Hi,
Reading the transducer datasheet I see a couple of relevant values:
* supply voltage: 16.8V ... 31.2V
* load (@ voltage output) >= 10 k Ohms
* open output: < 18V
--> So from this ...I see no problem for the transducer. You are within specifications on all three specifications.
But for accuracy reasons I'd avoid the 100 Ohms series resistor (at least at the position you use it)
But that's only half (if ever) of the truth!
There are a couple of things to consider.
First of all: I see no reason why the output of the transducer should be high impedance.
Please tell us why you consider this situation at all.
If you consider to disconnect the wires, then:
* Don't do this when the whole circuit is powered,
* if you do this with the circuit powered, then you need additionl protection circuitry. For both the transducer and the comparator circuit.
* the "pull up voltage" (12V in your case) is not relevant, because the transducer is not connected...
The biggest problem I see:
Is the comparator. It is supplied with 12V ... and your pullup is to 12V, too.
But the comparator's datasheet tells "common mode input voltage" is up to "VCC - 1.5V". Now your circuit violates this specification. In this case the comparator output state is unknown ... and this is exactly what you want to avoid.
Thus a more valid solution is to use two resistors: maybe use
* 200k as pullup (to VCC = 12V)
* and a 1M to GND.
With this the resulting "unconnected" node voltage will be 10V.
*****
Here a couple of additional hints.
* use the series resistor after the pullup/pulldown resistor. And use 10k.
* use two protection diodes. Maybe 1N4148 each one in parallel of the pullup and pulldown respectively.
* use a (foil) capacitor as LPF noise filter in the range of 1uF ... 10uF directly at the comparator input
* use a big bulk capacitor to stabilize the 12V supply. (Any weak supply may cause oscillations, because the relay current will drop the supply voltage...and thus the voltage at -IN, which causes negative feedback). Hopefully your hysteresis is higher than the negative feedback.
Klaus
Btw: I'd use only one power supply. This avoids power sequencing problems. But needs special care on circuit design.
Hello and thanks for the answer,
For practical reasons I will use separate power supplies, 24VDC for transducer and 12VDC for the comparator circuit.
I want to go through your suggestions and try to clarify what worries me.
First here below is the circuit I modified after your suggestions:
* The 100 Ohm Rout in my first circuit was to mimic the output impedance of the transducer which is not given. I estimated it but since not given now I deleted it in the new circuit. If you can refer the output impedance of the transducer from the datasheet let me know. I only needed it for simulations.
* Now following your suggestions;
I removed 100 Ohm Rout.(Transducer output resistance is unknown to me)
I added 10u and 100n capacitors for Vcc.
Added pull up and pull down R5 and R4 as 200k and 1Meg respectively to lower the input voltage not to violate the "VCC - 1.5V" spec.
Added two parallel diodes 1N4148. I added R8 as series 10k and C2 to form a LP capacitor.
[
***What do you think about the circuit now?(Hope I modified it accordingly with your suggestions)
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***But my main worry is when when the transducer output is seeing 10V applied to its output port. Transducer might be 0V up to 10v. In this case it will be like we are connecting a 9V battery to this transducer output through a 200k. So we are applying voltage to its output port where the transducer will sink around 100u. Now this reverse current will always exist. I hope I made where Im stuck better.
This confuses me because it is like connecting 9V battery to the output terminals of a function generator. An output port outputs voltage and sources current. How come we can apply voltage to an output port. So in this case the 10V at the node of R4/R5 is continuously applied to the transducer output port. Isnt this a problem?
Thanks!