120 VAC to 5 VDC

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stargazer3141
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120 VAC to 5 VDC

Post by stargazer3141 » Tue Jun 01, 2010 6:28 am

I am interested in the PIC microcontrollers, and have built several circuits that use batteries to power the microcontroller. Does anyone have a good method for stepping down the 120AC from the wall down to 5VDC? I would like a way to do this without using a "wall wart" tranformer.

dyarker
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Re: 120 VAC to 5 VDC

Post by dyarker » Tue Jun 01, 2010 9:21 am

Look at Mouser part numbers 418-CFM05S050, 709-NES15-5 and 709-PM05-5 for power supplies other than wall wart type.

A proper power supply does more than just step down the voltage, it also isolates the output from the power lines. NO ISOLATION IS DANGEROUS!


that first part number above would need to be put in an enclosure to keep fingers away from exposed parts.

Cheers,
Dale Y

stargazer3141
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Re: 120 VAC to 5 VDC

Post by stargazer3141 » Tue Jun 01, 2010 9:45 am

Thanks! I like the last one on your list - unfortunately Mouser has a 20 week lead time on all three of those! I get the idea - I bet I can find a supplier that has them in stock.

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haklesup
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Re: 120 VAC to 5 VDC

Post by haklesup » Tue Jun 01, 2010 10:32 am

The choice of a wall wart or an internal power supply in a project is based on preference and if there are any size restraints for the enclosure. For a manufacturer, it simplifies the design and increases reliability of the core function by disconnecting the reliability of the supply frm it.

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MrAl
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Re: 120 VAC to 5 VDC

Post by MrAl » Tue Jun 01, 2010 10:46 am

Hello there,


Alternate power supply techniques for circuits that draw small amounts of current such as with microcontrollers are discussed in at least one of the app notes on the Microchip site. These kinds of circuits have been used for years for other applications too.
The main idea is to use a capacitor to drop most of the voltage so that the power wasted is minimal. It is similar to the way we would drive an LED from the power line using a series capacitor and possibly one or two rectifier diodes. There is a little more to it than that, such as a series resistor to limit inrush current during startup, but it's not very complicated overall. It's true that there is no true isolation from the power line this way, but there are a number of ways to deal with that too such as using a potentiometer with a plastic shaft instead of a metal shaft, but that's only one issue that may come up in circuits that have to be adjusted by the end user.

If you can't find the app note then i could either find it or draw up a circuit...

Oh wait, here it is...
http://ww1.microchip.com/downloads/en/A ... 00954A.pdf

Just to note, you can use a simple series resistor, diode, filter capacitor and zener too but the resistor will most likely have to be a power resistor and it will heat up to some degree, while the series capacitor solution stays very cool and does not require a power resistor at all.
LEDs vs Bulbs, LEDs are winning.

renofalco
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Re: 120 VAC to 5 VDC

Post by renofalco » Mon Jun 21, 2010 9:43 am

Hi the app note from microchip is really great thanks! :grin:

In my project I need 24V DC from 120V AC source. Does the capacitive power supply would work for a 24V DC output?
Cause I think it's only for a 5V DC output? What would I have to change in the circuit?

Thanks for your help

gerty
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Re: 120 VAC to 5 VDC

Post by gerty » Mon Jun 21, 2010 10:47 am

[quote]What would I have to change in the circuit?quote]

To change the voltage you would need to replace the 5.1 volt zener diode with a 24 volt one.
You also need to make sure the capacitors are rated for the higher voltage.

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