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I'm new to this and i'm doing my first real build for a friend. What I got so far:

  • Amp: ZK-TB21 Here
  • BMS 3s 40A balance Here
  • 18 18650 battery on 3S pack
  • 2 x 25W generic speaker and 1x30w tangband mini-sub
  • One on/off switch
  • One 12V charge indicator
  • One 12V 5A DC charger

I've got an issue where i want to be able to charge the battery pack while using the amp.

The amp got 2 power source in, so I figure I would connect one directly to the DC in port, and the other one to the battery pack but I'm not good enought in schematic to figure out where I need to put my on/off switch to cut off power delivery from the battery to the amp without preventing the charge of the battery pack.

I do not have any schematic of what I got so far, but if needed, I'll do my worst on paint to explain it. Don't hesitate to question me and thanks for the help !

Thanks for the answers: here is a pic of the amp :amp

Left one is DC in, right one is just 2 pins: positiv and negativ

So there is two power input possible.

Thanks again for the answer. That was my guess too (because Aliexpress doesn't send the manuel with the amp ) but here's my problem: Schematic

Here's an oversimplified schematic of what i'm trying to do with the dc barrel plug in a 12v 5a power supply.

How can i make sure that the amp is using the power supply and not the battery pack ? And will the battery pack charge ? I'm really not sure of what i'm doing.

If this too much hassel, i'll just put two DC female power in: one for charging, one for using directly on the amp. But i would like to have the option to charge while i'm using it.

EDIT: Well, this is really helpful, thanks everyone for your answers and your help. I'm gonna go to a spare part electronic store near my place to ask for these.

I just have one question left:

Is my 12v 5a power supply enough to charge the battery bank and power the amp at the same time or will it blow ?

And

I thought BMS were charger module too, am i wrong ? Do i need something else to charge my diy battery pack ?

Once again, thanks everyone, i'm learning a lot.

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    \$\begingroup\$ "The amp got 2 power source in". You're going to have to explain that a lot better. \$\endgroup\$
    – Finbarr
    Commented Apr 13, 2021 at 14:24
  • \$\begingroup\$ From the picture on the AliExpress page, the amplifier only has one power input which can accept 12 to 24 volts. \$\endgroup\$ Commented Apr 13, 2021 at 15:38
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    \$\begingroup\$ It appears that you can use either the coaxial power jack or the screw terminals - I would expect those two power inputs to simply be connected in parallel. This should be documented in the user manual for the amplifier. \$\endgroup\$ Commented Apr 13, 2021 at 19:10

2 Answers 2

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I'll assume both input ports are connected in parallel (at least that's what makes sense to me), so that you can input power in any of those indifferently - If they have a warning saying you should not connect a power supply in both at the same time, then that's definitely the case.

About the charging and powering up at the same time, you should be looking at a PMOS transistor. Look up some power transistors. Also a Schottky diode.

As for the circuit to allow charging and powering up the circuit at the same time, here's a simplistic sketch: enter image description here

When you have 12V at the input, the battery charger will charge the battery, and the PMOS will not allow current to flow from the battery to the circuit, so the amplifier will be powered by the 12V directly. When you disconnect the voltage from the 12V input, the current will flow through the transistor's inner diode.

This allows you to power your amplifier and charge the battery at the same time when you connect the 12V at a single input.

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  • \$\begingroup\$ Well, this is clearly beyond my skills. I'll look into that in the morning. I've got some old electronics that i can scavandge for pieces, pretty sure there are some mosfet on them. Should i know something important about mosfet ? Like power rating ? Do they all do the same and i can use any of them ? Thanks a lot for the help, i'll do my best to be accurate, and maybe i'll do the complete schematic of the project. it might help \$\endgroup\$
    – Flexi
    Commented Apr 13, 2021 at 22:44
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    \$\begingroup\$ Adding a large resistor (say, 10 Mohm) between Vin and ground will ensure that the PFET gate gets pulled down when 12V is disconnected. This will ensure the PFET turns on when the battery supplies current, rather than dropping voltage across the body diode. \$\endgroup\$ Commented Apr 13, 2021 at 23:00
  • \$\begingroup\$ There are several types of transistors, in this case you need a MOSFET P-type transistor. A N-type will not work (you can even blow up something). And bipolar transistors will not work either. A PMOS type with that diode in series is required. Be sure to check datasheets of the components you find and check if the symbol is similar to the one in my previous sketch. Also, be sure that the power rating is large enough, for the transistor and the diode, look for something above 100W. Be careful since the wrong type of transistor can end up blowing your amplifier or your battery, or both! \$\endgroup\$ Commented Apr 13, 2021 at 23:01
  • \$\begingroup\$ Yes, also see previous comment from FrontRanger, is absolute value. I focused on the essential and forgot about that, but that resistor is a must! \$\endgroup\$ Commented Apr 13, 2021 at 23:03
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    \$\begingroup\$ @AntennaGuy, right, you need it, or else you might as well just use a diode instead of the PFET. I figured you meant to have it. Another potentially useful mod is adding a second PFET in series with the first, so that the sources are joined. This provides back-to-back diode isolation, which would be helpful for the event that the battery charger has boost capability, charging the battery above Vin. \$\endgroup\$ Commented Apr 13, 2021 at 23:41
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You have an extremely cheap "BMS" but you are missing a charger circuit. With a 12V supply, the amplifier produces stereo 12W per channel into 4 ohm speakers or 8W per channel into 8 ohm speakers. The subwoofer output can drive one 2 ohm speaker to 28W or two 4 ohm speakers to 12W each.

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  • \$\begingroup\$ I dont get it. I thought BMS were charging module too. What do i need to be able to charge my 3s18p battery ? I've look everywhere online, but a lot of information isn't for novice: i didn't do any electrical engineering studies so i just don't get it. \$\endgroup\$
    – Flexi
    Commented Apr 20, 2021 at 3:30
  • \$\begingroup\$ Is the extremely cheap no-name-brand BMS any good? Does it work properly? its instructions say the battery cells must match then its balancing is poor. How will you limit the 40A of charging current to safely limit the current for each of your many parallel cells? Why do you need 18 cells in parallel for your low amplifier power since the power supply voltage is so low? \$\endgroup\$
    – Audioguru
    Commented Apr 21, 2021 at 13:32

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