I have a sensor amplifier circuit that has been designed to give me a negative voltage until it reaches a certain temperature. I need to block this negative voltage as I need only the positive voltage to measure. Will a diode be enough. will the diode give me the same linearity as the sensor output.what else could i use to block the negative voltage.
thanks

 

A diode will not work without some additional parts.
What is the part number of your sensor? What power supply voltages do you have available?

 

diode will have a fixed forward voltage drop. this will introduce an offset into your readings. If you were to plot the expected voltage and the measured voltage on a graph of temperature vs voltage, the 2 lines would be parallel, but this is not a linear error. plot the error% vs the temperature and you will see the curve.

 

diode will have a fixed forward voltage drop. this will introduce an offset into your readings. If you were to plot the expected voltage and the measured voltage on a graph of temperature vs voltage, the 2 lines would be parallel, but this is not a linear error. plot the error% vs the temperature and you will see the curve.

A series diode also adds approximately -2mV/°C temperature sensitivity, which would wreak havoc with your measurements.

 

I am using an rtd sensor. I need my sensor to give me a positive output only at a certain temperature. That is why I have a negative voltage initially that I have to block as my voltmeter needle goes negative which I do not want. Is there a way i can short circuit the diode when the positive voltage starts. and hence it wil block only the negative voltage and not hamper with the positive? thanks.

 

A diode will not work without some additional parts.
What is the part number of your sensor? What power supply voltages do you have available?

I am using a normal pT sensor. with an amplifier circuit. My power supply is 12V. I have stated my objective in the previous reply. I want my voltmeter to start reading only at a certain temperature from the rtd. that is why I have a negative bias to give me an offset so that I will get a positive voltage only at a certain temperature and not at 0 deg.

 

A series diode also adds approximately -2mV/°C temperature sensitivity, which would wreak havoc with your measurements.

My diode will not be in direct contact with the heat. The whole item is insulated. Do you think I could use a series diode, without causing any errors in the voltage from the sensor amplifier circuit?

 

Nope. Your diode has to be somewhere, and if that somewhere gets hot or cold then it's forward drop can and will change.

Way back when I made a circuit to generate an exponential voltage from a linear voltage to run a voltage controlled oscillator so I could feed it voltages .1, .2, .3... volts and it would play A B C... notes. It used a diode to generate the exponential.

Believe it or not I could hear the diode getting hotter when I played a higher note, and could hear it getting cooler when I played a lower note just from the self-heating effect of the current thru the diode.

 

Post a schematic of what you have, and we will see if we can tell you how to get eliminate the negative output voltage.

 

An op amp precision rectifier might be a good circuit, it eliminates the diode voltage drop.
Google will find many references.

 

An op amp precision rectifier might be a good circuit, it eliminates the diode voltage drop.
Google will find many references.

That's what I was thinking, but we need to see how he has his RTD biased before we can design a circuit for him. He might be able to do it on his own, but...

 

This is the circuit. If you need any explanations about the circuit please let me know.

 

You appear to have your RTD biased with 400uA from the REF200P. Does the other end of your RTD connect to ground?
Does the output (after the diode) drive an analog meter (d'Arsonval movement)? If so, what is the full-scale voltage and current?
BTW, the pot R14 will not work predictably, as the only current flowing through it comes from pin 7 of IC7.

 

1. Yes the other end of the rtd goes to ground
2. The output after the diode goes to an analog voltmeter. The full scale is 0-10V.
3. About pot R14 it is a connection error. It is supposed to connect pin4 and pin7 of ic7 and not pin 5 and pin7.

 

1. Yes the other end of the rtd goes to ground
2. The output after the diode goes to an analog voltmeter. The full scale is 0-10V.

What is the full scale meter current? I need to know this for a circuit I am working on to solve your problem.

3. About pot R14 it is a connection error. It is supposed to connect pin4 and pin7 of ic7 and not pin 5 and pin7.

It still has the same problem that I mentioned.