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

Thread starter #22
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.

View attachment 114745
alright, got the diagram finally...

so on the MAP sensor, pin 1 is Ground. Pin 2 is the Air Temp Signal back to the ECU. Pin 3 is the 5V feed from the ECU. Pin 4 is the MAP signal back to the ECU, which is the one that we must keep below 2.51 volts at all times, but accurate up to 2 volts in its readings.

So there is no 12 volts from the battery, just a filtered/regulated 5 volts from the car's ecu.

So what is the least cumbersome way to limit this MAP signal... can't we just go back to a simple zener diode circuit now?
 

crutschow

Well-Known Member
Most Helpful Member
#23
So what is the least cumbersome way to limit this MAP signal... can't we just go back to a simple zener diode circuit now?
Depends upon how accurately you want to clamp the voltage.
 
Thread starter #24
my only 2 guidelines are i dont want to mess up voltage readings below 2 volts, and i dont want to go past 2.50 volts.

Anything else from 2 to 2.49 is fair game to be inaccurate.
 

crutschow

Well-Known Member
Most Helpful Member
#25
Anything else from 2 to 2.49 is fair game to be inaccurate.
For that wide a range, the Zener approach should work, if it's set to clip at about 2.25V.
 

crutschow

Well-Known Member
Most Helpful Member
#27
what are your thoughts about the one linked above in this thread that is set to clip at 2.4?
It would be problematic.
Look at the clip from the data sheet below.
The maximum zener voltage is 2.45V which is near your upper limit.
And its high dynamic resistance and soft zener characteristics may cause an error in the voltage around 2V and below.
Add in the high temperature coefficient and you could exceed 2.49V at low temperatures.

1538815881083.png

How about this slightly more complicated circuit (2 added resistors) that gives much better performance.
Below is the LTspice simulation of a clip circuit using a TLV431 1.24V programmable shunt reference (Zener).
It gives a more accurate, stable, and sharper clip point.
The TLV431 clips when the Ref voltage (red trace) reaches 1.24V, as determined by the voltage divider consisting of R2 and R3 [giving an OUT2 clipping value (yellow trace) of 2.24V for the values shown].

Note the much poorer clip at OUT1 using a standard 2.5V zener (green trace).
(I didn't have the model for the 2.4V zener but the performance should be similar).

1538819474295.png
 
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crutschow

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Most Helpful Member
#29
I'm guessing you never worked a GM.
Likely a good guess.
But I have.
Have you? :angelic:

But are you implying that GM's electronics are not designed to withstand voltage spikes on the 12V line?
If there weren't, they would likely be swamped by failures under warranty.
 
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shortbus=

Well-Known Member
#30
Likely a good guess.
But I have.
Have you? :angelic:

But are you implying that GM's electronics are not designed to withstand voltage spikes on the 12V line?
If there weren't, they would likely be swamped by failures under warranty.
I worked for their wire harness and terminal division, Packard Electric. And while I don't know about the electronics, I do know about them sending people from our plant to dealerships and distribution points to fix things that were a total mistake in design.
 
Thread starter #32
It would be problematic.
Look at the clip from the data sheet below.
The maximum zener voltage is 2.45V which is near your upper limit.
And its high dynamic resistance and soft zener characteristics may cause an error in the voltage around 2V and below.
Add in the high temperature coefficient and you could exceed 2.49V at low temperatures.

View attachment 114758

How about this slightly more complicated circuit (2 added resistors) that gives much better performance.
Below is the LTspice simulation of a clip circuit using a TLV431 1.24V programmable shunt reference (Zener).
It gives a more accurate, stable, and sharper clip point.
The TLV431 clips when the Ref voltage (red trace) reaches 1.24V, as determined by the voltage divider consisting of R2 and R3 [giving an OUT2 clipping value (yellow trace) of 2.24V for the values shown].

Note the much poorer clip at OUT1 using a standard 2.5V zener (green trace).
(I didn't have the model for the 2.4V zener but the performance should be similar).

View attachment 114764
Thank you, you've come through again with some amazing help with my questions. I appreciate your input very much. Now i just have to find someone to build this circuit for me.

Again, any and all ideas welcome on who could do it cost effectively.
 

crutschow

Well-Known Member
Most Helpful Member
#33
Again, any and all ideas welcome on who could do it cost effectively.
The circuit in post #27 is quite simple with only 4 components (3 resistors and the TLV431).
Anyone that can solder should be able to build the circuit for you on a small proto board, such as this.
 
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KeepItSimpleStupid

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#34
The output from the sensor is likely ratiometric with the actual value of the 5V supply.

Your reference should then be a % (~50%) of the actual real time vallue of the 5V supply voltage and not absolute.
 
Thread starter #35
The output from the sensor is likely ratiometric with the actual value of the 5V supply.

Your reference should then be a % (~50%) of the actual real time vallue of the 5V supply voltage and not absolute.

I am not sure what your point is exactly, can you clarify further?

I am trying to avoid 2.51 volts or above at all costs. That is where the error is set by the cars stock tune. If the car did supply 7 volts and the reading was 3.5 instead, that would still be a problem, as the ecu would still throw an error.

Back to 5 volts with a limiting circuit attached... The false reading of 2.4 will not hurt the car's air fuel ratios, as the car runs on the MAF (a different sensor) at ranges above 2 volts from the MAP sensor back to the ecu.

I must simply prevent 2.51 volts and above at all costs. Thanks
 
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Thread starter #36
The circuit in post #27 is quite simple with only 4 components (3 resistors and the TLV431).
Anyone that can solder should be able to build the circuit for you on a small proto board, such as this.
I need it to be packaged as small as possible, within reasonable cost, and with heat/moisture insulation... to some degree.
 
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KeepItSimpleStupid

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Most Helpful Member
#37
I am not sure what your point is exactly, can you clarify further?
Typical automotive sensors are not 0-5V; Meaning you can;t really connect them to a DVM and convert to engineering units.
The "5 V supply" varies with temperature, spikes etc. The uP reading the sensor, set the reference of the A/D converter to the supply voltage, not 5V.

The sensors have a really hard time outputting 0V and 5V or the power supply rails. They might be able to get within say 300-600 mV of the supply rail. In order to output 0V, the negative supply has to less than 0V and in order to output 5V, the positive supply needs to be greater than 5V.

Cranking COULD make the 5V supply dip as well.

The output of the sensor is actually a % of the supply voltage. The A/D converter can convert this because it's reference is usually 1/2 the supply voltage.

I picked a pressure sensor out of the blue. https://www.te.com/commerce/Documen...lishENG_DS_MS4515_B2.pdfCAT-BLPS0040#page=7

The transfer function described on PDF page 7 (link opens to that page).

It states that the output is 50% of the supply voltage when port 1 = port 2.

So, it's not correct to limit to say 2.5 V absolute when the supply is 5V or temporarily (when the reading was taken) 4.9V. So, you don;t use an absolute voltage of 2.5V. but a relative one of 1/2 the supply voltage. The supply voltage is constantly changing slightly. One such effect is temperature.

With this method, lots of stuff gets simplified.
 
Thread starter #39
I'm not positive that you are correct that the ECU is looking for a percentage returned.
We can clearly see in the tune the EXACT volts it is looking for that cause the ECU to make different decisions. No where in there does it speak of 50% etc. It is specified in exact volt values.

I agree the sensor limits the supply voltage by a percentage, and that percentage is returned to the ECU to notify the ECU of the pressure in the manifold.

I disagree that the ECU is programmed well enough to care one iota about the percentage, instead, it is looking for a specific voltage.

2.51 is what we are avoiding. 2.51 is what the ECU is looking for, not a percentage.

If you are positive the ECU is retranslating back to 2.51 volts and is instead reading percentage originally, then I will continue to listen, but so far, i see no evidence of that in the programming.
 
#40
Morning Irun .. .. ..

Can I ask .. where in the world are you situated ?

I would suggest a good next step would be to acquire a BZX55B2V4 and try it out .. .. If it works, you're in the pound seats, if not you've eliminated the possibility.

My experience of MAP Sensors convinces me it will be an easy solution.

S
 

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