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Old 14th November 2013, 03:59 PM   #1
Jay517 is offline Jay517  Ireland
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Default Reacheargeable CMoy Amp??

Hi

So I have built my first electronics project ... a CMoy Headphone Amp. Sounds great, but I would like to be able to charge the 2x 9V NIMH recheargeable batteries while they are in the amp.

Can anyone suggest how to do this or point me in the right direction?
Thanks.
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Old 14th November 2013, 07:51 PM   #2
agdr is offline agdr  United States
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Here is a circuit that would do the job.

When the DPDT switch is in one position, the non-grounded end (middle connection) of the two two batteries feed their respective CMOY power rails, just as the CMOY normally does.

But with the switch in the other position the negative end of the bottom battery is cut off from the negative CMOY power rail. The positive end of the top battery is cut off from the positive CMOY power rail and instead connected to the charger circuit. This effectively unhooks the two batteries from the CMOY and puts them in series with the charger circuit.

The charger circuit works by putting the 24Vdc from the wall wart across the two "9V" batteries, in series with a 470 ohm 2W resistor, which trickle charges them. Can take a long time this way to charge, like 8 hours or so (best to do it overnight).

The jack is a standard 2.1mm center-pin 5.5mm barrel DC power jack. The schottky diode is there in case someone accidently plugs in a tip-negative 24Vdc adaptor rather than tip positive. The wall wart adapter must be rated at 24Vdc - no higher or lower - and rated at least 100mA DCoutput current. And it must be tip-positive or the diode will block it.

The resistor limits the charging current for trickle charging. In sizing that I've essentially "stolen" the specifications that NwAvGuy built into the dual 9V NiMH charger in his O2 headphone amp. That amp uses 220 ohm resistors on each battery for a 7mA charging current when the batteries are fully charged, which is 1.45V per cell times 7 cells for an 8.4V "9V" NiMH cell. So here the math would be 24Vdc from the wall wart, minus 0.3Vdc for the schottky diode, minus two batteries fully charged at 10.15Vdc each across the resistor. That gives a resistor of (24 - 0.3 - 10.15 - 10.15) / 0.007A = 485 ohms. 470 is the closest standard 5% value.

Note than a NiMH "9V" cell really does charge up to around 10.15Vdc. The nominal voltage (voltage while being used) is around 1.2V a cell, or 7 * 1.2 = 8.4Vdc, which is what is usually printed on the battery. The discharged voltage shouldn't go much lower than 1Vdc per cell, or 7 * 1.0Vdc = 7Vdc. There is such a thing as an 8 cell NiMH "9V" cell, which charges to 8 * 1.45Vdc = 11.6Vdc. Don't use those here. They are harder to find anyway.

The resistor must be rated at least 1.5W, with 2 watts being more standard. You could also use two 1W resistors in series that add up to around 480 ohms, like 200 ohms + 270 ohms. The resistor won't dissipate anywhere near 2W in normal usage. The problem comes if one or both of the batteires dead short, then the resistor is right across the 24V. And yes, that most definitely does happen. In the last batch of 8 brand new Tenergy 8.4Vdc cells I bought one was dead shorted right out of the package. A 2W resistor can dissipate all the heat in a dead short situation.

The switch is absolutely required. The switch makes sure the CMOY's innards are disconnected during the charging process, so that shorted batteries, uneven charging between the two batteries, and loose battery clip contacts don't have any effect on the CMOY. The switch should be the "non-shorting" type, also called "break before make".

The big problem you will have with this circuit is size! It won't fit in your Altoids tin for the CMOY. It probably would fit in another, seperate, Altoids tin though, especially if you use the two 1 watt resistors in series. The switch, a miniature DPDT toggle switch, would have to fit and be mounted in the CMOY's tin somewhere/somehow. Then the problem is how to run and connect the two wires from the charging circuit tin to the connections on the switch and negative bottom battery lead. You could actually use a sub-minature 2.5mm TRS jack mounted on the CMOY tin somewhere and a matching 2.5mm plug on the charger wires. The TRS jack would short the charger wires for an instant while being inserted into the jack, but keep in mind your charger has that 470 ohm resistor to limit current in exactly that situation.

The png image and PDF file below are the same thing, but you can zoom the image up as much as you want in the PDF to make it clearer.
Attached Images
File Type: png CMOY charging.PNG (9.3 KB, 147 views)
Attached Files
File Type: pdf CMOY charging.pdf (38.3 KB, 15 views)

Last edited by agdr; 14th November 2013 at 08:21 PM.
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Old 14th November 2013, 08:15 PM   #3
Jay517 is offline Jay517  Ireland
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Thanks for the detailed response agdr.

I am using an enclosure bigger than an Altoids tin, so there could be enough room for that circuit.

Are there any issues leaving the charger plugged in longer than the recharge time of the batteries? Can it damage the batteries in otherwords to leave it charging for to long?
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Old 14th November 2013, 09:44 PM   #4
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Better to copy the charging circuit from the O2 lock, stock and barrel, since you can run the amp while charging:-
https://docs.google.com/viewer?a=v&p...jY2Y4&hl=en_US

Nothing wrong with copying a circuit, especially one that's in the public domain like this one...
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Old 14th November 2013, 10:03 PM   #5
Jay517 is offline Jay517  Ireland
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Thanks for the reply.

Quote:
Better to copy the charging circuit from the O2 lock, stock and barrel, since you can run the amp while charging:-
https://docs.google.com/viewer?a=v&p...jY2Y4&hl=en_US

Nothing wrong with copying a circuit, especially one that's in the public domain like this one...
Hmmm... looks a bit more complicated. I will have give it some study.

I will be using two 9 Volt Nimh batteries. Can I copy the circuit exactly or will it need to be modified in any way?
Sorry for the newbie questions.
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Old 14th November 2013, 10:23 PM   #6
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You can use the circuit exactly as shown, the section above the text - Power Management - Required for AC & Battery Operation - Do Not Omit. The 2 wires bringing power from that section run above and below the text I quoted. You might want to include C15, 0.22uF.

Be aware that this arrangement requires an AC wallwart between 12VAC and 20VAC, not a DC one.
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Old 14th November 2013, 10:36 PM   #7
Jay517 is offline Jay517  Ireland
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Quote:
Be aware that this arrangement requires an AC wallwart between 12VAC and 20VAC, not a DC one.
So it doesn't matter that the power supply is AC and the CMoy is DC?

Quote:
The 2 wires bringing power from that section run above and below the text I quoted. You might want to include C15, 0.22uF.
Do I connect a wire from each end of C15 to V+ and V- in the CMoy?
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Old 14th November 2013, 10:52 PM   #8
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I use a circuit like this to charge my battery pack.

And one like this to let me know when I need to recharge.
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Old 14th November 2013, 11:24 PM   #9
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This is a very simple trickle charging circuit
Trickle Chargers - Page 3

easy to knock up on a piece of strip board, no bells and whistles but works well

cheers
FRED
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Old 14th November 2013, 11:46 PM   #10
Jay517 is offline Jay517  Ireland
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Quote:
I use a circuit like this to charge my battery pack.

And one like this to let me know when I need to recharge.
Thanks Avro

The low battery indicator is a great addition.
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