auto switch circuit design

[Dr_Evil]

I'm working on another project. Basically an ammo can "boom box" similar to this.

Ammo Box Speakers

I am wanting to have it auto switch between battery and outlet power without a delay. I know using a power adapter similar to that of one used for a laptop might cause a slight delay as capacitors discharge. I came up with this and wondered if it might work or is possible. The amp and battery are both 12v.

 

[Cyparagon]

Why not schottky diodes? One on either source, both sources connected to load in parallel.

"no delay" is not possible. Even the fastest electronic switching takes several nanoseconds or microseconds. You'll have to clarify your requirement here.

 

[Dr_Evil]

Nano or microseconds is short enough for me to consider it no delay. Will those diodes eat up power from a battery though? I want the simplest circuit possible that will not cause additional drain on the battery. I was thinking a relay that has a closed circuit while unpowered will produce the lowest drain on the battery.

I realised that drawing above will produce a small noticeable delay due to the capacitors charging or discharging. I'd need one that runs off of 12v to energize the coil.

Another thing I need to mention is that I want the battery to charge while plugged into the mains. I plan on using one of these to monitor battery voltage.

https://www.amazon.com/gp/product/B00CJRE1PM/ref=ox_sc_act_title_5?ie=UTF8&psc=1&smid=AFHAE9RJVUMB

 

[lasersbee]

What you want to do is run your circuit from the battery and
the use the AC adapter's dc output to charge the battery
through an in line Schottky diode to prevent the battery
supplying power to the AC adapter like Cyp suggested.
This way there is no switching time involved and you don't
need a mechanical relay that actually will take time to switch.

Keeping it simple is the way to go.... IMO


Jerry

 

[Benm]

I'd agree on that: just make a permanent connection from battery to amplifier, and connect the charging circuit to the battery.

As long as the charger provides more current than the amplifier draws it will charge the battery.

Since you mention 12 volts i assume this is a 6 cell lead acid battery, and the only precaution you need to take is that the charger limits its output voltage to 14 volts or so.

If the battery is lithium you may need a different approach as you don't really want to keep those at a float voltage forever when the charger is connected.

 

[Dr_Evil]

This is one of the possible chargers I'm looking at. The OD and ID needs to be 5.5mm and 2.5mm. This one says the ID is 2.1 though. I've seen some that say 2.5/2.1. I figured 6 to 7 amp should be enough

https://www.amazon.com/gp/product/B00VA2K0MK/ref=ox_sc_act_title_2?ie=UTF8&psc=1&smid=AYOMIWFVJV81A

This is the amp I bought.

https://www.amazon.com/gp/product/B006AMF2R8/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1

Battery I'm looking at.

https://www.amazon.com/Tempest-TR9-...ie=UTF8&qid=1480148510&sr=1-1&keywords=tr9-12

 

[Astralist]

This should works as per OP requirement of switching, the only thing is that it can't charge the battery simultaneously:

You "can" connect the ac adapter to the battery and run the system, but remember that the battery has its own charging cycle, especially those lead acid battery.
Additionally, you need about 14.1V to charge those type of battery (you cannot charge those 12V battery with 12V adapter), but the amp system itself only needs a 12V (as you said).
Knowing that, the complete and safe circuit for this requirement are a bit more complicated then. It will requires some DC-DC converter and additional safety measure.

 

[Benm]

That would work indeed, but there is no problem leaving a lead acid battery charging for a long time as long as the voltage isn't too high.

Something like that sealed lead acid battery could be kept at 13.7 volts indefinitely. This is actually how they are used in many applications serving as backup batteries for alarm systems, emergency lighting and such.

One downside is that lead acid batteries generally don't respond well to being discharged deeply, unless designed for that (traction batteries and such). I think the one in the link is a typical lead acid intended to be kept full unless there is a power cut or something like that, and may not fare that well if you discharge it deeply many times.

In cars batteries are usually charged with a bit more voltage, something in the order of 14.4 volts. This is useful because they charge more quickly, but could reduce their lifetime somewhat if charged all the time. In a car this is less of a problem: most cars don't really run more than an hour a day, and there also is some intelligence in the alternator regulation circuitry these days.

 

[Dr_Evil]

What about using something like this and come up with a way to charge the battery while only providing 12v to the amp?

https://www.amazon.com/UpBright-Ada...68127&sr=8-1&keywords=14v+5.5mm+power+adapter

 

[Cyparagon]

The amp will work at 14V just fine.

You can't use a 12V power supply to charge a 12V SLA for two reasons. First, the voltage isn't high enough. You need about 13.8V ideally. Also, it isn't current limited, so the battery will shut down the power supply on overcurrent. You need a dedicated charger, or charger circuit, which provides 13.8V with current limit.

 

[Benm]

You do need some type of current limiting indeed, not to protect the battery, but to protect the charging circuit.

In old school battery chargers this was done in the transformer: it was simply short-circuit proof in the sense that shorting out the output caused the magnetic flux in the core to be the limiting factor in current, resulting in no damage to the transformer nor shorting out mains breakers if you did.

Modern toroid core transformers do not have this 'feature', and switchmode supplies certainly need current limiting as well.

To the battery it doesn't matter though, you can connect that to a very hard 13.7 volt supply. The charge rate would be limited by the internal resistance, and gas formation around the electrodes in severe situations (completely drained battery on a very high current capable supply).

Lead acid batteries are tanks when it comes to charging, as long as you don't go over 13.7 volts for a 6 cell model no bad will come to it.

Their downside is when actually using them to supply power: the number of charge cycles is very limited, and deep discharges usually damage them unless made for that purpose.

Car batteries are a prime example of this: you can start your car several thousands of times from the battery, but leave some lights on that result in the thing going flat and it will fail on you.

 

[Dr_Evil]

I decided to go with a 12v 10Ah battery. After looking around at chargers more, I decided to find one made for SLA batteries. This is the one I plan on getting.

12v 8amp BC-12-8000F***Tempest Smart Battery Charger $39.00

You can pick which type of leads you want. I'll probably go with the one the uses spades for the battery connection and has an additional lead coming out of the spades for whatever it is being connected to.

 

[lasersbee]

You can use that 14VDC power supply with a high current Schottky
Diode (to stop the battery voltage getting into the Power Supply and
a 12VDC car (filament type) brake light bulb as a current limiter in all
in parallel connected to the 12VDC battery which will run your load.
If the current to the battery is too high just add another brake light.
The light will get dimmer as the battery gets charged fuller drawing
less current from the power supply.
I still use this system to maintain charge on bike batteries over the
winter months.

Or you can buy a battery Trickle charger like this......
MotoMaster Eliminator Intelligent Battery Cha | Canadian Tire


Jerry