside the current limiting series resistors (10K each) meaning it does very little to provide an adequate load for the Bosch VR
The max9926 has a high impedance 100K+ Ohm input internally and the internal shunt drops the impedance the VR loads against down to 25K

The Bosch VR is quite strong and much happier with a decent load to play with
an external shunt of 1K @ 2 watts should do the trick and give the VR sensor in the distributor some load to work against

The purpose of the shunt is to change what would be a high impedance input in to a low impedance input. One that the relatively low output impedance VR sensor can still easily drive, but that induced noise cannot significantly drive.

[hberwald]

Hi,
Did you ever get to the bottom of this issue? I am having the same issue at higher RPM's, only I have been messing with the resistors to raise the breakup point. Now at around 6200 rpm. using 3K series and 1k shunt.

[flightlead404]

Originally Posted by hberwald

Hi,
Did you ever get to the bottom of this issue? I am having the same issue at higher RPM's, only I have been messing with the resistors to raise the breakup point. Now at around 6200 rpm. using 3K series and 1k shunt.

Sort of. I purchased a VR conditioner from the speeduino project. Pretty cheap. Connected that up inline. Works like a charm.

[Kram]

There is SO much misinformation about this I had to leave a reply to save others from going around in circles.

Firstly, the correct place for a shunt resistor is where indicated in the Megasquirt schematic, rather than directly across the VR sensor. That is, after the input resistors, which in the Megasquirt example are 2 x 5k resistors in series.

The reason there are 2 x 5k resistors instead of 1 x 10k is to do with the voltage rating of the resistors. Generally, resistors will have a voltage rating of about 200V. Using 2 in series allows 400V input.

One of the reasons the resistors are there is to provide protection for the inputs of the MAX9926. The inputs of the MAX962x series are protected with diodes connected to the supply voltage and ground. Thus any input that tries to head above supply or below ground will be clamped by these diodes. However the diodes can only withstand +/-40mA absolute maximum. (Safe design would allow for half that). Poking a VR sensor straight up the inputs of the chip will destroy these protection diodes causing peaks of almost 300mA. The result will then likely be erratic output (I traced a "will not start" fault in a Volvo to this exact issue after the car was due to go to the scrap yard because the dealers could not find the problem).

So a 10k input resistance allows for 200V nominal (400V worst case) on the input with no shunt present.

Secondly, the 10k input resistors and the 1nF capacitor act as a differential input filter. These values provide a cut off for frequencies about about 8kHz which is designed to get rid of high frequency noise. Similarly the 10nF capacitors act as a common mode filter. All designed to get rid of external noise.

This circuit works by keeping the signal path of both the VR+ and VR- identical. The MAX992x subtracts one from the other (difference amplifier) and compares the result to an internal reference (2.5V). The idea is that noise will be induced equally into both lines, therefore the difference amplifier will cancel out the noise leaving the true signal to be amplified.

This is also why the wires from the sensor should be twisted together (called a "twisted pair") and posisbly also screened.

This circuit arrangement is almost exactly the same used in Bosch Motronic ECU's used on millions of cars. They don't use the MAX992x, rather a special Bosch ASIC, but the input arrangement is very similar. (Using a VR on the crank and Hall on the cam)

If you place a shunt directly at the input, it will need to be about 5W to 10W, depending upon your expected input voltage. Your VR sensor will also dissipate heat from its internal winding resistance. None of that is a good thing.

If you place the shunt directly at the chip input, the power rating of the resistor only needs to be 0.008W (ie a 1/4W resistor is very comfortable). It will achieve about a 4x voltage reduction at the input, although this is only really relevant at lower speeds where the VR output is less than about 30V (because at higher voltages the signal gets clamped by the protection diodes inside the MAX9926.

If you are going to believe some of the internet wisdom and fiddle with resistor dividers etc, then try to make them balanced (i.e. equal in each leg) that is, a series resistor in each leg (10k?) and a shunt resistor (4k7?) between the two. This is effectively exactly the same as just using the shunt on the input to the MAX9926.

I am sorry this is so long. Can I put it another way? If you have noise problems, then you likely have a noise being induced into the input that is [I]different[I] in each of the VR+ and VR- wires. Or very large in both. Either way, look at the wiring path, look at current paths in the engine block and bell housing around the starter motor. Make sure both wires are the same lenght(!) and twisted around each other. This is important at any connectors making sure NOT to have a big loop of wire in one leg and a short wire in the other. Shield if necessary and ONLY earth at the ECU end. Make sure the engine and transmission are well earthed. Make sure you haven't damaged the MAX9926 inputs with inadvertent overcurrent stress.

If this is a starting related issue note that the starter often finds earth in the bell housing and this is then bolted back to the block to make its way back to the battery. Dirt under the bell housing is known to cause crank sensor noise problems in some cars. Manufacturers have even issued instructions to clean and rebolt the bellhousing faces to cure misfire and starting issues. An earth strap directly to the starter might help.

[hberwald]

Kram, thanks for the insight.

I did fix my issue with adjusting the noise filtering in Tuner Studio. It was set just enough to mess with me at the higher RPM's.
Thanks, herb