Infrared Thermometor - battery dies after few days...Low Power LDO the cause?? - Page 1
Forum section: electronics / repair

author: LiftedTrace
author_rank: Regular Contributor
timestamp: January 09, 2021, 06:14:04 pm
content: Hello,I have this infrared thermometer that i got off amazon if I can remember a while ago. I put my battery in and used it and it was decent enough. Went back to use it a few days later and the battery was dead. It was cheap enough i figured its not worth returning, Ill just unplug the battery when not in use.Well fast forward a few years later, I decide to figure out why its killing batteries and if there is anything i can do to fix it. Tied directly to the battery input is a STO-89 HT7530-1 LDOhttps://www.digchip.com/datasheets/parts/datasheet/196/HT7530-1-pdf.phpI dont know that much about electronics, but I know enough to tinker. I wanted to ask here if this is normal behavior?Measuring the voltage across Vout and GND pins with a variable power supply attached where the battery should be, shows the voltage mirror the input.Now this is being powered by a 9V battery, but as soon as I used my variable power supply, I immediately started to smell that "something is getting hot". I touched around and the HT7530-1 was about to go nuclear. Looking at the current being drawn, it was about 350mA. I pulled the supply off and connected the battery with a meter in series to see what the battery was delivering. The battery was delivering 225mA when this is supposed to be off. Pulling the trigger caused the current to go up almost negligible. SO now I know why my battery is getting killed so fast.Looking at the datasheet, I see it can take an input voltage up to 24v, and my chip number says it should be a 3v output.So, am I right to suspect this chip as bad? Perhaps its shorted internally? This had to be like this brand new. Whats strange is, since the output was mirroring the input, I applied 3V to the input and got 3V on the output. I pulled the trigger and it did not function until I increased the voltage to around 5v. So I am not sure whats going on here.Any suggestions?

author: Manul
author_rank: Super Contributor
timestamp: January 09, 2021, 08:42:45 pm
content: My idea would be that the LDO pass element is shorted, but that is likely a result of another failure. I conclude this, because if the information you provided is accurate, the thermometer should work from 3V. It does not. Probably something is sinking huge current on a 3V rail and creates voltage drop. So down the line the 3V is not anymore a 3V and the MCU or whatever does not function. When you increase to 5V, you compensate for voltage drop. It is hard to imaginge such a drop over PCB traces, so it should be some component in line. Anyway, I think something is shorted (around 15 Ohms to gnd) on the 3V rail + LDO is toast. LDO pass transistor is for sure shorted if it mirrors. I suggest to desolder LDO and check.

author: LiftedTrace
author_rank: Regular Contributor
timestamp: January 09, 2021, 10:52:45 pm
content: Ok, so I was suspecting what you also said, that something else down stream caused it to fail. So I just removed the chip and now I am even more confused, lol.I applied 3V with very low current limit to where the Vout was connected to and I drew "zeeeeero" lol. I pressed the button, and it drew .08A and back down to zero after it shut off. So this seems to be working perfectly. There was another trace that buggered off to some transistors that was tied directly to B+, so I stuck that with 3V and it also had zero current draw. No clue what thats for as it would be seeing 9V on good battery, and the devices seemed to work just fine without it being powered.....perhaps it goes into the infrared section?I connected 9V to the input of the chip and Gnd (removed from board), and it was pulling 300mA, Just like when it was installed.I think this thing must have just been dead on arrival. I guess that what ever brains are in this and other chips must be able to handle 9V cause its always worked fine, its just always killed my batteries.Which makes me wonder, can I just keep that thing gone and connect up a single cell lithium battery?

author: wraper
author_rank: Unknown
timestamp: January 09, 2021, 10:58:02 pm
content: Quote from: LiftedTrace on January 09, 2021, 10:52:45 pmNo, just replace it.

author: LiftedTrace
author_rank: Regular Contributor
timestamp: January 09, 2021, 11:56:25 pm
content: Ok, Ill replace it.I would prefer not to buy from china, and the only place I found that chip was on ebay coming from china. (Edit: Looking back, this thermometer was probably from china)I found this:https://www.mouser.com/datasheet/2/115/AP7383-1379744.pdfon mouser. Would this be a good replacement? I see the sample schematic uses a 1uF instead of 10uF like the original.

author: wraper
author_rank: Unknown
timestamp: January 10, 2021, 12:29:40 am
content: Quote from: LiftedTrace on January 09, 2021, 11:56:25 pmYes, it's a good alternative. 1uf cap is shown because it can be used with low capacitance MLCC unlike older LDO. It should work with 10uF tantalum or other polarized type too.

author: LiftedTrace
author_rank: Regular Contributor
timestamp: January 10, 2021, 12:34:13 am
content: Fantastic, thanks. Ill order a few and see if I can get this working the way it should have always worked.

author: LiftedTrace
author_rank: Regular Contributor
timestamp: January 17, 2021, 11:22:01 pm
content: Just a follow up.I installed that new chip and my battery has not died in a week, so all is well. Thanks again.