help powering a circuit

[warwatt]

I am trying to power three different devices in one circuit. They are a small audio amplifier rated 6 volts. Draws 70 ma peak. A usb recorder board 4.5 v draws <45 ma operating and a display motor with led and small lamp in parallel drawing <100 ma. I want to use two 4aa battery holders. I have this circuit wired and everything works fine but there is a very short life on the batteries. I was thinking of using one pack for the amp and the other pack would be used for the usb and motor using regulators. The usb board and amp would work together intermittently
I'm looking for at least 30 hours of battery life. Please note it is working now with a 4aa and 2aa battery packs. The 4aa pack powers the amp and usb. I'm wondering what is the best combination of battery packs.

[KMoffett]

Check this discharge rate for AA batteries:http://www.powerstream.com/z/AA-100mA.png
The AA batteries will never give you the requested 30 hours.
D-cell battery would be a better choice: http://ww2.duracell.com/media/en-US/pdf/gtcl/Product_Data_Sheet/NA_DATASHEETS/MN1300_US_CT.pdf

Ken

[warwatt]

Thank you for your reply. I was afraid of that. I don't have a covered battery box for the D batteries. What about NIMH batteries, I can get 2000mah batteries but I would be sacrificing some voltage. This is a table top display so I'm trying to avoid a wall transformer. Any other ideas?

[pebe]

You say you are using a 2AA and a 4AA battery pack. The 2AA will only give 3V; are you powering the USB board from this?

You may do better with one or two larger capacity lithium cells and a buck/boost converter. Can you say what the supply voltage tolerances are on the various components?

[warwatt]

I was using a 4AA and 2AA but I'm now using 2 4AA one for the amp and the other will share the usb and motor led combo. Unfortunately I don't have any specs on the amp it is a surplus item from ALLelectronics. It comes with a 4AA holder but some users (at web site) said it was under powered and they used it up to 9volts. I was sharing the 4AA batteries with the USB recorder (tapping the 3 batteries to get 4.5V)and I measured the voltage drop at around .5 volts. After only about 3 plays, the first battery voltage went down to .8 volts. The amp sound started to be static. The usb seems to tolerant of lower voltage but not of a higher voltage (chips will heat up). Before I used the amp and usb, I would run the motor (Hankscraft display 3v 20ma no load) and the led and the battery life (2AA) was exellent. You mentioned a buck/boost converter, I'm not familiar with that?

[pebe]

You said the amp was rated at 6V and required 70mA max. I cannot find it on the ALLelectronics site, but a consumption of less than half a watt seems a bit small.

The individual components seemed to work happily on separate supplies, yet the batteries failed after a few playings (a few minutes?). So something must be passing a heck of a lot of current. Could it be that combining the amp and the other circuits by feeding them from a single supply has created an extra load somehow? I think you need to check the current drawn from the 4.5V tapping and from the full 6V of the battery to see what the culprit is.

I don’t like the idea of tapping the third cell for a lower voltage takeoff, because the three cells will discharge faster than the fourth cell. A better method would be to provide a battery for the highest voltage (the amp) and use a buck converter to change that voltage to a lower one for the USB card/motor.

A buck converter is a switch mode device with an adjustable output voltage and is more efficient than a dropping resistor or series regulator. Converting 6V down to 3V with a buck converter is about 92% to 95% efficient, as against 50% of a series dropper. The converters are inexpensive and reliable.
http://www.ebay.co.uk/itm/DC-DC-Buck-Co ... 27e663ec1b

If you want to stay with alkaline cells then it’s best to use C types as Ken suggested. But if you want to use rechargeables, then I would suggest Li-ion 18650 cells rather than NiMH cells. An 18650 gives 3000mAh to 5000mAh at a nominal 3.7V (3.5V – 4v), which is more than 5 times the energy capacity of an AA NiMH which gives only 2000mAh at a nominal 1.35V.

So you could use two 18650s in series to power the amp, with a buck converter taking the voltage down to a suitable level for the USB/motor combo.

As you have had the motor working OK on 2AA, then if the USB will run on 3.5V, an alternative would be to use a single cell 18650 (or 2 in parallel for extra capacity) for the USB/motor and use a boost converter to raise its voltage high enough for the amp.
http://www.ebay.co.uk/itm/1Pcs-XL6009-D ... 3f389c4c9a

I hope that may be of help.

[warwatt]

Thanks for your reply Pebe. Sorry it took long for my response. I didn't receive an email and thought no one had responded. The amp is item AMP-5 on the website. Yes I think it was because it was supplying the AMP and USB on the same pack. I made a test setup and ran the AMP on one pack (4) 2000mah NIMH. Even with the batteries down to 5.1 volts the audio quality is still quite good. I ran the USB on the other pack with a diode in series to drop voltage to 4.9 v. I read that a .7 volt drop is expected from a 1N400x diode but I only got .36 volts. The USB works fine though, no heat on the chips and again audio quality is great. I haven't hooked up the gearmotor and LEDs yet but am not suspecting any surprises. I intend to use an LM317 regulator to drop the battery voltage from 5 volts to 3 volts. I know that a 2 volt differential is close. I also intend to go back to the original AMP and check that to make sure it matches the AMP in my test setup, it could be a faulty AMP in the first place. They ARE surplus. Anything you could help me with this would be much appreciated. These amp/usb's are going into music boxes that I have been making (the mechanical ones) for over 20 years. In past years copy writes have caught up with that industry (mechanical movements)and 80% of the songs are unavailable. I can create a wave file song (up to 5 minutes long) and write to the USB and play through the music box. Because of this I have to design this with at least 200 play times at 5 minutes per play. The lithium batteries have a great shelf life and power but they are not rechargeable. I have done another prototype using a 1.2 ah gel cell and supplied charger but that is expensive.
I do intend to look into the boost converter but price is the bottom line.

[pebe]

It would probably be best to delay your decision until you have tried the motor.

BTW, lithium non-rechargeable cells give 3V. The 18650 cells I mentioned are rechargeable and give 3.7V

[warwatt]

Yes you're probably right. I'm going to work on it this weekend. I purchased 10 of the Lm2596S buck converters on ebay. They were a great price and I got them in the USA.
They supposed to arrive DEC. 3rd. Thanks for the info. I will look at the lithium cells also for availability in the USA and a charger for them. As I said before, this will be the basic design for all my music boxes in the future. Some of my designs are complex enough to require an Arduino or similar device to control them. This is something else I will have to learn in the near future. I have purchased an Arduino starter kit but haven't had time to learn it yet. Thanks again Pebe

[pebe]

Great.

If you are going to use lithium cells you need to get the 'protected' variety (to prevent overdischarge) or use them with a protection pcb.

Sorry I cannot help you with the Arduino. I use PICs.

[warwatt]

hi Pebe,
I didn't get the shipment of buck converters (they went USPS , very unreliable)
I'm forced to use 2 voltage dividers to drop it for my motor and USB. Not the greatest but it will work. My customer wants it by Monday the 8th. My fault for selling an R&D
item. My plan is to convert 3 more music boxes and test them till I'm satisfied with the
design then swap one of them for the my customer's original prototype. I would also like to create my own tactile switch overlay for the panel. The amplifier control panel is that design so all I would have to do is add that board to another board with my switches on it. My first trial would be to reverse print on a mylar transparency then back that up with another clear mylar panel. I'm now using those small push buttons that look cheap and I already had one fail testing the panel.(see picture). Well that's my plan but I wanted to thank you for all your help. You put me in touch with the buck converters that I didn't know existed.