https://www.google.co.in/search?q=b...Ep48KHda-AzEQsAR6BAgFEAE#imgrc=TIoe2AdpNO5g1M:

The image is from this page:
http://www.circuitdiagramworld.com/alarm_circuit_diagram/Wire_loop_alarm_12054.html

Moderators note : linked to site

 

Welcome to AAC!

Please post the schematic on this forum so we don't have to click on links of unknown trustworthiness, and to avoid broken links in the future.

 

Welcome to AAC!

Please post the schematic on this forum so we don't have to click on links of unknown trustworthiness, and to avoid broken links in the future.

how to edit this post?



MOD: added your circuit image.

 

Look at the ratio of the resistors. What percentage of the supply voltage will appear at their junction when the loop has continuity? Will that voltage turn the device on? What happens to the capacitor voltage when the loop is cut? What will happen to the device?

 

Look at the ratio of the resistors. What percentage of the supply voltage will appear at their junction when the loop has continuity? Will that voltage turn the device on? What happens to the capacitor voltage when the loop is cut? What will happen to the device?

why capacitor does not charge initially??

 

It is charged initially, but to a very low voltage. You can calculate the initial voltage across the capacitor using Ohm's Law twice.

ak

 

how to edit this post?

I think capabilities are limited for new members until they reach 10 posts.

 

The C1 and R2 should be works like as the noise filter to avoid the unwanted voltage to causes the Vgs too high and turn on the Vds, when the loop wire is connected to Ground then the Vgs will be keep on around 1.87V(Vth = 2~4V) that it doesn't reach up 2V and close to the threshold voltage of Vgs, when the loop wire was cut then Vgs will be going up to 9V and the time will be depends on the R1 and C1, when the Vgs up to 4.5V then the Vds will be turn on since the Vbat is 9V, so the Vgs is using a 4.5V logic level is better .

 

The C1 and R2 should be works like as the noise filter to avoid the unwanted voltage to causes the Vgs too high and turn on the Vds, when the loop wire is connected to Ground then the Vgs will be keep on around 1.87V(Vth = 2~4V) that it doesn't reach up 2V and close to the threshold voltage of Vgs, when the loop wire was cut then Vgs will be going up to 9V and the time will be depends on the R1 and C1, when the Vgs up to 4.5V then the Vds will be turn on since the Vbat is 9V, so the Vgs is using a 4.5V logic level is better .

Thank you so much ScottWang !!! Great explanation..

 

I think capabilities are limited for new members until they reach 10 posts.

How to increase them??

 

How to increase them??

Ask the EE questions that you don't understand as you did in this thread or help other members to solve their problem, but you can't post the useless or meaningless contents, otherwise our Moderation Team will delete the post and given a warning.