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CMOY Rechargeable Solution

post #1 of 10
Thread Starter 
Is there any cheap and efficient way of making a rechargeable CMOY. My problem is, running off of 9V can be especially costly (at 5$ per battery, cdn). I do have many NiMH 1.5V battery, but I'm not sure how many of these in series I would need to properly run my amp. I'm using the OPA2107 opamp with 32ohm phones. Also, my source is an iPod.

Are there any good writeups out there for this kind of solution, or does anyone have any suggestions?
post #2 of 10
Quote:
Originally Posted by ex0du5
Is there any cheap and efficient way of making a rechargeable CMOY. My problem is, running off of 9V can be especially costly (at 5$ per battery, cdn). I do have many NiMH 1.5V battery, but I'm not sure how many of these in series I would need to properly run my amp. I'm using the OPA2107 opamp with 32ohm phones. Also, my source is an iPod.

Are there any good writeups out there for this kind of solution, or does anyone have any suggestions?
There is a pretty easy trickle charger using a LM317, a resistor, and two diodes, that was done in the PINT and some homemade chargers. You should even be able to get parts at radio shack. Tomb's post in this thread has a really nice and easy layout.

Most people have theirs set up for charging a pair of 9V NiMH batteries. I made one to charge AAA cells, I needed more battery life than a 9V has. Using different capacity battery means changing the resistor value.

From Tangent's PINT writeup:

Quote:
This resistor sets the current used by the NiMH battery charging circuit.

The formula for computing the proper resistor value is R=1.25÷I, where I is the desired charging current. I recommend that you use the standard trickle charge rate: 0.1 times your battery's mAh rating. (The notation for that is "0.1C".) Since the default resistor value gives a charging current of about 17 mA, it's suitable for NiMHs of 170 mAh or higher.

Charging current is a trade-off between charge time and battery cycle life. You can choose to charge faster than 0.1C, but your battery will become useless after fewer charges than if you didn't abuse it so. If you charge much greater than 0.1C, you must use some sort of intelligent charger, which the PINT does not offer. Contrariwise, you can charge slower than 0.1C to get more cycle life from your battery, but I wouldn't go lower than 0.05C. Below that, the battery's self-discharge rate becomes too significant: the battery won't charge as fast as it "should", and it may actually fail to charge.
Since my AAA cells were 700mAh, the resistor value needed to be different than the one used for a 9V. I calculated the value I'd need like this: R = 1.25 ÷ ( Battery mAh rating ÷ 10 ). I then picked a resistor as close as I could.

If you take 8 of your batteries (I'm guessing they are probably AA sized since it is the most common) and connect then in series, you'll have about the equivalent of a 9V battery and it will last a lot longer. The only thing is, they take up a ton of room.
post #3 of 10
The downsiede of this is the internal resistance of the cells which would be added in series... haveing a damping effect on current delivery... unless I got it totaly wrong.

Lets say in theory you had 2 2V Cells each capable of delivering 500mA (i.e. 10Ah) lets say they had a fictitious internal resistance of 4 ohms each.

In series you have 4V over 8 ohms which leaves you with the original current power of one cell...
post #4 of 10
Quote:
Originally Posted by Nordic
The downsiede of this is the internal resistance of the cells which would be added in series... haveing a damping effect on current delivery... unless I got it totaly wrong.

Lets say in theory you had 2 2V Cells each capable of delivering 500mA (i.e. 10Ah) lets say they had a fictitious internal resistance of 4 ohms each.

In series you have 4V over 8 ohms which leaves you with the original current power of one cell...
If I understand what you are trying to say, then you have this completely wrong. A 9v battery is made up of several smaller cells, so a NiMH one is between 6 (7.2v) to 8 (9.6v) individual cells, wired in series. Making your own pack works the same way, but they arnt prepackaged. Also, 4 ohm internal resistance is way too high, series wiring is to increase voltage, not current.
post #5 of 10
Thread Starter 
how many volts are actually needed to run the amp?
and will the LED matter....my LED is a 3V -_-
If i could get away with 4 AA or AAA batteries that would be pretty sweet.

Oh, I also wanted to ask...does the quality of the sound generally increase by increasing the gain?
I'm currently at gain 2, since it's more than enough volume than I need, with little noise. Is there any point to moving to gain 3 or higher?
post #6 of 10
That opamp needs about 10 volts if I remember correctly.
post #7 of 10
I know the internal resistance is very low... fractions of an ohm...
But still isn't this why the rateing on a 9V baterry is alot less than say 1 1.5V AAA... I just always thought If I put 2 equal cells in parallel, I keep the same voltage and double the current, and in series I get twice the voltage but only the current of 1 cell
post #8 of 10
Thread Starter 
Nordic, I think you are correct.

You can only have 1 independant current source in series, and only 1 independant voltage source in parallel. Therefore in series, your batteries should be acting as independant voltage sources, where Vtotal = V1 + V2. Same thing in parallel, but your batteries are acting as independant current sources where Atotal = A1 + A2.

Now where people were saying that batteries in series will have decreased current due to the internal resistance, this may be true (I have no idea as to the value of the resistance of the batteries) but it really shouldn't matter since the CMOY, I think, only draws a small portion of the current of a 9V battery, and I think AA or AAA batteries already have higher current ratings than 9V do.


Now...I don't know how to figure out how much voltage the opamp requires, the only thing I've used opamps for in class so far is generating differently shaped wavelengths (viewed on an oscilloscope). My amp seems to run perfectly fine with a single 9V battery, and I can't say I notice a difference by adding a second 9V in series.
post #9 of 10
The TI website show the Vs(Min) for an opa2107 as 9V. Will it run a little lower? Probably. To roughly figure out where your minimum acceptable voltage is, measure the voltage, with the amplifier turned on, at the battery terminals when the 9v is almost dead.

NiMH batteries are 1.2v per cell (nominal). What this means is that in calculations, figure each NiMH cell as 1.2v. In real life, with a trickle charger, my batteries charge out to right around 1.35-1.4v per cell. 1.5v NiMH batteries are single NiMH cells and to get the equivalent of a 9v battery you will need 7 (8.4v) or 8 (9.6v) 1.5v (AAA, AA, C, or D) batteries in series. I find that 7 cells in a trickle charger (.1C) charge out to right around 9.45v and 8 cells charge out to right around 11.1v.

Quote:
But still isn't this why the rateing on a 9V baterry is alot less than say 1 1.5V AAA
I am not sure what you are talking about here. If you are talking about absolute current output capability, we really aren't concerned with that at all as most of these designs don't even come close to those rates. If you are talking about the mAh rating of a 9v vs 1.5v cell, this is based on size of the cell. That whole AA battery is only 1 large NiMH cell where 7 or 8 NiMH cells must be packed into a 9v battery case. More physical quantity of reactants allow for more energy storage.
post #10 of 10
With that opamp, you are gonna need a decent amount more than 9 volts to get any usage out of it. If your batteries top out at 9.2v, its not gonna stay at 9.2v for very long. It starts dropping with usage. The amp apparently is listed for a minimum of 9v, which in a CMOY, you will most likely notice lack of power issues well before a design with a buffer, as the opamp is actually driving the load.

I recommend swapping to a AD8620 opamp or one of the low voltage LM's and using a 9v of any type. A normal 9v Energizer or Duracell battery will run you almost 100 hours like this.
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