• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Help to improve this filtering circuit

Status
Not open for further replies.

DDT

New Member
The following shows the diagram for a sensing and filtering input of a break wire alarm. The logic input is held low by the break wire and goes high once the wire is broken. The triggering voltage for the logic high is around 6V. R2 needs to be very high ( min 1M ) to minimise the current consumption, the R1 makes a low pass filter combined with C1. Once the wire is broken, the voltage on C1 needs to be as high as possible ( over VT+ 6v) to maximise reliability. Considering TVS has 1uA leakage, how would you choose R1 and R2?

IC is CD4093BE with Schmitt trigger.



Kindest regards
 

Attachments

Grossel

Well-Known Member
I don't know what you mean by TVS.

But I do have a troubling question - you're talking about break wire. I do drive cars and bikes regularly and would never put anything at risc bu allowing a break wire to snap. What is the application here, I hope it is not something life critical.

Wouldn't it give more sense having a system that is capable of detecting a worn wire - but still in fair time before it goes snap?
 

DDT

New Member
Hi, TVS is a protection diode similar to zener but much better for removing RF and other induced voltage in the long breakwire. The break wire (0.1mm) is the sensor of the alarm which is for the protection of a camp site against intruders and animals in a remote area. When an intruder breaks the wire, alarm sounds. Regards
 
Last edited:

MikeMl

Well-Known Member
Most Helpful Member
The CMOS input leakage is worst case 1uA. That adds to the TVS leakage of 1uA, even when ignoring capactor leakage, so R2 must source >2uA with about 3V across it, meaning that R2 < 3/2u = 1.5meg, so your 2meg is a bit high.
 
Last edited:
  • Like
Reactions: DDT

DDT

New Member
OK thanks, do you think R2= 1M is good value if I want to be sure of a reliable operation? Also regarding the R1, will it affect anything? I think the max value for R1 would be desirable to enhance the RF suppression. Regards
 

MikeMl

Well-Known Member
Most Helpful Member
loop.png
 
  • Like
Reactions: DDT

DDT

New Member
Thanks for the calculation. Can I1 and I2 be substituted with equivalent resistor (9M) in the calculations? Since I don't know to use spice program, I'll be using manual calculation to find the sweet spot for the resistor values. Regards
 

MikeMl

Well-Known Member
Most Helpful Member
No, since V(loop) is 7V, then the equiv resistance of I1 is 7V/1uA = 7megΩ.

You can solve the circuit using algebra, KCL and Ohm's Law, however. For example, you can see by inspection that the voltage drop across R1 has to be 2V because there is 2uA flowing through it.
 
  • Like
Reactions: DDT

AnalogKid

Well-Known Member
Most Helpful Member
The R1/C1 time constant is 10 ms. If you eliminate R1, then R2/C1 forms a lowpass filter with a time constant of 200 ms. Decreasing C1 to 10 nF and decreasing R2 to 1.0 M gets you back to a 10 ms filter while addressing the leakage current issues mentioned above. And saves 1 resistor. Unless millisecond response time is important, I would stay with the larger cap for a lower corner freq.

ak
 

DDT

New Member
Removing R1 would be problematic as the long (60ft) break wire needs all the help that it can get for filtering. I'll decrease R2 to 1M or even lower to overcome leakage current and voltage drop issues. The fast response time is not critical, the reliability and battery life are most important requirements. Regards
 

MikeMl

Well-Known Member
Most Helpful Member
R1 (your numbering, not mine) serves another important requirement, which is to limit the surge current that flows into the CMOS input during a big transient. The CMOS input will tolerate +-10mA, so you need more current-limiting than otherwise is provided by (your) R3.
 

DDT

New Member
If you'r referring to transients induced in the break-wire by outside elements, wouldn't TVS and R1 be barriers against that? Please note that this is a battery operated device and not connected to mains.
 
Status
Not open for further replies.

Latest threads

EE World Online Articles

Loading
Top