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18 years ago
There is a way to configure a standard regulator as a constant current source. I suspect this is what you mean. Only ever did it once for EMI reasons.
For a positive output you need a negative regulator and you use it 'back to front'.
A power resistor ( R ) is placed between Common/Sense and Out. I = R/Vref ( Vref = 5 for 7905 for example or 1.25 for LM337 ) . DC in + goes to Common / Sense and the current out comes from 'In'.
Graham
And then I recall that there's a way to make a shunt regulator to IIRC ! Uses the 'opposite polarity' chip again in a wacky configuration.
Graham
anyone
it's
Nokia
my
I think allot of voltage regulators can be used like this, but the LM317 seems one of the most popular for the task.
The datasheet is also a good reference source for cutesy circuits like this.
The key is that the output pin and the ground pin will always have 1.25V across them. By floating the ground and connecting it to the output pin via a resistor, you can create a constant current source. The resistor is sized using Ohms law (R=E/I). So for a 25mA current source, you would need 50 Ohms of resistance between the pins. Be sure to calculate the power being dissipated by the resistor and size it accordingly. In this case, the power is 31.25mW so a 1/8W resistor should be plenty big enough. If you want to supply 1A, then you would need a resistor of
1.25 Ohms that would be dissipating 1.25W, so use at least a 2W and don't burn yourself. ;-)
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.
A current limited regulator isn't what you're looking for here - all that will do is make the power supply to your device droop, and it won't operate properly. You need either a USB with a beefier supply, or an auxiliary power supply, like a wall wart.
Good Luck! Rich
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.
use the regulator to regulate the voltage in a resistor.
usually works more economically with a 5V or lower regulator. as the 5V etc is wasted
Bye. Jasen
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