Battery Charger switches off the PCB

Thread Starter

lannocm

Joined Jul 14, 2021
6
Hi,

I have a wireless camera that works from a rechargeable battery. It works as expected when the battery is on by itself. It also works when the battery is connected and plugged in to the charger. The problem is when the camera is on and the battery is charging; but then I remove the charger the PCB switches off and I have to manually flick the on/off switch to get it back on. I've tried it on two cameras of the same type and it is the same on both.

What is happening here? Does the PCB charger need a holdover capacitor?

I have another similar camera that doesn't have this problem.

I tried to upload a short video but I can't see how I can do it.

Thanks for looking.
 

Thread Starter

lannocm

Joined Jul 14, 2021
6
Since writing the post, I've done a bit more digging and it would probably help if I gave a bit more information.

The camera works from a 3.7v LIPO battery but the charging is done through a standard 5v micro USB connector. I have installed the circuitry, battery and camera in a model train (oo gauge) and have included a simple circuit to derive power from the rails. On a digital system, there is a constant supply of 15v AC. On a non-digital system, it is 0-12v DC.

I have connected a DB107 bridge rectifier to the rails and fed the DC output through a buck converter to provide the 5v to the charging circuit. This seems to work fine for both AC and DC systems. What I want is that, when the rail power is disconnected, the battery will take over without the PCB switching off.

I have seen another couple of posts on this form (eg https://forum.allaboutcircuits.com/threads/automatic-switching-between-two-power-sources.156924/) and it looks like maybe I need simply two diodes in series with the DC feeds from the battery and the rails.

Please can someone confirm that and what I need to get and why this works (assuming it does)?

Thanks
 

Lo_volt

Joined Apr 3, 2014
219
That sounds like it should work. Can you draw a schematic of what you want to do just so we can make sure you are doing it correctly?
 

Irving

Joined Jan 30, 2016
2,097
It sounds like the disconnection causes a glitch that turns the camera off. The article you linked to was at the level of a much higher voltage battery where the voltage drop in the diode(s) wasn't an issue.

Your problem is different - turning the 12v and therefore the 5v off momentarily disconnects the battery from the camera which turns off. At that point you don't have an alternate supply to feed the camera so the 2-diodes solution won't work.

Is the charging circuit outside the camera and the battery inside? Can you post a picture of the main components (dc feed, charger, battery & camera) and links to the charger board, camera, etc..
 

Thread Starter

lannocm

Joined Jul 14, 2021
6
Hi,

I have found a solution but it's a bit 'clunky'. I have a second battery charger that also powers the pcb 5v input. If power is removed, the pcb takes power from the battery connected to the second battery charger. ie the pcb's own battery charger is not used.

Attached are a couple of pictures showing the pcb (without the second battery charger), the battery, the 5v power and the camera.

Thanks
 

Attachments

Irving

Joined Jan 30, 2016
2,097
Do you have a link to that PCB? It seems the 5v from the USB goes to the camera and charges the battery and theres a boost converter from 3.6v to 5v when the USB is disconnected, but there's nothing there to hold the voltage up during the transition. You could try a large capacitor (470uF/10v) between +5v and ground, but you might find the boost converter won't start reliably from cold due to the load it puts on the converter when discharged.

Is there anything on the back of the PCB? Photo please..
 
Last edited:

Thread Starter

lannocm

Joined Jul 14, 2021
6
Do you have a link to that PCB? It seems the 5v from the USB goes to the camera and charges the battery and theres a boost converter from 3.6v to 5v when the USB is disconnected, but there's nothing there to hold the voltage up during the transition. You could try a large capacitor (470uF/10v) between +5v and ground, but you might find the boost converter won't start reliably from cold due to the load it puts on the converter when discharged.

Is there anything on the back of the PCB? Photo please..

Hi,

I don't have any more information on the board itself: It's a generic endoscope from China.

I tried a couple of capacitors across ground and 5v and I THINK the second one was 470uF but it made no difference. Is it worth pursuing that?


I've attached another couple of photos of the board, though, if that helps. Thanks.
 

Attachments

Irving

Joined Jan 30, 2016
2,097
It would help, if the one of the back of the board was in focus so I can read the chip numbers! :) However it did answer one question I hadn't understood... The camera is Wi-Fi capable hence you can use it disconnected!
 

Irving

Joined Jan 30, 2016
2,097
OK, so this is what's happening on the back of the board...
pcb.jpgWhen the USB connection is live power flows via the DC-DC boost chip to the rest of the board via the on-off switch, and the battery is charged as well. When USB is turned off the chip stops charging the battery with the Buck converter portion and starts the boost converter portion of the DC-DC converter - this takes a few tens of milliseconds to start hence the camera shuts down. The chip is not intended to be an uninterruptible power supply!

A quick calculation suggests you need 20000 - 30000uF capacitor to hold up for the 50mS or so it takes to switch over, which is why 470u didn't appear to work...

I have 2 suggestions, neither of which are painless...
  • #1 is to use a supercapacitor. Unfortunately you can't just connect it across the 5v rail, you need to control its charging regime, so a little extra circuitry is needed...
  • #2 is to stop using the onboard charger and use an external board, but I'm not sure any of them gaurantee a hold-up during transition without trying them. Several on Amazon/eBay use the same chip.
 

Thread Starter

lannocm

Joined Jul 14, 2021
6
OK, so this is what's happening on the back of the board...
View attachment 245114When the USB connection is live power flows via the DC-DC boost chip to the rest of the board via the on-off switch, and the battery is charged as well. When USB is turned off the chip stops charging the battery with the Buck converter portion and starts the boost converter portion of the DC-DC converter - this takes a few tens of milliseconds to start hence the camera shuts down. The chip is not intended to be an uninterruptible power supply!

A quick calculation suggests you need 20000 - 30000uF capacitor to hold up for the 50mS or so it takes to switch over, which is why 470u didn't appear to work...

I have 2 suggestions, neither of which are painless...
  • #1 is to use a supercapacitor. Unfortunately you can't just connect it across the 5v rail, you need to control its charging regime, so a little extra circuitry is needed...
  • #2 is to stop using the onboard charger and use an external board, but I'm not sure any of them gaurantee a hold-up during transition without trying them. Several on Amazon/eBay use the same chip.
Ahh...

So I was on the right lines with the capacitor, just not chunky enough. #2 is what I have done and it is working fine using the extra board. If #1 needs extra circuitry, then I might as well stick with #2 and just buy the board (which cost £1 or something)

https://smile.amazon.co.uk/gp/product/B08DQXPNR1/ref=ppx_yo_dt_b_asin_title_o08_s00?ie=UTF8&psc=1

Thanks very much for clearing that up.
 
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