noob power switching question

Discussion in 'Electronics & Electrics' started by joomax, Jan 18, 2019.

  1. joomax

    joomax Member

    Joined:
    Nov 1, 2016
    Messages:
    102
    Location:
    Melbourne
    Hey again, working on my speaker project and recently shipped in a nice cheap chinese amp for it. Chucked out the huge old hifi boards and this works just as well!

    Final thing I'd like to do is hook up a 12V lead acid battery I have here, so that I can make it portable on occasion. This amp has a barrel jack for a wall adapter, but I'm wondering if I can simply solder on a switch to the jack/diodes and wire it to the battery terminals, so I can switch between off/battery/wall adapter. Here's some pictures, with the easiest access to the board I have at the moment. Could take it completely apart if required I guess.

    [​IMG]
    [​IMG]
     
  2. Technics

    Technics Member

    Joined:
    Apr 29, 2002
    Messages:
    1,507
    Location:
    Brisbane, AU
    A lot of DC sockets have a switch built in for disconnecting a battery whenever a plug is inserted. It's hard to tell from your pictures if that is the case here but it does look that way. Essentially the socket has two "negative" pins (assuming negative is on the of the jack which it is here). When the jack is not inserted the two pins are connected together and when the jack is inserted it breaks the connection. See here for an example http://www.farnell.com/datasheets/2606983.pdf.

    The issue is that the two terminals of the switch are often connected together on the PCB when the switch is not needed. This is done because connecting them both helps with the mechanical integrity of the socket. So you would need to cut the track joining the two pins. Then you can connect the battery positive to the socket positive and the battery negative to the socket switched negative. That way the battery connection is broken automagically whenever the jack is plugged in.
     
  3. OP
    OP
    joomax

    joomax Member

    Joined:
    Nov 1, 2016
    Messages:
    102
    Location:
    Melbourne
    ah right that makes a lot of sense. Slowly figuring this out as I go. I understand the switch concept now, but just to confirm:
    In this picture: https://i.stack.imgur.com/tcQ8n.png I understand pin 1 should go to the positive terminal of the battery, and pin 3 to the negative. So pin 1 is the tip, am I right in thinking that pin 2 would be the sleeve of the jack? It seems to make sense that if the pin 2 bump thing contacts the sleeve, then pin 3 would be not in use, so the battery disconnects.

    I've got the pcb out now, here's a picture: I'll cut away the track like so (red line)? I've just noticed that jumper connection, this could be an issue...

    [​IMG]
     
  4. merlin13

    merlin13 Member

    Joined:
    Apr 12, 2003
    Messages:
    1,806
    Location:
    back in BrizVegas
    Ummm - why not save some grief and simply make up a lead from the battery to that power socket?

    Although on saying that, just be aware the voltage range on a nominal 12 volt lead acid battery can range from just over 14 fully charged down to under 10 if you let it discharge too far. So before going too far with the Stanley knife and soldering iron I'd be inclined test test the low end of the actual voltage range that that amp will happily run on first.
     
  5. Technics

    Technics Member

    Joined:
    Apr 29, 2002
    Messages:
    1,507
    Location:
    Brisbane, AU
    Excuse the MS Paint but it would be more like... Cut the the track as shown in red and connect your battery as per the yellow. merlin13 makes some good points but I suspect it will be fine on a battery because the design was probably based on something intended for car stereos by the looks of it.

    [​IMG]

    Edit: The way to confirm the switch in the socket is working properly is to power it off the battery and then connect the plug without the power supply plugged into mains. It should disconnect the battery and shut the amp down when the plug is inserted and only power back up again when the supply is connected to mains.
     
    Last edited: Jan 18, 2019
  6. OP
    OP
    joomax

    joomax Member

    Joined:
    Nov 1, 2016
    Messages:
    102
    Location:
    Melbourne
    Whoops, I forgot the separated switch pin would still complete the circuit as it routes back to the sleeve pin in the socket. For some reason I thought I'd still have to keep that joint connected to everything down the line haha

    I did read up about voltage ranges, but it's a relatively low powered device (made originally for cars it seems, yes) so I figured if it's running on a drained battery it would just lose power and not necessarily harm it. Plus it's a good indicator of low battery :p

    Great suggestion for testing - I'll do this. Really grateful for your help here! I feel like I should be able to understand this quickly and easily but I just don't have the confidence of mind yet. Lack of experience!
     
  7. DarkYendor

    DarkYendor Member

    Joined:
    Feb 25, 2008
    Messages:
    3,195
    Location:
    Perth
    ^^This. Will cost $2, take 5 minutes, good flexibility (and doesn't involve drilling into the traces on the PCB).
     
  8. Technics

    Technics Member

    Joined:
    Apr 29, 2002
    Messages:
    1,507
    Location:
    Brisbane, AU
    Just remember that for lead acid this will probably lead to deep discharge and be doing more harm to the battery than the amp. A low voltage cutout is a good idea if you want to retain the capacity. It may not matter much for your use case if the capacity drops a little but if you really flatten the battery then its capacity will drop substantially.

    You'll get the experience by doing. Hopefully without letting the smoke out.

    I'm definitely not discounting that either. Less effort and quick to make up. Still needs a trip to Jaycar or an Element14 order though unless you have the plugs already though.
     
  9. OP
    OP
    joomax

    joomax Member

    Joined:
    Nov 1, 2016
    Messages:
    102
    Location:
    Melbourne
    fair point on the battery life. Voltage cutoff might be my next learning project.

    I've just used my dremel to cut the traces, just tested and it works fine. Also used the dremel to cut a neat hole under the dc socket for the battery wires to come out! All coming together now. Here's a picture: https://i.imgur.com/GL0FnDs.jpg

    I've just powered it on using the battery, played some music, then plugged in the unpowered dc jack - and voila! - it switches off!

    Seems to cut out a bit when I put it near max volume, perhaps there isn't enough current to power it from the battery.
     
  10. OP
    OP
    joomax

    joomax Member

    Joined:
    Nov 1, 2016
    Messages:
    102
    Location:
    Melbourne
    Hmm, looks like I messed something up.
    In an effort to fix the volume on the battery, I resoldered the wires to the PCB, and now it doesnt work...

    Red power LED comes on, but no sound. Still works fine with the DC Jack. I used some shitty chinese flux this time though which has caused a problem I think.

    Interestingly, it also doesn't work when I connect the battery to the pins the DC Jack uses...
     
  11. Technics

    Technics Member

    Joined:
    Apr 29, 2002
    Messages:
    1,507
    Location:
    Brisbane, AU
    Have you checked the battery voltage?
     
  12. OP
    OP
    joomax

    joomax Member

    Joined:
    Nov 1, 2016
    Messages:
    102
    Location:
    Melbourne
    Oh whoops, you could be on the money. My multimeter battery died, left it at home. Will try this afternoon with a new battery in it.
     
  13. OP
    OP
    joomax

    joomax Member

    Joined:
    Nov 1, 2016
    Messages:
    102
    Location:
    Melbourne
    Found a battery...
    Ah yeah shit it's just below 10... And quickly pops up to 11.45 after a 30second charge, and the charger led goes off indicating it's stopped charging. It should be reading like 14 right?

    Looks like I'm off to Jaycar!
     
  14. Technics

    Technics Member

    Joined:
    Apr 29, 2002
    Messages:
    1,507
    Location:
    Brisbane, AU
    Around 13.5V to 14.4V is typical depending in the exact chemistry and charger/mode. If it shoots up quickly it's a sure sign the internal resistance is high and the battery is toast.
     
  15. cvidler

    cvidler Member

    Joined:
    Jun 29, 2001
    Messages:
    11,927
    Location:
    Canberra
    lead acid batteries - unless specifically designed for deep cycle usage - are not meant to be run flat, they'll rapidly sulphate the plates and that's almost unrecoverable (especially on sealed/maintenance free batteries).

    Even if specifically deep-cycle, leaving them flat will kill them too.

    You'll find most discharge protection circuits for lead acid batteries have a cut off at somewhere between 11v and 11.5v. If you don't want to keep killing batteries (get's expensive fast), get/build a low voltage protection circuit for it too.
     
  16. OP
    OP
    joomax

    joomax Member

    Joined:
    Nov 1, 2016
    Messages:
    102
    Location:
    Melbourne
    Thanks for the insight here again!
    Went to Jaycar and they confirmed the battery was dead with a tester thing. Thank god, was doubting my soldering abilities for too long back there!

    All installed and wired up now, works a treat. I'm going to mount the AC adapter in it so I can plug it in through the case wall. Unfortunately there's no neat way to avoid having to open the case up to plug/unplug the dc jack when I want to do this. As such, I think I'll just leave the battery leads disconnected as a surety that it doesn't drain over time. Then I'll periodically check the voltage with the multi and charge it up again.

    I'll chuck up some pics when it's finished tomorrow :)
     
  17. cvidler

    cvidler Member

    Joined:
    Jun 29, 2001
    Messages:
    11,927
    Location:
    Canberra
    what's the specs on the plug pack and the battery you're using?
     
  18. Technics

    Technics Member

    Joined:
    Apr 29, 2002
    Messages:
    1,507
    Location:
    Brisbane, AU
    Just bear in mind a low voltage cutout module can be had for only a few bucks on eBay if you don't mind waiting for the slow boat from China. It never forgets to check the battery so it seems like cheap insurance. The default cutout for the module below seems to be 10.5V and you would probably want to wind that up to somewhere between 11.5 to 11.8V by adjusting the trimpot. Higher means it will run for less time on a charge but the battery will last for many more cycles.

    https://www.ebay.com.au/itm/Low-Vol...ecovery-Module-for-DC12V-Battery/183177972947
     
    Last edited: Jan 23, 2019
  19. OP
    OP
    joomax

    joomax Member

    Joined:
    Nov 1, 2016
    Messages:
    102
    Location:
    Melbourne
    Ah good one, just ordered. I think the final part of this project is to add bluetooth connectivity - will do some ebay investigating.

    Here's some pics if you're interested:
    https://imgur.com/a/JLUZI2l

    I'll install the voltage cutoff in that corner between the battery and amp.
     
    Technics likes this.

Share This Page