LM7805 & batteries charging batteries question

[Jack.Straw]

Hello. I've been building fun little LM386 amplifiers. They are powered by 6x AA NiMH rechargeable batteries, with an internal MAX713 IC charging circuit built in. Recently I decided i wanted to add bluetooth capability. I found some inexpensive options on ebay and have ordered a couple different ones. Both options are powered by 5 volts USB, but one of them has a built-in rechargeable battery so that it doesn't have to be plugged in all the time. Here is an overview of how I was thinking of implementing the bluetooth add-on:

The two bluetooth modules i have already ordered are:
**broken link removed**
**broken link removed**

And finally, my questions:

1) The LM7805 says it requires 7 volts input. I know my 6x AA rechargeables only supply 7.2 volts fully charged... what happens as the batteries start to loose charge?

2) Which bluetooth option would conserve overall battery life? I feel like the battery powered option might be it, but then we enter into a batteries charging batteries situation, which I know nothing about. Currently the 6x AA batteries last a really long time powering the amplifier alone. How much impact do you think the bluetooth will have on battery life? I could add a separate on/off switch for the bluetooth option, but would really prefer it to share the amplifer's on/off switch since i think i will be using the bluetooth option a majority of the time.

Thanks in advance for any advice you can offer!
-Scott

 

[tpetar]

Re: LM7805 & batteries charging batteries question

LDO voltage regulator will be good if you have smaller difference between Vin and Vout.

To prolong battery usage consider to use bluetooth modules which have good range with low power.

Plus on this that amplifier and rest circuit will make additional battery voltage drop.


Best regards,
Peter

;-)

 

[Jack.Straw]

Re: LM7805 & batteries charging batteries question

Tpetar, thanks for the response. Unfortunately i have no more knowledge than i did before... please elaborate:

LDO voltage regulator will be good if you have smaller difference between Vin and Vout.

Are you saying that the LDO is not a good option for my situation? Is there a better option?

To prolong battery usage consider to use bluetooth modules which have good range with low power.

The modules are what they are, and none of them list power usage... only volts required. Both of them have a 10 meter range. Would the extra battery included with the first bluetooth option improve overall battery life of the amp?

Plus on this that amplifier and rest circuit will make additional battery voltage drop.

Good to know. So you're saying the pull of the amplifier circuit will reduce the voltage below the 7 volts needed for the LDO? Is there a better for me to supply the bluetooth module with 5v from the existing 7.2v power supply?

Thanks again,
Scott

- - - Updated - - -

Maybe i'll just look into using 8 AA NiMH batteries instead of 6? that might be tricky for my situation, but it might be do-able...

 

[tpetar]

Re: LM7805 & batteries charging batteries question

Tpetar, thanks for the response. Unfortunately i have no more knowledge than i did before... please elaborate:

Are you saying that the LDO is not a good option for my situation? Is there a better option?

LDO regulator will be better for your situation. This type of regulator can have very small voltage difference 0,1V-0,3V between input and output voltage. For 7805 is minimum 2,5V-3V. Plus LDO usually have lower quiescent current (can be few uA) then 7805 which have 3-5mA.

The modules are what they are, and none of them list power usage... only volts required. Both of them have a 10 meter range. Would the extra battery included with the first bluetooth option improve overall battery life of the amp?

Maybe you get good modules. If you raise voltage with extra batteries you will have longer stable voltage, but used current will be the same.

Good to know. So you're saying the pull of the amplifier circuit will reduce the voltage below the 7 volts needed for the LDO? Is there a better for me to supply the bluetooth module with 5v from the existing 7.2v power supply?

Usually when batteries are used voltage goes down under pressure of electrical load. To avoid bigger voltage drop use bigger batteries (bank or pack) with higher battery capacity. Better check specification of your bluetooth module to see allowed voltage range.

Maybe i'll just look into using 8 AA NiMH batteries instead of 6? that might be tricky for my situation, but it might be do-able...

Yes, this can be good. But for final conclusion do the real tests of usage.




For other additional thinking, you can use voltage booster to stepup voltage from Li-Ion/Po or few NiMh to 5V. I use often Li-Po phone and Li-Ion 18650 batteries with small compact stepup PCB to power device.



Best regards,
Peter

;-)

 

[kam1787]

Re: LM7805 & batteries charging batteries question

LDO voltage regulator will be good if you have smaller difference between Vin and Vout.

depending on the current drain, i would use a simple resistor and zener diode or series pass regulator!

why bother with an IC? does it really need such exact regulation?!

 

[tpetar]

Yes that is true. Zener with resistor will do the job for smaller power needs, and if this switch from circuit is used.

I personally do not rely on zener for this purposes, I never use zeners for this role, except for ADC input or similar situations for signal leveling.


Best regards,
Peter

;-)

 

[Jack.Straw]

Thanks for the input guys. I started building the new charger to support 8 batteries. I think the LM386 amp will be ok with the 9.6v. Is there a better amp chip out there that works at this voltage that might be a little louder?

I already have a LM7805 & heat sink, so i'll probably stick with that unless you see a problem there. I'm not familiar with using zener diods for this purpose. I'm certainly willing to give that a shot if you think there is some significant advantage to it though..

Thanks again,
-Scott

 

[kam1787]

9.6V will not give you much audio power in a conventional class A or AB amp. You could bridge the amp, or you could go to class D.

As others have stated the 7805 needs a "high" voltage in. It MIGHT be unstable or not regulate. If you have problems with the Bluetooth as the batteries are draining [not fully charged], I would suspect the 7805 first.

How does current does the bluetooth receiver need?

 

[Jack.Straw]

Thanks Kam. I started looking at class D amps on ebay to save myself some trouble. A lot of them are 7.5v-15v, optimal at 12v. I'm considering going with a battery pack of 10 batteries (instead of 8) for 12 volts (10x 1.2v NiMH 2000mAh). If i went with a 12v class D option, would 12v be enough to assure no problems with the 7805? Neither of the bluetooth receivers that i purchased show how much current is needed. They simply say 5v, usb powered.

 

[tpetar]

In that case its not good to use linear regulator, you will have high loss on 7805. If you increase input voltage you need switcher power supply.


Best regards,
Peter

 

[Jack.Straw]

Peter, thanks. Would this **broken link removed** be acceptable? It's 1A, which I would guess is more than enough for the bluetooth module?

 

[tpetar]

Peter, thanks. Would this **broken link removed** be acceptable? It's 1A, which I would guess is more than enough for the bluetooth module?

Yes that will be enough. TO220 Its little large with dimesion, you can also look regulators in SO8 case.


Best regards,
Peter

:wink:

 

[kam1787]

Neither of the bluetooth receivers that i purchased show how much current is needed. They simply say 5v, usb powered.

The normal convention for USB is 100mA per device as a maximum................

7805 at 12V in [assuming max of 100mA] would have to dissipate 0.7W - not a problem, assuming you have a good sink - hint use thermal paste

5V differential is fine for the 7805 as a minimum of 2.5V to 3V is recommended

 

[tpetar]

The normal convention for USB is 100mA per device as a maximum................

7805 at 12V in [assuming max of 100mA] would have to dissipate 0.7W - not a problem, assuming you have a good sink - hint use thermal paste

5V differential is fine for the 7805 as a minimum of 2.5V to 3V is recommended

USB port can supply more current then 100mA. I suggest reading of this thread or reading newer date spec updates :

https://www.edaboard.com/threads/292882/



With what current you plan to charge battery pack ? 100mA ? Even USB keyboard drain more current from USB port.





If batteries are used its good to save any amount of power what we can save.


Best regards,
Peter

:wink:

 

[Jack.Straw]

With what current you plan to charge battery pack ? 100mA ? Even USB keyboard drain more current from USB port.

During fast-charge, the current for charging the 2000mAh batteries will be between 750mA and 1000mA, depending on the power source i choose. I have done 1A in the past for the same batteries. I'll probably end up with a 1A power supply, which means I'll probably drop it down to a 750mA charge so as not to max the supply.

If batteries are used its good to save any amount of power what we can save.

Amen to that! Thanks for all the advice guys, i really appreciate it.

 

[Audioguru]

An LM386 amplifier has low output power so it uses low battery current. Its datasheet shows that it clips at an output power of only 288mW into 8 ohms when it has a 7.2V supply.
The datasheet shows that it is only 50% efficient then it draws a total of 288mW x 2= 576mW from the 7.2V battery.
Then the maximum battery current is only 576mW/7.2V= 80mA which is very low for an AA Ni-MH battery and its voltage will not drop until it is almost dead.
Then there is plenty of voltage for a 7805 regulator for the Bluetooth receiver. Maybe the current for the receiver is low enough that you can use a little uA78L05 regulator.

 

[kam1787]

USB port can supply more current then 100mA. I suggest reading of this thread or reading newer date spec updates :

Even USB keyboard drain more current [100mA] from USB port.

The USB specifications, per 2009, are quite clear.

While some devices do supply current higher than the specifications, I suggest it is very poor engineering practise to rely on that. To suggest that one can expect higher than the specs on all devices is not responsible whatsoever.

Are you stating, without qualification or conditions, that ALL USB ports can supply over 100mA? perhaps you can tell us, exactly what current we can draw from ALL USB ports?




and BTW most USB keyboards consume around 60mA, and not over the 100mA as was so distinctly implied. It is a keyboard, and not a hand-warmer.

 

[tpetar]

The USB specifications, per 2009, are quite clear.

While some devices do supply current higher than the specifications, I suggest it is very poor engineering practise to rely on that. To suggest that one can expect higher than the specs on all devices is not responsible whatsoever.

Are you stating, without qualification or conditions, that ALL USB ports can supply over 100mA? perhaps you can tell us, exactly what current we can draw from ALL USB ports?


Most people are honest, so you would let anyone hold your wallet?


and BTW most USB keyboards consume around 60mA, and not over the 100mA as was so distinctly implied. It is a keyboared, and not a hand-warmer.

I suggest reading thread what I mentioned in post #14:

Post #10 in this thread:
https://www.edaboard.com/threads/292882/


USB Power Delivery
https://www.usb.org/developers/powerdelivery/

Power Delivery Specification Rev. 1.0 ver 1.2






Best regards,
Peter

:wink: