Ionization smoke dedector IC guard pin

Thread Starter

ilginsarican

Joined Jul 13, 2017
142
Hello,
I want to use that IC for smoke dedector :part number: RE46C180S16F
I've seen guard pins on IC, but Ionization chamber has 3 pins.
Could you explain function of "guard" pins and how can I connect?

upload_2019-6-17_16-52-4.png
 

DickCappels

Joined Aug 21, 2008
10,240
Notice that the guard traces run on both sides of the Detect trace, and connect nowhere else.This kind of arrangement protect the high impedance Detect input from leakage currents from other parts of the circuit board.
 

Thread Starter

ilginsarican

Joined Jul 13, 2017
142
It's a Screen guard for the Ionisation chamber to prevent false triggering.


Datasheet link...
http://ww1.microchip.com/downloads/...Vaw26eXMQOaVfU_8O9wIWDGHx&cshid=1560780855732
Hello,
I could not download the datasheet.
I've found a datasheet, It says "An active guard is provided on both pins adjacent to the detect input. The voltage at these pins will be within 100 mV of the input signal."
I understand function but I don't understand connection.
Where sould I connect these pins?
 

Thread Starter

ilginsarican

Joined Jul 13, 2017
142
Thanks for reply,
I've opened the smoke dedector box and looked at pcb:
Pin number 16 and 14 are connected and pcb trace creates a circle connection.
Isn't that what you're talking about?

upload_2019-6-17_17-44-40.png
 

MisterBill2

Joined Jan 23, 2018
19,107
Guard pins and traces on a circuit board are to prevent surface leakage currents from causing problems. The intention is that they are held at the same voltage as the line that they are guarding, so that no current will flow to the protected trace. The problem is that the surface of the PC board becomes slightly conductive due to contamination from stuff in the air, and dust. And in this case the circuit being guarded can be influenced by nearby voltages if those guard traces are not used.
 
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