#Circuit#
Consider the following U-Shaped Micro Photoelectric Sensor *Panasonic PM-T54P* with two PNP Open-Collector Transistor outputs:
[![Photoelectric Sensor Input/Output Circuit Diagram][1]][1]
[![Photoelectric Sensor Wiring Diagram][2]][2]
[![Photoelectric Sensor Output][3]][3]
I chose the supply voltage to be 12V DC. In the place of the two loads I want to place two voltage meters (to be more precise the [NI 9401][4]), which can endure a maximum voltage of 5V DC.
After some calculations, I came up with the following first schematic for Output 1 (the one for output 2 is identical, so I leave it out):
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Some explanations:
- The connection to ground at the pull-down resistor R_GND is supposed to prevent a floating input into the NI 9401.
*It should be possible to get a switch into the schematic between V_A and V_In, which physically opens or closes the connection, but I did not know how to do this.*
- The resistance R_NI is supposed to limit the current into the NI 9401 to less than 250uA in case the circuit is compromised and the resistors R_A and R_GND are bridged, so that R_NI alone has to deal with the full 12V of V_A.
#Questions#
1. Do I really need the connection to ground at R_GND? Can one tell from the internal circuit on the I/O circuit diagram if e.g. Output 1 will float or will have 0V if the transistor Tr1 is non-conducting?
2. Let's assume that V_A rises due to a control error of the user up to 30V, but I still want **exactly** my 12V at V_In.
Could I accomplish this task with a 12V Zener-Diode between V_A and V_In as shown in the circuit below?
3. Finally I want an additional protection for V_Out, which should be capped at **exactly** 4.7V independently of what happens before it.
Could I accomplish this task with a 4.7V Zener-Diode between R_NI and V_Out as shown in the circuit below?
This is my draft for both question 2 and 3:
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And here is the result for a DC Sweep of V_In between V_In = 0V and V_In = 30V. It seems to be OK, right?
[![Voltage Protection Result][5]][5]
#Update#
Now it states [here][6] that the input impedance of the NI 9401 is 47kOhm, so I tried to create a new schematic accordingly, based on the remarks from [Bruce Abbott][7] and [rioraxe][8]. Would that be correct?
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Here's the result:
[![Voltage Plots][9]][9]
[![Current Plots][10]][10]
[1]: https://i.sstatic.net/IKNXQ.png
[2]: https://i.sstatic.net/SJ7vr.png
[3]: https://i.sstatic.net/RE4Yf.png
[4]: http://sine.ni.com/nips/cds/view/p/lang/en/nid/208809
[5]: https://i.sstatic.net/pdQFi.png
[6]: https://forums.ni.com/t5/Real-Time-Measurement-and/impedance-NI-9401/td-p/1476184
[7]: http://electronics.stackexchange.com/users/49222/bruce-abbott
[8]: http://electronics.stackexchange.com/users/60144/rioraxe
[9]: https://i.sstatic.net/vkp0L.png
[10]: https://i.sstatic.net/V1Lb4.png