Re: Using Voltage Regulator for current limiting - John Fileds ?
- From: "kell" <kellrobinson@xxxxxxxxxxxx>
- Date: 10 Jan 2006 19:49:31 -0800
William A. Bong wrote:
> I think that it was John Fields that posted a circuit using a voltage
> regulator IC such as the 7812 as a current limiting device - John or anyone
> else that could help out with this please repost it - I searched but it's
> obviously no longer on my server.
>
> I was thinking of using this as a current limiter to build a USB to Nokia
> phone charger - I have
> the hardware and connected it up straight, but it trips the power to my
> notebook USB port requiring a reboot to reset the USB.
>
> All advice and most welcome.
>
> Bill.
Are you connecting your Nokia phone directly to the USB? It's actually
not such a bad idea, in principle -- the USB is a 5 volt power supply,
and you can charge a Nokia cell phone from a 5 volt power supply. But
if your Nokia is like mine, it doesn't draw a steady low current when
charging. Mine draws its power in pulses. And I'm sure these pulses
would be enough to trip a USB, which can only put out very small
currents.
Try putting a resistor in series with the USB and allowing a capacitor
to charge from the limited current coming through the resistor. Charge
your phone from the capacitor. Start with a few dozen ohms for the
resistor, and 1000 uF cap. With a correctly sized resistor, you may be
able to pevent the USB from tripping and yet still get the cap to
charge up between pulses. You should be able to make this work if the
total power drawn by your phone, over time, does not exceed the amount
of steady state power the USB can deliver. But there's no guarantee of
that.
.
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- Using Voltage Regulator for current limiting - John Fileds ?
- From: William A. Bong
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