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Making MP3 Player capable of charging batteries

syrevo

Dec 3, 2013
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Dec 3, 2013
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I'm trying to convert non-chargeable mp3 players into charger and mp3 player at the same time so the user can simply plug the mp3 into any USB to charge his battery

To lower the cost of converting as much as possible my idea is to use a 2.4v zener diode (the device hold 2x AAA batteries) connected between the USB connector positive pin and the battery positive pole.

My question is since the device will try to charge the batteries constantly as long as it's connected to USB (5V - up to 500mhA) will that setup harm the batteries if left for a long period of time and is there any other options?

I'm thinking about using Ni-Ca batteries since Ni-Ca can endure overcharge better than Ni-Mh and Lithium
 

davenn

Moderator
Sep 5, 2009
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Sep 5, 2009
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hi there
welcome to the forums :)

a zener isn't connected in series like that
have a look at this Wiki Page and scroll down to Uses
to see there is a current limiting resistor in series and the zener is in parallel with the load

cheers
Dave
 

syrevo

Dec 3, 2013
2
Joined
Dec 3, 2013
Messages
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Thank you for your reply

I see, so i need to add a resistor. is 5.6 ohm good for the circuit?
 

davenn

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Sep 5, 2009
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Thank you for your reply

I see, so i need to add a resistor. is 5.6 ohm good for the circuit?

from that page ......

In this circuit, a typical voltage reference or regulator, an input voltage, UIN, is regulated down to a stable output voltage UOUT. The breakdown voltage of diode D is stable over a wide current range and holds UOUT relatively constant even though the input voltage may fluctuate over a fairly wide range. Because of the low impedance of the diode when operated like this, resistor R is used to limit current through the circuit.

In the case of this simple reference, the current flowing in the diode is determined using Ohm's law and the known voltage drop across the resistor R;
IDiode = (UIN - UOUT) / RΩ
The value of R must satisfy two conditions :
1.R must be small enough that the current through D keeps D in reverse breakdown. The value of this current is given in the data sheet for D. For example, the common BZX79C5V6[7] device, a 5.6 V 0.5 W zener diode, has a recommended reverse current of 5 mA. If insufficient current exists through D, then UOUT is unregulated and less than the nominal breakdown voltage (this differs to voltage regulator tubes where the output voltage will be higher than nominal and could rise as high as UIN). When calculating R, allowance must be made for any current through the external load, not shown in this diagram, connected across UOUT.
2.R must be large enough that the current through D does not destroy the device. If the current through D is ID, its breakdown voltage VB and its maximum power dissipation PMAX correlate as such: I_D V_B < P_MAX

A load may be placed across the diode in this reference circuit, and as long as the zener stays in reverse breakdown, the diode provides a stable voltage source to the load. Zener diodes in this configuration are often used as stable references for more advanced voltage regulator circuits.

The diode wattage rating will also need to be chosen to handle the current

Dave
 
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