XTR117: XTR117

Hi Amruta,

You need to translate the resistance to an electrical signal to be able to drive the XTR117. If you have a resistance that changes with valve position, then one approach you could take would be to place this resistance in a wheatstone bridge configuration. If you are unfamiliar with wheatstone bridges, you may find this presentation useful: 

After the bridge, you will need an amplifier to convert the differential bridge voltage into a single ended voltage that will drive the XTR input. You will likely want to use an instrumentation amplifier to accomplish this, because they have high input impedance and will not load your bridge. Alternatively, if the impedance of your bridge is not too low, you could use a device like the XTR105, which already has an instrumentation input. The XTR105 would also be useful because it provides a more precise reference for your bridge than the regulator on the XTR117, but it all depends on what kind of accuracy you are looking for. If you choose to use the XTR117, your design will look something like figure 1 in the datasheet (see below). The exact implementation will depend on the sensitivity of your bridge and how much gain you need.

Thank you sir,
Is there any other way to implememt 4-20 mA circuit instead of wheatstone bridge.

Hi Zak,

I have a similar requirement of converting resistance value to 4-20mA signal but with cost effective solution. The project is to make a low cost 4-20mA transmitter. I have gone through your suggesting and it is realy helpful. The pdf file for wheatstone bridge prsentation is not landing to any page as it is expired or so. I have tried to summarise the circuit and made as below. I have chosen LM136 2.5 Ref as reference voltage to wheatstone bridge and LM324 as a low cost Opamp. For zero adjustment added trim pot of value 10K in series with valve position pot and for span adjustment a 10K pot in line at Q1 emitter. Request plaese review the circuitt and guide and suggest the changes required.    

  4_20_mA-Tx.pdf

 

Amruta,

There are certainly other ways you could interface with the XTR, but the Wheatstone bridge is the most commonly used. As I mentioned, you need to translate the resistance into an electrical control signal. Another approach you might consider is forcing a constant current through this resistance and conditioning the resulting voltage with an op amp to achieve your desired scaling into the XTR. This will require a current source though, and the accuracy of your input will depend largely on how accurate this source is. Let me know if there is anything I can help with.

Hi Zak,

Thank you very much for the valuable input. I have gone through XTR106 and updated my schematic. I have ordered few XTR115 and will receive in day or two.

Meanwhile request to please have a look on the schematic attached and please guide the changes required.

I have few points to get clarity on the bridge configuration-

1. The bridge resistance as a single value (350Ohm in one example) or is this the same value for three leg and fourth one is the variable resistance input.

2. Please review the Opamp LM358 input/output used for basic amplification.

3. The three leg on the bridge I have chosen 100Ohm and fourth one changes 100Ohm @ valve closed position to 230Ohm @ valve open position.4-20mA_Tx_PCB_Rev1.0 - Project.pdf

Thanks 

Niranjan

Hi Niranjan,

One leg of your bridge needs to be a variable resistance, however as you have it shown your bridge is going to produce a negative voltage. To remedy this, you should make R4 your valve resistance or reverse your bridge connections. In addition to this, given that your bridge impedance is so small, you will need to add resistors at the top and bottom of the bridge to limit the current drawn. With 100 Ohm resistances you are going to draw 25mA from Vref, which is more current than the reference can supply. You also cannot have an output current less than the total current drawn by your input circuitry, so you will need to scale this accordingly.

With regards to the LM358, you cannot configure an op-amp in this way. If you are using the XTR115, what you really need is an instrumentation amplifier, such as the INA333.

Hi Zak,

Thank you for the quick reply and valuable suggestions, I have noted the bridge configuration. will update the schematic accordingly.

For the instrumentation amplifier I can go for INA332 as low cost option, please suggest any concern.

Will share the updated design soon.

Thanks,
Niranjan