Reacheargeable CMoy Amp??

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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.
 
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.
 

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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?
 
Thanks for the reply.

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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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.
 
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?

I've asked that question directly to tech support at two NiMH "9V" battery makers. They both said the same thing. Under 10mA charging current is considered to be safe indefinitly for NiMH. In the circuit I posted the initial charging current, with "dead" batteries at 7Vdc or so each, would be around (24 - 0.3 - 7 - 7) / 470 = 21mA. Then as the batteries charge that decreases down to the continuous trickle amount of about 7mA, inside that 10mA safe zone.

Hey I'm glad FredFred posted! He is a CMOY wizard. :) I haven't seen that thread he linked to before but it looks similar. I'll have to give it a read-through.

Good luck!
 
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So it doesn't matter that the power supply is AC and the CMoy is DC?

Do I connect a wire from each end of C15 to V+ and V- in the CMoy?

The circuitry (the diodes closest to the AC inlet) acts as a rectifier, specifically the kind called a voltage doubler. It produces DC from the AC. The fact that it's AC in means that it's possible to generate both rails and a ground.

Yes, C15 has the output positive and negative rails (the 7812 is +ve, the 7912 is -ve), bring the cmoy ground to the junction of C6 and C7.
 
NiMH don't really like trickle/float charge - they can tolerate a very low rate - so low as to be impractical for actually recharging in less than a week

if you really want the 100s of cycles life and overnite charging for NIMH you really need a smarter circuit

http://www.ti.com/lit/an/snva557/snva557.pdf there's lots more literature, charging depends on battery chemistry
 
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NiMH don't really like trickle/float charge - they can tolerate a very low rate - so low as to be impractical for actually recharging in less than a week

if you really want the 100s of cycles life and overnite charging for NIMH you really need a smarter circuit

http://www.ti.com/lit/an/snva557/snva557.pdf there's lots more literature, charging depends on battery chemistry

NiMH are perfectly happy with a trickle charge at 10% of capacity, this allows for overnight charging of the amplifier so a 280mAh battery will charge at 27-29mA a 47ohm resistor will set that current
10% means that if the recharger is left connected for a few extra hours if will have little or no detrimental effect on the batteries.
I have used the simple circuit in hundreds of amps with no issues ;)
 
How does an 'Energizer Universal Recharger' charge? Would it be possible to connect wires to the + and - terminals of the 9V battery area, and plug them into the amp using some sort of switch to complete the circuit?

Just a thought. If that worked, it would also turn itself off when the batteries were fully charged.
 
Sorry but read the spec sheets from the battery manufacturers - NiMH are not "perfectly happy" at C/10 "trickle charge"

"trickle charge" is by definition the continuously applied charge rate, for weeks, months, years, that doesn't degrade battery capacity/life

hobbyists can do many things that major companies would be killed by customer complaints for

you may be happy if you get a few dozen recharges, not notice until capacity drops by 1/2 or more, just accept the hit to battery life if you forget and leave it on charge for a week
 
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Sorry but read the spec sheets from the battery manufacturers - NiMH are not "perfectly happy" at C/10 "trickle charge"

"trickle charge" is by defintion the continuously applied charge rate, for weeks, months, years, that doesn't degrade battery capacity/life


I said charge over night not plugged in for months

If your amp is going to be plugged into trickle charge for months why have batteries ?

A rechargeable amp needs to be charged so it can be used in a portable fashion
 
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