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Topic: Decrease line voltage by clamping (Read 1 time) previous topic - next topic

xgrapher

Nov 27, 2017, 04:00 am Last Edit: Nov 27, 2017, 04:13 am by xgrapher
I have a sensor that outputs 0~5V and I want to be able to hook up a circuitry onto that signal wire in order to reduce a dynamically defined percentage of the voltage. I DO NOT WANT TO CUT THE WIRE. I just want to splice it and join it with my circuitry.

I have already made a successful board that reads the sensor, applies the correction and outputs the corrected voltage thru a DAC. So please don't suggest this idea to me it's already done ;-)

I tried adding a pull down resistor to the line of different values but that didn't drop nothing. Is it even possible?


aarg

#1
Nov 27, 2017, 05:11 am Last Edit: Nov 27, 2017, 05:22 am by aarg
What is the output impedance of the sensor? Is it constant and stable regardless of the sensor reading? Trying to drag down a low impedance output that isn't designed for it is a bad engineering practice.

What is this mystery sensor? Is there some reason you must conceal its identity from us? Not fully explicating the problem will definitely impact the quality of the answers you get.

Also, this forum is for Arduino questions. Where is the Arduino in this circuit?
  ... with a transistor and a large sum of money to spend ...
Please don't PM me with technical questions. Post them in the forum.

TomGeorge

Everything runs on smoke, let the smoke out, it stops running....

TomGeorge

#3
Nov 27, 2017, 10:32 am Last Edit: Nov 27, 2017, 10:34 am by TomGeorge
Hi,
What you are doing in the first diagram is scaling the output of the sensor to make it suitable for the factory controller.
That is you did using a serial type system.

The second diagram you want to use SHUNT LOAD to scale the output for the factory controller so you don't have to open circuit the signal wire.

As @aarg has pointed out what is the output impedance of the sensor?

Can you tell us your electronics, programming, Arduino, hardware experience?

Quote
to reduce a dynamically defined percentage of the voltage. 
What is the characteristic you need to apply to the 5v sensor signal to make it suitable for the factory controller.
Do you just need to reduce the signal by a fixed percentage?

Thanks.. Tom... :) 

Everything runs on smoke, let the smoke out, it stops running....

xgrapher

What is the output impedance of the sensor? Is it constant and stable regardless of the sensor reading? Trying to drag down a low impedance output that isn't designed for it is a bad engineering practice.

What is this mystery sensor? Is there some reason you must conceal its identity from us? Not fully explicating the problem will definitely impact the quality of the answers you get.

Also, this forum is for Arduino questions. Where is the Arduino in this circuit?
Why would I conceal a sensor? Is that what you would do? I'm keeping my post as short as possible. Speak for yourself and stop being a grandma. I'm not a teenager who is looking for general advice on living. I'm specifically looking for a technical advise so keep yourself confined to it, if you may. What's the deal with wether this is for Arduino or not? You love policing people don't you. Please don't answer me I'ld rather have my question answered by someone else.

It's a delphi MAP sensor and according to the spec sheet: The load impedence seen by the sensor shall be 51kOhm.

I'm making a piggyback system for stock ECUs. In the first diagram, I'm simply reading the map signal thru Arduino's analog pin and outputting an adjusted voltage thru a dac module (MPC7425) so that the ECU thinks that the MAP is greater/lesser than actual thereby making the ECU increase/decrease the fueling.

The above mentioned circuit works perfect but like I said if there is a possibility to add a load or something to the line to reduce the voltage I wouldn't have to cut the wire.

But let's forget about all that. I'm not worried about toasting anything I'll find that out later. Can somebody please show me a way that for instance, I can simply tap into a 2.5V line and reduce it to let's say 2.2V. Is it possible?

JohnRob

Hi xgrapher,

From your first post I think you want to scale the output as a function of something an arduino calculates.

So for instance if your scale factor goal is 0.9 then a normal 5V out would be 4.5V and a 3V output would be 2.7 V.   Am I understanding you correctly?

I've worked with this sensor some years ago in a fuel tank leak system and recall the output was proportional to input voltage.  So I see two options:

  • If you don't require a large range of scale factor you could simply modify the input voltage (Vcc).   This may not meet your "don't cut a wire" goal.
  • Load the sensor output with resistors.  Some DAC's can be wired this way but I don't know if they can handle the current needed (which is unknown at this point).  Alternatively you could 4 Mosfets, each grounding a resistor tied to the sensor output.  This would give you 16 combinations.   I believe it will  not damage the sensor as all automotive sensors must be capable of extended short circuits on their outputs.  Caveat, the sensor output impedance may not be constant for all conditions (especially temperature)


Good Luck.
Please do not PM me with thread based messages.  If your thoughts are worth responding,  the group should benefit from your insight.

tinman13kup

Many auto systems that utilize a reduced voltage system for sensors use an isolated supply to do so. Connecting a resistor to body ground will not complete the circuit.
Tom
It's not a hobby if you're not having fun doing it. Step back and breathe

Southpark

#7
Nov 28, 2017, 04:57 am Last Edit: Nov 28, 2017, 04:58 am by Southpark
I have a sensor that outputs 0~5V and I want to be able to hook up a circuitry onto that signal wire in order to reduce a dynamically defined percentage of the voltage.
Are you saying that the "line voltage" (or estimated voltage of the line with respect to ground) is dependent on the sensor's output voltage (in the range 0 to 5 volt), and you want to be able to adjust the output voltage of the sensor?

When you indicate 'reduce a dynamically defined percentage of the voltage', does this mean you want to alter the sensor's output voltage (which is the voltage of the line)?


MorganS

But let's forget about all that. I'm not worried about toasting anything I'll find that out later. Can somebody please show me a way that for instance, I can simply tap into a 2.5V line and reduce it to let's say 2.2V. Is it possible?
Yes, that is possible. But then you no longer get any data from the sensor. The Arduino is loading it down to 2.2V and you cannot possibly know if the sensor has changed from 2.5 to some other value.

Now there may be some ways which this is useful - for example, you never want to see any value greater than 2.2V so you can brick-wall it at that value, allowing any lesser voltage to pass through unmolested.

The thing is, it's not just a sensor. It's a delicate sensor which produces a signal in the microvolt range plus an amplifier which brings it up to the volt range. That amplifier will have feedback. That means that if its output is not at the right voltage, it will apply more power to bring it up. You can load it down by exceeding the power available to that amplifier. And that is the point where you're no longer reading the sensor, you're just applying the voltage that you want to that wire, using your more powerful amplifier.

You could add another sensor and use that output to overload the original sensor but I suspect if your goal is not cutting wires, then bolting a new sensor on is not in the picture.
"The problem is in the code you didn't post."

TomGeorge

#9
Nov 28, 2017, 08:56 am Last Edit: Nov 28, 2017, 08:57 am by TomGeorge
Hi,
Can somebody please show me a way that for instance, I can simply tap into a 2.5V line and reduce it to let's say 2.2V. Is it possible?

A potential divider will work, but you will have to cut into the wire.
Using shunt method is not recommend as the sensor is not designed for that sort of loading.
Here is the typical output characteristics of your map sensor.

It has an output impedance of <50R.
It can only Source 0.1mA

As you can see, if you use shunt scaling you will be sourcing 6mA.

Tom.. :)
Everything runs on smoke, let the smoke out, it stops running....

xgrapher

Many auto systems that utilize a reduced voltage system for sensors use an isolated supply to do so. Connecting a resistor to body ground will not complete the circuit.
I can ground to the same point (ie. sensor ground) not that difficult is it? No brother, I disagree with you. I work on classics to conventional cars as late as 2018 models, japanese to american to european and I've not come across a single car that has isolated power supply for engine sensors. They do have a dedicated sensor ground that meets with the general chassis ground at the ECM.

xgrapher

Hi xgrapher,

From your first post I think you want to scale the output as a function of something an arduino calculates.

So for instance if your scale factor goal is 0.9 then a normal 5V out would be 4.5V and a 3V output would be 2.7 V.   Am I understanding you correctly?

I've worked with this sensor some years ago in a fuel tank leak system and recall the output was proportional to input voltage.  So I see two options:

  • If you don't require a large range of scale factor you could simply modify the input voltage (Vcc).   This may not meet your "don't cut a wire" goal.
  • Load the sensor output with resistors.  Some DAC's can be wired this way but I don't know if they can handle the current needed (which is unknown at this point).  Alternatively you could 4 Mosfets, each grounding a resistor tied to the sensor output.  This would give you 16 combinations.   I believe it will  not damage the sensor as all automotive sensors must be capable of extended short circuits on their outputs.  Caveat, the sensor output impedance may not be constant for all conditions (especially temperature)


Good Luck.

That's exactly what I'm trying to do.

Actually there's a digital potentiometer that I have it's a 10k pot except that it works digitally. Can you sketch me something?

xgrapher

Yes, that is possible. But then you no longer get any data from the sensor. The Arduino is loading it down to 2.2V and you cannot possibly know if the sensor has changed from 2.5 to some other value.

Now there may be some ways which this is useful - for example, you never want to see any value greater than 2.2V so you can brick-wall it at that value, allowing any lesser voltage to pass through unmolested.

The thing is, it's not just a sensor. It's a delicate sensor which produces a signal in the microvolt range plus an amplifier which brings it up to the volt range. That amplifier will have feedback. That means that if its output is not at the right voltage, it will apply more power to bring it up. You can load it down by exceeding the power available to that amplifier. And that is the point where you're no longer reading the sensor, you're just applying the voltage that you want to that wire, using your more powerful amplifier.

You could add another sensor and use that output to overload the original sensor but I suspect if your goal is not cutting wires, then bolting a new sensor on is not in the picture.
Wow thank you very much! You went deep. That's what I was thinking. I'm very close to giving up.

xgrapher

Hi,A potential divider will work, but you will have to cut into the wire.
Using shunt method is not recommend as the sensor is not designed for that sort of loading.
Here is the typical output characteristics of your map sensor.

It has an output impedance of <50R.
It can only Source 0.1mA

As you can see, if you use shunt scaling you will be sourcing 6mA.

Tom.. :)
Thank you very much. Now I have established that this is not achievable. I'll push thru with my DAC method

aarg

Why would a simple voltage divider not work?
  ... with a transistor and a large sum of money to spend ...
Please don't PM me with technical questions. Post them in the forum.

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