Signal conversion for monitoring system

[2PAC Mafia]

Hi guys,

I´m working on a boat where there are two pressure sensors to detect water level on tanks. Sensors are 4 to 20 mA output and we have checked that maximum output when tank is full is 8 mA in one tank and 10 mA in the other.

From the ohter side I have two different monitoring system which need 0,4V to 2V signal to interpretate 0 to 100% scale at LCD.

This means I need to convert 4 to 8 mA to a 0,4 - 2V and 4 to 10mA to a 0,4 - 2V. As power I have 24V from batteries boat but as it´s changing values I guess I would have to use a DC/DC converter to work with my adapter with a constant value. I´ve been thinking to make it using a simple transistor circuit that depending on mA at base it drives more or less power from collector, is it possible? Can you help me with design? It´s only a quick idea but I accept any easy idea to have a circuit which converts these signals.
I´ve been thinking about operational amplifiers and comparators also but I´m not sure how to do it. Thanks in advance.

 

[JimB]

Sensors are 4 to 20 mA output and we have checked that maximum output when tank is full is 8 mA in one tank and 10 mA in the other.

Are you sure that the sensors cannot be adjusted or calibrated in some way to give 20mA whenthe water tank is full ?

I have two different monitoring system which need 0,4V to 2V signal to interpretate 0 to 100% scale at LCD.

If your sensors could be adjusted to give 4 to 20mA, all that you would need to convert to 0.4 to 2v is a 100 Ohm resistor.

JimB

 

[MikeMl]

...I´m working on a boat where there are two pressure sensors to detect water level on tanks. Sensors are 4 to 20 mA output and we have checked that maximum output when tank is full is 8 mA in one tank and 10 mA in the other.

What do they read when the tanks are empty?

 

[2PAC Mafia]

Thanks,

sensors can not be adjusted, you supply 24Vdc and they send you 4 to 20 mA from a scale of 2 meters deep, depending on pressure of water.

The 100 ohm resistor to convert to voltage I know thanks.

When the tank is empty sensor sends 4 mA.

 

[MikeMl]

What LCD display module are you planning to use? **broken link removed** where you only use 0 to 100 (half its useful range)? Call the units % of full. Then the task becomes converting 4-8mA to 0-100mV.

 

[2PAC Mafia]

The monitoring system with LCD´s is already there, also analog to digital converters modules, I don´t have to change anything buying new modules. I´ve been repairing some of these modules, also adapting signals, I´m pretty sure the system installed on board was never working properly because of its configuration... I solved a lot of wrong things at this yacht.
Now, to finish the complete job I have to adapt this sensor signal, in this case 4 to 8 mA to a scale 0,4V to 2V used by the inputs of A/D converter modules. I usually repair and I don´t design a lot so I don´t remember very well when at school we calculated transistor circuits (Ib, Ic, Vce...).
My idea was to use a DC/DC to have a constant voltage, for example 5V, then a transistor circuit where changes the output voltage depending on the input at base (sensor signal), if necessary I can amplify the input with an operational amplifier which would drive the transistor, I can play with variable resistors to get the correct output scale, what do you think?

 

[MikeMl]

I am confused by your nomenclature. Is 0,4V = 400mV?

 

[MikeMl]

Try this circuit. The output is constant even though the boat's battery voltage varies from 22 to 28V. It puts out 0.4V with a loop current of 4mA. It puts out 2.00V with a loop current of 8mA. Change R4 to 14.0K for the 4mA to 10mA version. Note that this requires a precision voltage reference, and an opamp that will run from up to 40V which has a common-mode input range including ground....

Also notice the caveat about grounding one side of the loop.

 

[2PAC Mafia]

Hi Mike,

I am confused by your nomenclature. Is 0,4V = 400mV?

Yes, it´s 400 mV.

Try this circuit. The output is constant even though the boat's battery voltage varies from 22 to 28V. It puts out 0.4V with a loop current of 4mA. It puts out 2.00V with a loop current of 8mA. Change R4 to 14.0K for the 4mA to 10mA version. Note that this requires a precision voltage reference, and an opamp that will run from up to 40V which has a common-mode input range including ground....

Also notice the caveat about grounding one side of the loop.

It seems good, did you test it with computer simulator? May be I can use at R4 a potentiometer just in case I have to adjust some value. To test it, I have to buy components, as soon as I have it I give you a feedback. Thanks for your help.

 

[MikeMl]

It seems good, did you test it with computer simulator? May be I can use at R4 a potentiometer just in case I have to adjust some value. To test it, I have to buy components, as soon as I have it I give you a feedback. Thanks for your help.

If you have trouble procuring the Linear parts, there are some substitutes. Who do you order from?

R1-R2 could be replaced with a 50K pot. That would control the offset (move the purple line in the simulation up and down). R4 could be replaced with a 50K pot wired as a rheostat. That would control the slope of the purple line. Problem is that the two adjustments interact a bit, and it takes many iterations between the adjustments to hit the endpoints simultaneously. I would just go ahead and get the 1% standard value resistors...

 

[2PAC Mafia]

OK, I have a Farnell account and I found all components.

So if I need less output voltage, for example 1,5V instead of 2V, should I change value at R4?

 

[MikeMl]

OK, I have a Farnell account and I found all components.

So if I need less output voltage, for example 1,5V instead of 2V, should I change value at R4?

Here is a series of plots of V(out) vs I(i1) at different values of R4, green is 10K and purple is 50K

 

[2PAC Mafia]

Perfect!!! I have a program for PCB design, it´s called PCB PIA, not installed yet, may be it has option for circuit simulation but I need time to do everything I want. I also have Arduino kit for starting programming microcontrollers, not open yet... A lot of work. Thanks again for your help.

 

[MikeMl]

One minor correction. Increase R5 to about 4.7K (value not critical). I just noticed that with a 28V supply, the current through the reference should be lowered from ~11mA to about 5mA.

 

[2PAC Mafia]

OK

 

[2PAC Mafia]

Hi Mike,

I made the unit, I tested it at workshop and it works fine, I have to go on board to install and test it.

http://www.restoretronic.com/descargas/pcb/Señal ma a V Super Tatico/Esquema señal mA a V.jpg
http://www.restoretronic.com/descar...o/Placa adaptadora señal cara componentes.jpg
http://www.restoretronic.com/descar...Tatico/Placa adaptadora señal cara pistas.jpg
http://www.restoretronic.com/descargas/pcb/Señal ma a V Super Tatico/Placa adaptadora señal.jpg

 

[KeepItSimpleStupid]

Very nice. You just forgot the mounting holes?

 

[2PAC Mafia]

No, I still have to make it and put the board into a protecting box, thanks anyway for the advice.

 

[2PAC Mafia]

No, I still have to make them and put the board into a protecting box, thanks anyway for the advice.

 

[2PAC Mafia]

I´ve got a problem. When I finished the board I tested it at workshop before installing it on the yacht. I don´t know why the LT1013 exploted. I tested it again carefully and everything worked fine so I didn´t find out why the first time the LT1013 was damaged.
I installed the unit on the boat and final connection was power supply, again the IC was damaged. Now I´m waiting for new IC´s to replace it, I checked in the datasheet the maximum power supply is +22V and -22V so 22 or 28V is correct and accepted. I was thinking may be the problem is about having input before connecting the power supply because I don´t see any other reason, it shouldn´t be a problem but... When I receive new IC´s I can make the test, if that is the problem I can install a relays to detect first the power supply and then bypass the signals input. Do you think it has sense that the problem is because of having input before power supply ins connected?