PCB current draw - Page 1
Forum section: electronics / beginners

author: Joegh6
author_rank: Newbie
timestamp: January 18, 2023, 09:46:50 pm
content: Hello,I have been working on a Clas Ohlson "North Light 1531" solar-powered wall light with motion sensor (AS312), which prematurely drains its double 18650 (7.4V) Li-Ion battery.The PCB is connected to the battery (upper right), the LED light itself (lower right) and the solar panel (lower left). There is also a pot for defining the duration of light during detected motion and a button switch for selecting one of the three modes (or off).What I'm seeing is a constant 1.5 mA draw even when turned off, which I'm unable to attribute to any of the valid main components.When turned on in any of the three modes, I see a 4.0 mA draw, even when the LED panel is not lit, either due to daylight or no motion detected. This effectively drains the battery in a couple of weeks.What I have done so far:- Removed indicator LED1 and LED2, on both sides of the PIR sensor.- Desoldered and checked C1 and C2 for partial short and capacitance.- Desoldered and checked EC1 for partial short and capacitance.- Disconnected the solar panel.- Examined the board with an IR camera for identifying components warming up. Even with the light turned off, the 5.1Kohm resistor Ra does get a little warm, but I'm assuming that is due to the current being drawn from somewhere else. Other than that, nothing is visibly warming up.I'm sorry I don't have a schematic. I'm hoping someone could give me an idea on what to check for next or let me know if you see anything in the design which would make these lights draw this much power constantly.Any tips are appreciated!Thanks,JG

author: Thunderer
author_rank: Regular Contributor
timestamp: January 19, 2023, 02:19:59 am
content: Hei JG,Can you specify the LED type, please? Or the VF when fully lit.Some thoughts:1. The voltage regulator (U4 = HT7350-1 = 5V regulatorhttps://www.holtek.com/documents/10179/116711/HT73xx-1v120.pdf) is low power, but it is not saying how low for given output currents, but should be few uA for few mA output.2. Another voltage regulator (U1 = HT7530-1 = 3V regulator for PIR sensorhttp://www.e-ele.net/DataSheet/HT75XX-1.pdf).3. AS312 PIR sensor other 0.5mA.4. The LED is powered via a step-up driver (U2 = OC5022B, I hope your chinese is better than Googlehttps://www.chipmall.com/datasheet/OCX-OC5022B_5384634.html, we see the 0R33 shunt), thus the continuous power need for the driver, even in stand-by (they say 0.2mA).When you add each load, the 1.5mA may be what the electronics in stand-by draw from the supply.

author: MathWizard
author_rank: Super Contributor
timestamp: January 19, 2023, 01:57:19 pm
content: Could be some solder balls too or residue from soldering adding to any idle power. Maybe give it a sruc with a toothbrush and soapy water.

author: Joegh6
author_rank: Newbie
timestamp: January 19, 2023, 10:28:19 pm
content: Quote from: Thunderer on January 19, 2023, 02:19:59 amHello, I appreciate your input.Unfortunately, I don't think I can identify the LED type without breaking the panel enclosure. It seems to be a panel consisting of 30 white LEDs. When fully lit I see 6.4V across positive and negative.Thanks for pointing out the draw from those components. I hadn't realized the PIR sensor draw was actually that big.Thing is, though, the draw in stand-by is actually 4.0mA. The 1.5mA draw comes just from connecting the battery. I must still be missing something.Thanks,JG

author: Joegh6
author_rank: Newbie
timestamp: January 19, 2023, 10:29:53 pm
content: Quote from: MathWizard on January 19, 2023, 01:57:19 pmThanks, I'll be sure to try that. I actually spotted some solder residue before but that wasn't the culprit.JG

author: drvtech
author_rank: Regular Contributor
timestamp: January 21, 2023, 08:32:54 pm
content: Could be a damaged ceramic cap. Try freezing (with freezer spray) or heating (with a soldering iron) each component individually while monitoring the current for changes.

author: Joegh6
author_rank: Newbie
timestamp: January 23, 2023, 10:37:48 pm
content: Hello and thanks for all the suggestions.I have still been working on this without any major breakthrough.I'm still puzzled by resistor Ra warming up. This is really a newbie question, but could anyone explain to me what the purpose of it might be? As I'm seeing about 8V across it and it's a 5.1KOhm resistor it comes out suspiciously close to the 1.5mA draw I'm seeing.Thanks again,JG

author: Joegh6
author_rank: Newbie
timestamp: January 27, 2023, 09:52:54 pm
content: Quote from: Joegh6 on January 23, 2023, 10:37:48 pmJust as an experiment, I desoldered the Ra resistor and idle draw actually does decrease from 4,0mA to about 2,7mA. Also, the draw when turned off decreases from 1,5mA to about 1mA.So the resistor does seem to play a role in this. Does it have some safety feature, or could I just permanently leave it out? Perhaps increase the value to make the draw even smaller?Thanks,JG