negative dc curent to possitive (for analog input)

[caslor]

Hi
is any cheap-easy method to convert negative dc curent to possitive?

i have a board that has analog input and convert it to digital but it must be possitive analog

in the other hand i have a car sensor that is ground it to negative (works like a resistance ) that give also negative curent as an output signal

example :
lets say that the car has -12v ground curent, the temp resistor takes that curent and as it works gives output also negative but with diferent curent
bigger the temp then bigger and the output volt from the sensor(resistance)

 

[ronsimpson]

I am confused. Please draw a picture. A car (-12V) is only negative as compared to the + on the battery. The -12 is zero compared to the chassis.

 

[caslor]

this is the setup as it is working in my car (old car)
i have one pcb that can reads analog inputs 0-3v (+) so i need something to invert this ( i will reduce the voltage from 12 volt maximum to 3volts maximum with a resistance)

 

[crutschow]

A resistor from the plus side of the battery to the temp sensor (to ground) will generate a positive voltage across the temp sensor.

I think you are confusing current flow direction (from the plus battery terminal to ground) with the voltage polarity (which is always plus to ground). You can't get negative voltage from a battery if its negative terminal is grounded.

 

[KeepItSimpleStupid]

Or, he could be confusing "reverse acting". i.e. Higher temperatures are lower voltages across the meter. This would happen with an NTC (Negative Temperature Coefficient) thermistor. Resistance decreases with increasing temperature.

Aside:
Normal people say current flows from positive to negative. That's called conventional current.
Since Ben Franklin was wrong, it usually doesn't matter. FWIW: electrons go the other way and it matters in chemistry ans solid state physics.

 

[caslor]

negative pole of the battery is in chassis

engines temp sensor is grounded in engine case and has just one terminal that goes to analog gauge
gauge has one terminal at the back for possitive and one for the sensor

my pictures is as it is normal works the cars sensors from the factory

 

[KeepItSimpleStupid]

caslor:

Usually true. The gauge acts a current meter and the transducer is a resistor, Most thermisters are what they call NTC or the resistance decreases when the temperature increases.

So, I think you want to measure the voltage across the sensor to chassis and I think that voltage will decrease as the temperature increases,

Some cars had regulators internal or external to the gauge. I remember a 1968 vehicle that pulsed about 2.5 V to the gas sender.

 

[caslor]

for my temp i get this

60 c / 146R = 0,25v
70 c / 110R = 1,25v
80c / 90R = 2v and after some other readings i have the last one that is
14oc / 22R = 4,25 v

this is what the sensor sends to the Guage and reads the temp

i want to make it work with a pcb like andruino ... in the analog inputs that it has
will it work if i just place the wire there from my sender? i think that will not

 

[crutschow]

for my temp i get this

60 c / 146R = 0,25v
70 c / 110R = 1,25v
80c / 90R = 2v and after some other readings i have the last one that is
14oc / 22R = 4,25 v

this is what the sensor sends to the Guage and reads the temp

i want to make it work with a pcb like andruino ... in the analog inputs that it has
will it work if i just place the wire there from my sender? i think that will not

I don't understand you "R" and "v" values. How did you measure them?

 

[caslor]

i measure them with the help with my gauge...
i connect it with a trasformer i had 12 v dc and with the use of an adjustable resistor and a multimeter i get the readings

lets say that the guage show a 70 c reading.... then i was measuring the value of the resistance and the volts

 

[crutschow]

Still not sufficient info. Where is the resistor connected and where are you measuring the volts?

 

[caslor]

the resistor was connected to the negative of my source before the gauge (where the sender had to go normaly)
the volts where measured after the resistance (one connector of my multimeter in the possitive of my source and the other at the negative but after the resistance)

 

[caslor]

Here is a simple draw how o measure my gauge (calculate the values of the resistance in my sender)

 

[KeepItSimpleStupid]

That set-up won't necessarily tell you much. The LIKELY arrangement is that the temperature gauge is an ammeter with a high internal resistance.

The ammeter resistance can be measured INDIRECTLY using a potentiometer and a voltmeter,

With the gauge in place, the voltage across the sender can be used as an input to your

We can measure the resistance of the ammeter although you may not understand the procedure.
You would start out with a voltage source and a large value resistor (1.e. 1 M ohm). Yo would insert this in series when the resistance is maximum. You will then low the resistance until the meter reads 1/2 scale.

Take the pot out and measure it's resistance, That is the resistance of the ammeter.

Now you might have to repeat with say a 10K, 50K or 100K pot for better resolution.

i.e. your putting a variable resistor where the sender is at a high R initially. Then, you will adjust that pot until the meter reads 1/2 scale. Needle straight up.

Remove the pot and measure it. Say you got 4.5k. You could then repeat with a 10K pot to get better accuracy. The pot must be inserted when the value is maximum and then reduced.

 

[caslor]

ok
but as i say in my first post my interesting is other... (ok i will do that when i have to measure the value to put in my program)

i want to learn if there is any method to measure the reading from my sensor to my analog input of andruino pcb (not exactly andruino but similar ... says that has to put analog readings from 0-3 v only)

 

[KeepItSimpleStupid]

If you had JUST the sensor and you knew it's characteristics (for now just range of resistance), you can condition in a manner suitable for 0-3, by using something less than a 3V source.

Linearity, you will have to compensate for in your program R(x) = t(x).

Depending on the value of the sensor, a simple voltage divider can be used. You pay attention to the impedances in the circuit, so they don't introduce large errors.

The sensor is a variable impedance and so is your a/D converter. Thinks you can't do is measure a 1 Meg sensor with a meter with a 1 Meg input Z easily.

==

Your system, as it stands:

1) probably has a resistor inside the temperature meter.
2) The sensor is a NTC thermistor.

Now, I haven't a clue what the voltage from the sensor to ground is, nor do i have a clue what the meter is doing. Nor do I know if your keeping the meter.

So, it requires some measurements: Suppose the measured voltage was 0-7 V across the sensor and the sensor was 100-500 ohms. We could divide the voltage by a little less than 2 and if we appropriately sized resistors, we don;t compromise accuracy.

Voltage sources we think of ideal. They have an output Z of zero. Meters, like your DVM probably has an input Z of 10 meg ohms.

10 megs in parallel with 0 is still 0, but 10 megs in parallel with 10 meg ohms is 5 meg ohms. The meter here affects our measurement.

If you plan to keep the original sender, then it may be possible to clamp the output, so when the sensor opens, the A/D doesn't see 12 V.

Hesitance vs temperature can be dome in a bucket with an ohmmeter and a thermometer.

We cannot do much of anything until:
1) Are you keeping the original gage?
2) Range of sensor? Area of interest? Accuracy required?
3) Input Z of A/D? Max voltage: (3V)? Ratiometric?
4) Voltage across sensor vs temperature especially if #1 is a yes.
5) Resistance vs temperature if #1 is a no.

 

[crutschow]

Here is a simple draw how o measure my gauge (calculate the values of the resistance in my sender)

Your measurements do not seem to give a consistent answer for the meter resistance.
Try redoing the measurements as follows:

Set the supply voltage to 14 V and record its value.

Adjust the variable resistance to give the first temperature reading (60°C) on the meter.
Record the voltage across the variable resistance.
Remove one connection to the variable resistance and measure and record its resistance

Repeat the previous paragraph for 80°C, 100°C, and 120°C.

From those measurements I can determine the meter resistance and how to convert the sensor voltage to readings from your micro.

 

[KeepItSimpleStupid]

Do what crutschow says.

The EQUIVALENT circuit of what you have is:

1) A voltage source of 12-13.6 V
2) A fixed resistor (the meter)
3) variable resistor (the sensor)

The scale does the linearizion and the meter measures a current. The fixed resistor of the meter turns the voltage source into a current source per Thevinin and Norton's theorems.

You cannot read the resistance of the meter directly without popping the meter. You can read the sensor resistance directly.

You can cause the meter to read whatever and through the magic of ohms law figure out what you/we need to know.

It's not a good idea in this case to insert an ammeter UNLESS it is a feedback ammeter.