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LM324: Struck with Circuit to measure -15VDC to +15VDC analog voltage

Part Number: LM324
Other Parts Discussed in Thread: TLV9001

Hi,

 

We need to measure both Negative and Positive analog Voltage range from -15VDC to +15VDC in the same pin. Measured Voltage should be displayed on the LCD.

 

We know only Inverter circuit and Voltage Follower individually.

But we don't know the circuit to measure both Positive and Negative Voltage. Kindly suggest/Provide circuit/schematic for this.

 

Is it possible to implement this on LM324 or could you suggest better part ?

 

Kindly share Schematic for this application or Kindly advise us How to Proceed for the implementation.

 

Thanks & Regards,

Naveen K

  • Hello Naveen,

    For non-inverting, see my blog on voltage scaling. The excel calculator picks components for the voltage scaling.  

    Three ways to scale an analog input signal

    Download the three resistor scaler Excel calculator.

  • Hi Ronald,

    Thank you very much for the suggestions, We are using Solution#3 without amplifier for CT application to scale up. We are having some doubts for this application.

     Kindly provide your support for all the questions individually for more understanding.

    1. Can we use Clamping circuit as shown image instead of Solution#2 or #3 ? Clamping circuit is only for AC or it can also be used for DC ?

    2. Regarding to Solution #2, Why gain circuit (we can avoid 80K,10K on inverting input) is required instead of Voltage Follower here? Without gain, output will 0 to 2.222, with gain 0 to 2.5V. What is key advantage for adding gain circuit?

    3. Solution#2 and Solution#3 both scaling circuits are same right? Only Resistors values differ. For both the cases amplifier is optional right?

    4. Amplifier is Optional, but it may be used for current gain right? Current gain is necessary here or is there any key advantage?

    5. Figure-7, amplifier output will be 0 to 5.8V(due to VDD=3.3V, it will clip to 3.3V)) not 0 to 2.5V right?

    6. Iam thinking to go with figure 8 with some modification as shown. Kindly suggest which one is more efficient A. Voltage Follower or B. With Gain?

    Thanks and Regards,

    Naveen K

  • Naveen,

    1) Clamping circuit is to level correct a AC signal whose DC level is lost. The most popular example is restoring the horizontal sync pulses in analog television. Clearly the input series capacitor totally blocks DC voltage.

    2) There is no advantage to blog solution 2 unless gain is really needed. For example if solution 3 has no real solution.

    3) Solution 2 requires and op amp. Solution 3 does not mandate using an op amp. 

    4) The op amp buffer prevents output load (not known) from changing the scaling function. 

    5) Forget about figure 7; It is just there to make sure the were TI parts in the blog. Otherwise it would only be about resistors.

    6) 'A' is better when using a rail to rail input & output op amp. 

  • Hi Ronald,

     

    Thank you very much for your advise and appreciate your quick response.

     

    1. Is there any other alternative methods except Solution#1(Need –Ve supply, -ve Resistor), Solution#2/3(Need ~3.3V supply to non-inverting pin)?

     2. Iam having doubt here, The same input signal(-15V to +15V) is reading by some other device(Master as shown in Figure1/2/3) and we(Slave-Device) are also using . We are scaling-up the signal by adding ~3.3V voltage Supply.

    Whether these things will effect to the behavior of the signal and it will cause to the reading operation of the microcontroller in the Master-Device?

    3. Our Scaling function should not modify the signal level/values in the Master-Device and the microcontroller reading operation should be same as before(Without SLAVE-DEVICE). So Kindly confirm it(Slave-Device) will not effect to Master-DEVICE and Master-Device will work as before like without SLAVE-Device right?.

     4. Master-Device GND, Slave-Device GND, Amplifier Ground, all 3 Grounds should be connect right?

    5. If we connect Master Device GND and Slave-Devise GND, there is No isolation like optocouplers right?Is it possible to make isolation?

    6. Without Master-Device GND, Amplifier will not work right?

    7. For over input voltage protection & ESD protection, where i can place the Diode. Figure 1 is correct or 2 is correct? Figure-1,Higher voltage will effects the Slave-Device including amplifier, microcontrollers right? Can you suggest protection diode?

    8. How i can protect from more negative voltage like -20V?Is it possible or i have to consider this in scalup calculations only?

    9. Figure1,2,3 Which one you will suggest?

    Sorry for some basic questions, i need your confirmation. Kindly advise.

     

    Thanks and Regards,

    Naveen K

  • Naveen,

    I select figure 1 and make the resistors 10X bigger, 100k and 22k. Add a 1nF capacitor across D2. The op amp will be protected from +/-20V , even +/-50V. Now change LM324 to a TLV9001 instead. LM324 is not rail to rail on input nor the outputs.

     

  • Hi Ronald,

     

    Thank you very much for your all advise and appreciate your support. You have supported for all my query it will great helpful. This project is very important, i should not make single mistake. This is my last query which is also sent in previous text.  Kindly help us with individual answers.

     

    1. Is there any other alternative methods except Solution#1(Need –Ve supply, -ve Resistor), Solution#2/3(Need ~3.3V supply to non-inverting pin)?

     2. Iam having doubt here, The same input signal(-15V to +15V) is reading by some other device(Master as shown in Figure1/2/3 above) and we(Slave-Device) are also using . We are scaling-up the signal by adding ~3.3V voltage Supply.

    Whether these scalup function will effect to the behavior of the signal in the master -Device and it will cause to the reading operation of the microcontroller in the Master-Device?

    3. Our Scaling function should not modify the signal level/values in the Master-Device and the microcontroller reading operation should be same as before(Without SLAVE-DEVICE). So Kindly confirm it(Slave-Device) will not effect to Master-DEVICE and Master-Device will work as before like without SLAVE-Device right?.

     4. Master-Device GND, Slave-Device GND, Amplifier Ground, all 3 Grounds should be connect right?

    5. If we connect Master Device GND and Slave-Devise GND, there is No isolation like optocouplers right?Is it possible to make isolation?

    6. Without Master-Device GND, Amplifier will not work right?

    7. As you suggested, we will go with Figure 1 with multiply 10X to all Resistor's(100K,22K,22K),TLV9001.

    If >3.3V comes on point 1,it will go through D1 to Device VDD. It will not effect to the Slave-Device including amplifier, microcontrollers right? Can you suggest protection(ESD) diode?

    Thanks and Regards,

    Naveen K

  • Naveen,

    The simulation answers most of those questions. Here is normal usage output voltage, input current, and current to 3.3V supply. The circuit does draw some current that the master must supply (-150uA to +120uA). Is there a specification for allowable current draw?

    Below is same signals for over voltage input. 

    The ground is the return signal and must be connected. There is no isolation for the ground. The slave power should have some isolation so the slave ground can use the master ground. 

    TLV9001 pm15 to 3V.TSC

  • Hi Ronald,

    Thank you very much for your kind support.

    We are tapping the signal from amplifier output(LM224-4 Channel), Is per our customer confirmation We can drawn up to 1mA from Master Device. we need -150uA to +120uA. So nothing will happens to Master Device signal right?

    For isolating the Power can we use LC Filter as shown in the figure? or could you give any suggestions?

    For Signal and GND line we are using 330R ferite bead in series with Master and Slave device.

    Thanks & Regards,

    Naveen K

  • Naveen,

    The draw is less than 1mA so the requirement is met. I see no reason to isolate the 3.3V power, but your circuit is acceptable.

  • Hi Ronald,

    Thank you for the suggestions.

    I have very clear for everything now. I will go with the same circuit.

    For isolating the 3.3V or any signal, we need to use only Transformer Barrier right? Ferritebed is only for filtering right?Could you suggest any Isolation Barriers?

    Thanks and Regards,

    Naveen K

  • Naveen,

    Tell me more about how the master and slave are powered. How are they powered?

    It this a single slave or multiple slave system?

  • Hi Ronald,

     

    Thank you for the suggestions.

     

    We are connecting multiple sensors, optocouplers from Master(Client Machine) to our Slave. For sensors we are supplying 24VDC from Slave. Sensors are mounted inside/outside the Master-Machine.


    Our concern is to protect our Slave device from Master Device for some unexpected cases like Short Circuit due to wire damage, Noise, EMI.

    1. We can Protect Slave-device(MCU/IC) from Nois/EMI from LC filter as shown in the circuit image.
    2. From Short circuit and from AC power we have to protect the Slave(MCU/IC). Could you suggest any Isolation method for this?
    3. LC-Filter circuit is sufficient or could you suggest any Isolator?

    Thanks and Regards,

    Naveen K

  • Naveen,

    I see that the cabling contains various signals that vary from ground to 24V. The power comes from master, so the master must limit current for 24V supply to protect from intra cabling shorts. I assume all GND points are the same. I also assume the the opto-coupler has the series resistance in the path that is not shown in the block diagram.

    When you say you need protection from "AC power", do you means a direct connection from 3-Phase AC to any slave cable wire?  I do not think that is possible.

  • Hi Ronald,

    Thank you very much for your suggestions.

    Yes,AC power comes from master to power supply. Power supply having input fuses to limit the current.

    Master GND is connected with Slave GND only because of Amplifiers, Rest of all sensors/interfacing common GND is not required.

    Optocoupler having series resistor.

    Protection, I mean:

    Example1:, For proximity sensor we are supply power from Slave, If Proximity sensor wires accidentally get shorted with AC line or with any High-DC line in the Master, we have to protect the Slave- MCU & Power Supply from this case.Is it possible?

     Example2: In master device, due to EMI the signal line (Proximity Output / amplifier input / Switch detection) behavior may change. For this case, LC filter will take-care right?

    Thanks and Regards,

    Naveen K

  • Naveen,

    For example 1 , I don't know how to protect other than fuses that might not stop damage but will prevent far worse damage.

    For example 2, LC filters could work (but they can resonate), maybe only RC filter needed.

    Shielded cable is always helpful. Because signals are single ended, twisting pairs won't be helpful.

    For the analog input with the 100k resistor, that I suggested, could survive short to 240VAC if 100k resistor is sized properly. Power is E^2/R = 240^2/100000 = 576mW. Two 49.9k resistors would lower power to 288mW each.

  • Hi Ronald,

    Thank you very much for your suggestions.

    Yes Shielded cables are very helpful.

    1. Shielding should be connected to GND on both Master and Slave side or only slave side is sufficient?

    2. Slave-Optocoupler input is a 24VDC from the Master-Switch.When switch is close, optocoupler will get 24V input. Its not a signal. Can you suggest Shielding cable? If you suggest, shielding will connect to GND on both Master & Slave Side right?

    3. For Amplifier Input, we are connecting Signal and Common GND from Master to Slave. Shielding will connect to GND in both Master & Slave side right?

    4. Could you suggest Shielding cable for Power supply Output side (24VDC, GND)? If yes, Shielding is connected to GND or earthing?

    5. Could you suggest Shielding cable for Power supply Input side (3-phase AC, 400VAC)? If yes, Shielding is connected to earthing right?

    6. For increase the MCU life, better we should use >1K ohm current limiting series resistor to all GPIO's right? Example: connecting optocoupler output to MCU pin, UART pins, SPI pins, Switch interface, etc.

    Thanks and Regards,

    Naveen K

  • Naveen,

    I suggest that shield be terminated to ground on only the side that receives the signal for each cable.

    From your drawing and text, I assumed the 400VAC does not go the the slave module. So why do you need to address this?

    1k resistors could provide some benefit during fault conditions.

  • Hi Ronald,

    Thank you for your advice.

    In some machines no space is available to mount Power Supply, so in this cases Power Supply will be mounted in the slave Device panel box.

    1. So Kindly advice to handle this situation, How to protect MCU/Slave from Power Supply EMC/Noise,etc.

    2. Could you suggest Shielding cable for Power supply Input side (3-phase AC, 400VAC)? If yes, Shielding is connected to earthing right?

    3. Could you suggest Shielding cable for Power supply Output side (24VDC, GND)? If yes, Shielding is connected to GND or earthing?

    Thanks and Regards,

    Naveen K

  • Naveen,

    These questions are outside of my expertise. I have no futher advice.  

  • Hi Ronald,

    Thank you very much for your all advice's. Its very helpful for us and its solves our issues.

    I really appreciate your support.

    Thanks a lot !

    Regards,

    Naveen K