I recently tried (and failed) to make a battery charger for my Monolith
MX-7010 MP3 player. The player normally charges via it's USB connection
and I wanted a standalone charger for use when I am not near a
computer. The MP3 player reports to my computer that it requires 90mA
when it is connected so I built a circuit which had a 5V dc output
limited to 100mA. I connected the output of my circuit to a USB plug
and tested the output voltage and current limit. When I connect the USB
cable to the MP3 player it behaves as if it is charging (red LED on)
but doesn't charge (battery still flat after many hours) and only 5mA
flows (probably only due to the MP3 player LED coming on)
Anyone have any ideas? Maybe the device has to initiate some kind of
communication with the host computer to tell it how much current it
will be pulling before it will start to work or maybe I need to
terminate the USB data connection pins with a pull-up or pull-down
resistor? Has anyone ever tried charging such a device using a powered
USB hub which is not connected to a computer?
Thanks for any help.
It probably needs to 'talk' to the computer before switching itself into
charge mode. It could be quite difficult to simulate the dialogue, using a
Otherwise, if it's just logic signals, you need to hold the other lines in
the right state.
Do you have a powered hub? If so, and if it charges when the hub is
connected to a computer but not when it isn't, investigate what happens to
each line when you plug and unplug it.
Thanks for your help. I don't have a powered hub to check with but I
have now made some progress: There isn't any data communication
required. I confirmed this by cutting the data wires in a spare USB
cable and checking that the unit charges properly from a PC. I also
found that when charging it draws around 130mA (not the 90mA that it
reports to the host computer). I upped the current limit in my home
made regulator circuit but this also failed to work. The problem seems
to be that the output voltage of my supposedly regulated charger
circuit was dropping from 5.0Vdc to 4.5Vdc with the MP3 player
connected even though only a few milliamps are being drawn. The circuit
works fine into any resistive test load but the MP3 player seems to
upset it. I can only guess that some internal switching regulation is
taking place in the MP3 player itself and this is upsetting the
stability of my voltage regulator and current limiting circuitry. I'll
redesign the circuit for improved stability and give it another try.
For a test you might try running off 4 AA cells with a diode in series
for ~5.3V. Batteries can't get confused and if there's an internal
regulator it won't notice the extra .3V. (I've just measured my USB
voltage to 5.1).
Spamtrap in use
To email replace 127.0.0.1 with blueyonder dot co dot uk
Thanks for the good ideas.
Last night I rebuilt the circuit using just a LM317 regulator set to
output 5.0Vdc without current limiting cicuitry and everything appeared
to work OK. The MP3 player drew around 150mA when "charging". I noticed
after about 15 minutes the current was about the same but the output
voltage had fallen to 4.5V or so. I find this surprising since the
regulator should be able to maintain the output upto at least half an
amp (it is mounted on a decent sized heatsink). The supply input to the
regulator circuit is an unregulated 12Vdc @ 500mA max wall-wart type
thing so should be quite suitable. I left it "charging" like this for a
couple of hours but when I tested the MP3 player I found that the
battery had actually discharged completely! Could it be that the output
voltage dropped so low that the regulator started to sink current from
the battery? I think the problem may be a defective LM317 since I used
it in both versions of the circuit (and it was pretty much the only
common component). For rev 3 I plan to use a new regulator and install
an output diode to prevent any possible current leakage back to the
charger, but it might have to wait until next week before I get the
You don't need constant voltage charging for a Ni-Cad - that's for lead
acid. You need constant current. The actual voltage doesn't matter
provided it is adequate - within reason.
I could send you a very simple cc circuit for a 14 hour charge.
* What do they call a coffee break at the Lipton Tea Company? *
Dave Plowman email@example.com London SW
I'm sure you could (one transistor and a couple of resistors, right?).
But once you're into building, I'd find it hard not to reach for a Maxim
charger IC, which will do the Right Thing for a Ni-Cad (fast charge,
detection of voltage levelling off/dipping to stop the fast charge, and
all the rest of the NiCad voodoo) - one-off prices around a fiver, if
Depends on whether it includes LED indictors. But the basic circuit is one
transistor, two resistors and two diodes. And if the DC supply is near
enough the ideal voltage you'd probably not even need a heatsink for the
*Why is "abbreviated" such a long word?
Dave Plowman firstname.lastname@example.org London SW
Pity it's not a bit more flexible.
The option to have a higher voltage (12/30V?) would be nice.
Only for devices that request it of course.
So that (for example) instead of 2.5W, 15W would be available, while
keeping within the current limits of the connector.
Thanks for all the replies. Problem was indeed a defective LM317. I
found that the peak current demand could be in excess of 500mA (so
strictly speaking it actually exceeds the limits of the USB electrical
specification). Whilst searching for a new voltage regulator IC in my
garage I came across a compact DIN rail 5V 1.5A switch mode power
supply. This works perfectly so I've ditched all the nasty bits of
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