Gas Detector Circuit Low impedance between ground - Page 1
Forum section: electronics / repair

author: sfiber
author_rank: Regular Contributor
timestamp: November 30, 2017, 08:51:28 pm
content: Hi Friends;I am having a problem with gas detector circuit. Circuit has standart transformer-diode bridge and filter capacitor combination.In addition there is a 7805 voltage regulator for sensing side. The problem is about low resistance between 7805's 5V output pin and ground pin,its value like 26.3 ohm. I have a scope to measure what s going on this circuit,but 7805's temperature increase critically in the seconds. I check all the resistors and diodes,here is the photo of the pcb. What would you offer to me?

author: Armadillo
author_rank: Super Contributor
timestamp: November 30, 2017, 09:13:58 pm
content: That means that the 7805 is defective.Replace it will be fine.

author: sfiber
author_rank: Regular Contributor
timestamp: December 01, 2017, 08:32:21 am
content: I have replaced with the new one before posting and I measured the resistance of the nodes without 7805,very low resistance again.

author: Armadillo
author_rank: Super Contributor
timestamp: December 01, 2017, 09:22:48 am
content: Quote from: sfiber on December 01, 2017, 08:32:21 am26.3 ohms is not low ohm.It's only 200mA.Your 7805 is able to handle 1.5A.The 7805 will be warm to the touch but not HOT.Replace it and measure Pin 2 and Pin 3. It should read 5V steadily.Edit: There is a internal heater inside the sensor. Hence the ohms will be in this range.

author: sfiber
author_rank: Regular Contributor
timestamp: December 01, 2017, 11:08:01 am
content: So,you say temperature increment in seconds is not matter.Do you think paralleling to 2 7805 can effect to heating.?

author: Armadillo
author_rank: Super Contributor
timestamp: December 01, 2017, 11:17:45 am
content: Quote from: sfiber on December 01, 2017, 11:08:01 amYou need to quantify somewhat your temperature otherwise we cannot understand what you are trying to say.If the 7805 is just warm to the touch is OK but not hot.Your original 7805 may be faulty, so you need to replace with a new one. And do watch the orientation when you put the new one back in.I think is just plain wrong diagnosis of the problems you are having. Instead of just hypothetical about seconds and rise, do the measurement. Please measure the voltage when you do write back.Pin 1 to Pin 2 is about 9vdc.Pin 3 to Pin 2 is 5vdc.Otherwise, don't expect the sensor to last forever.Edit: The designer don't even bother to put a heatsink to the 7805. So don't even think of paralleling 2 7805. It's not the problem!Edit: to avoid language barrier, I attached a picture to differentiate sensor and 7805.

author: sfiber
author_rank: Regular Contributor
timestamp: December 01, 2017, 05:33:52 pm
content: its like 60 C degree in 15 seconds.My concern is about long time working. I am not sure this circuit is safe to work at kitchen all the time,because of the temperature.

author: Armadillo
author_rank: Super Contributor
timestamp: December 01, 2017, 05:44:56 pm
content: Quote from: sfiber on December 01, 2017, 05:33:52 pmIn the kitchen, your refrigerator compressor and condenser are even hotter.Go to the internet, do more reading about the "The fire triangles or combustion triangles".But to appease your fear, always engage a gas detection professional to install the system for you.Only when you part away with money, your heart will be more at peace.

author: sfiber
author_rank: Regular Contributor
timestamp: December 02, 2017, 06:57:48 am
content: Thank you very much for all your helps. I am gonna work to improvement this circuit for less heating.Thanks again.