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Please, how do I limit a peak DC voltage to 2.45 volts in a car MAP circuit?

Thread starter #1
Hi guys, I need help limiting a circuit to a MAP pressure sensor. As pressure increases in a boosted manifold application, voltage from the MAP sensor increases. I need to limit that voltage from ever hitting 2.51 volts. I figure 2.45 volts is a safe spot to limit to. This will effectively limit the car from knowing a higher boost level is ever achieved (its a long story, but confirming for inquiring minds that is indeed my goal)

I do not want to scale the voltage signal, i want to trim or clip it... only eliminate anything above 2.45 volts and leave everything below 2.45 volts unchanged and not scaled in any way. I believe the original supply voltage is 5 volts, if that matters.

If someone can point me to how I can package a small device for easy replication and installation, into the MAP sensor circuit, please let me know. Thank you!
 

ronsimpson

Well-Known Member
Most Helpful Member
#2
I need to know more so I found this:
How it works is 5 volts DC is supplied to the sensor from the PCM (powertrain control module). Inside the MAP sensor there is a resistor that moves in relation to intake manifold pressure. The resistor varies the voltage between about 1V to 4.5V (depending on engine load) and that voltage signal is returned to the PCM to indicate manifold pressure (vacuum). This signal is essential for the PCM to determine fuel delivery and is also used some times to determine if the EGR Valve is operating properly.
 
Thread starter #4
As i mentioned above, i am fully aware of the negative consequences this will have on the vehicle. However, the vehicle measures from the MAF at this level of boost for air fuel ratios, and the boost levels at the MAP are only used to throw an error code on the stock tune. We are staying below that error code throwing.

Please, I need to know how to do the circuit, not spend days explaining to others all the "why's" of my project or be taught by them how they assume my car works. Thank you.

This is for a scientific process / research based project. I am fully aware of the potential negatives. I just need to make the circuit properly. Would a Zener diode work, or would it need to be more elaborate than a zener diode? Thank you
 
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Thread starter #6
I spoke with an electronics manufacturer, and they suggested that the 2.4 volt limiter may still allow 2.51 to occur from time to time. Your linked sheets and tables seem to indicate the same thing. Thus, I guess the ideal setup might be a 2.3 volt limiter instead. My next obstacle is heat, and voltage drop for the lower voltage signals. I could put the zener diode in the cabin of the car, but then the wires to and from the zener and the stock MAP would have a voltage drop and mess up the car's idle signals/voltages from the map to the zener back to the ecu.

So I need it to stay close by the stock MAP. The stock MAP is in the supercharger of a c7 z06, up under the blower which sits on top of the motor, in the valley of the V of the engine heads. The MAP is at the back of the blower.

Needless to say, it gets pretty hot. These engines reach temps of nearly 300F degrees and then enter into limp mode at that coolant and oil temp. I would want this sensor to survive and operate up to these temperatures. I don't know the exact ambient heat temp the sensor would see from the 300F degree engine, but that temp is a rare occurrence.

Someone suggested attaching the zener to the firewall and running a pigtail connector back to the stock MAP, and have the stock wire fitting plug into the aftermarket zener circuit on the firewall.
 
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Thread starter #7
I need to know more so I found this:
The car ecu supplies a steady 5 volts to the MAP sensor. The map sensor scales from vacuum up to 3 bar map pressure. Its max vacuum is around .5 volts and goes up to around 4 volts we think.

Surpassing 2.51 volts throws an engine light that we need avoided.

Anything above 2 volts is meaningless information for the car to calculate the air fuel ratios, it relies on the MAF at this +2 volts point.

It only uses the MAP reading when at lower boost figures and in vacuum, so voltages under 2 volts need to still be precise and accurate, and thus un-altered
 
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#8
Hi Irun

BCX55C2V4 gives 2.28min / 2.4nom / 2.56max

BCX55B2V4 gives 2.35min / 2.4nom / 2.45max ( see pg 3 of Datasheet )

S
 
Thread starter #9
good spot, i overlooked the pg 3 figures. That should work.

How would you suggest i package this so that it is resistant to heat of the engine and also avoids voltage drop/increase from the zener or the length of the wires used, which would potentially mess with lower voltages that I need to remain precise and accurate from the MAP to the ecu?
 
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#10
Mmmmm .. ..

Not easy to find stock though .. Mouser appear to have 17000 of them !

As for mounting it, I don't have much to offer .. what's the MAP sensor made of ? The one's I use for LPG conversion are polythene and have a conventional two pin plug attached .. .. no heat issues. I think the Zener needs to be as close as possible to the sensor, but the anode side length doesn't matter so much.

S
 
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#12
Well looking at the picture in the second link, there's no great heat insulation there .. .. .. looks like a standard waterproof plug & socket .. .. it should be possible to splice the Zener between the signal and Gnd and wrap it up in the cable ??

If you don't get the wiring diagram it shouldn't be too difficult to figure which pin is which .. .. .. .. then I think a bit of trial & error will solve the rest.

S
 

crutschow

Well-Known Member
Most Helpful Member
#13
Here's a three opamp (one LM324 package) circuit that will precisely clip the voltage at an adjustable reference input level.
The LTspice simulation is shown below.
The clip voltage is provided by a TL431 2.5V accurate and stable reference IC, and the level can be adjusted from 2.5V to 0V by pot U4.
As can be seen, the output is clipped at 2.45V for the shown setting of the pot.
If you don't need an adjustable clip level, you can replace the pot with two fixed resistors.

1538767817258.png
 
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Thread starter #14
Here's a two opamp circuit that will precisely clip the voltage at an adjustable reference input level.
The LTspice simulation is shown below.
The clip voltage is provided by a TL431 2.5V accurate and stable reference IC, and the level can be adjusted from 2.5V to 0V by pot U4.
As can be seen, the output is clipped at 2.4499V for the shown setting of the pot.
If you don't need an adjustable clip level, you can replace the pot with two fixed resistors.

so, I learn quickly, but this is all Chinese to me at this particular moment in time. But I am trying to learn what I am looking at.

I really appreciate you taking the time to put this together for me. How would I best go about getting this manufactured and packaged so it can be pugged into the stock circuit?
 

crutschow

Well-Known Member
Most Helpful Member
#15
I best go about getting this manufactured and packaged so it can be pugged into the stock circuit?
You will need to find someone who's familiar with laying out and packaging such a circuit.

Are you going to build more than one?
If so, you may want to design and build a small PCB (printed circuit board).

Also there should be some circuitry added to protect the circuit from voltage spikes on the 12V battery line.

The temperature where this will be mounted is also a consideration.
 
Thread starter #16
Also there should be some circuitry added to protect the circuit from voltage spikes on the 12V battery line.

.
You may be correct on this point. However, i would like to question the validity... i would think the vehicle computer which is sending out the 5 volts is already regulating this voltage to a consistent 5 volts. What are your thoughts?
 
Thread starter #17
Yes, i would like to have many built... but would want to test a single 1 first to make sure it behaves as intended. Then if testing goes well, be able to built 20 or so in the first batch. Depends on pricing.
 

crutschow

Well-Known Member
Most Helpful Member
#18
i would think the vehicle computer which is sending out the 5 volts is already regulating this voltage to a consistent 5 volts.
True.
But I'm referring to the 12V the circuit requires for power.
 
Thread starter #19
to the best of my understanding, the circuit is powered by 5 volts from the computer.

So all the more voltage it should ever see is 5 volts. The stock ecu is already the filter if you will.
 
Thread starter #20
a map pressure reading is not a signal that a 100,000 dollar 650hp boosted air car... can have fluctuate due to poor voltage quality.

im pretty confident it is very well regulated, as is the feed signal to the MAF

...otherwise, I assume the engine would frequently make choices to run lean from bogus corrupt false return signals, and experience catastrophic failure. ...A 20,000 dollar "oops".

So im guessing GM invested in making sure the circuitry is immune to voltage spikes from the battery/alternator
 
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