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Old 02-09-2019, 11:20 PM   #1
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Electronic help please


Im not sure if this is the right place, but I need some help with a project, i want to set up something for an led, with a battery backup, i can't give specifics because it is for an invention, i plan to use a charger for a cell phone as the plug in, but i dont know how to make the backup power supply, and i would like it to be rechargeable so in the event of a power outage it will still give light. Can someone tell me how to make the battery backup?
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Old 02-10-2019, 05:40 AM   #2
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Re: Electronic help please


Adapt one from any battery powered device using Ni-cad batteries, and a circuit with a control switch, that will actuate whenever there is an outage.

Look into uninterruptable power supplies for Computer applications, and copy them.


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Old 02-10-2019, 08:06 AM   #3
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Re: Electronic help please


Design the circuit to only run from a battery then add in a charging scheme for the battery.
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Old 02-10-2019, 08:53 PM   #4
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Re: Electronic help please


Check out the circuity of an emergency light. Does exactly like you want.
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Old 02-10-2019, 09:18 PM   #5
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Re: Electronic help please


Uh, the way you have the potentiometer drawn, it doesn't adjust anything, and you won't be putting an AC source in series with your battery. Not sure what's going on with that schematic...

Running from a rechargeable internal battery or from line power is a very standard thing to do, and there are a few different ways of doing it. Which way is the best for your application depends on all the details - which you have not provided. With the information you did provide, all I can say is that there are standard off-the-shelf solutions for this exact problem. Exactly which one you need depends on voltage, current, battery type, charging and discharging time, form factor requirements, and other stuff you're not telling us.
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Old 02-11-2019, 05:15 AM   #6
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Re: Electronic help please


Quote:
Originally Posted by mpoulton View Post
the way you have the potentiometer drawn, it doesn't adjust anything,
mpoulton got in before me with the comment concerning the potentiometer.
This potentiometer is "tapped resistor" in the position where one would expect to find the current limiting resistor for a LED supplied from a constant voltage source.
In addition, potentiometers tend to be devices with very low Wattage dissipation - such as 100 mW, or less. Also, they tend to be of a greater resistance value than would be required by your "design", since the lowest available value which I can descern from a quick glance at a certain catalog is 1 kΩ.

Since you wrote "i would like it to be rechargeable so in the event of a power outage it will still give light" it seems to indicate that it may be that you intend to run the LED concerned at close to its maximum limits.

A 5 mm "Bright White" LED from that same catalog is rated at a maximum current of 30 mA with a voltage drop across the LED of 3.5 V.
If supplied from a 5.2 V source (because that would be about the voltage you would expect from "a charger for a cell phone"), the LED would require a current limiting resistor of at least 57 Ω - to dissapate about 51 mW.

Of course, as a designer, you should use a somewhat higher (Preferred Value) resister, to ensure that the LED is not supplied with more than 30 mA.

62 Ω, 68 Ω or 73 Ω half-watt resistors are possibilities in the case which I have described.

(A 1 kΩ potiometer used as a Rehostat would be much less suitable than a half-watt resistor and would be virtually certain to "burn out" within a short period.)

Quote:
Originally Posted by mpoulton View Post
you won't be putting an AC source in series with your battery. Not sure what's going on with that schematic...
Of course, you wrote "i plan to use a charger for a cell phone as the plug in", which will supply DC and not AC.
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