240vac to 5vdc

[justin_t]

Hi im looking to convert 240 vac to 5vdc in a small enclosure for an FM reciever.

Ive been lookin at one method from another small FM reciever unit i had laying around. There is a physically large capacitor (approx 15x20x8 mm) of 0.33uF connected across the live and neutral supply, i think this is for noise suppression from the supply.

There are at lease 2 diodes and i think there are also 2 zener diodes so i guess that must be for the rectifier but the components are tiny and most dont have values printed on them.

Does anyone have a circuit diagram of something similar? im gonna open up a 5vdc adaptor plug now but im sure the noise suppression capacitor is a vital component that wont be in the 5vdc unit.

any help appreciated

justin_t

 

[rjvh]

What you see is a transformerless power suply which are cheap to build and provide small amount of power (for your aplication it will do)

how ever this type is not realy a safe type of power supply as it is not sepparated from the net,if something goes wrong in this supply the chance is that the (deadly) net voltage ends up in other parts of your circuit that you consider safe

I would recomend to work with a small transformer rectifie the power from it ,smooth it with a capacitor and use a 78L05 to have a stable and safe 5 volt

Robert-Jan

 

[justin_t]

is the large capacitor to dampen the effects of noise on the reciever?
will i have to include one in the circuit you mention above for the same purpose or should the reciever be happy enough beside a transformer

 

[justin_t]

Ive found a circuit at the following link:

**broken link removed**

It seams identical to the circuit ive been looking at but with the exception of the voltage being 12 volts as opposed to 5v.
i might look into making the above circuit and attaching the 5v regulator to the output.

The entire circuit is going to be enclosed in a sealed housing so that should eliminate the danger of people interfearing with the components.

the 5vdc also needs to drive 5 LEDs should this pose any problems?

Is this a widely used method?

 

[AllVol]

Ive found a circuit at the following link:

**broken link removed**

It seams identical to the circuit ive been looking at but with the exception of the voltage being 12 volts as opposed to 5v.
i might look into making the above circuit and attaching the 5v regulator to the output.

Did you read the first sentence?

"If you are not experienced in dealing with it, then leave this project alone."

 

[rjvh]

it is a widly used methode for cheap products that don't have to pass a sertain level of safety

in case of powering a timer not that critical and usable if you know what you do

in case of your radio with a earphone jack not advisible as if something goes wrong there is a posibility that 220V or 120V (depens where you are ) will be on the jack output and the chance is than that you will electricute yourself

i don't want to make you scared but have comon sense and think of the risks it can be for the aplication and the users of it

Robert-Jan

 

[Sceadwian]

One of the main functions a transformer adds to a device is isolation, even with transients the core can saturate and only transmit so much energy. If the primary winding melts it will cause the entire transformer to cease to function likely before it can contact the secondary to transmit voltage, and even the depending on where it shorts the induction value and the basic DC resistance limit current flow.

Regardless of weather or not the device is shielded from an outside user if a component fails on it's own anything it's attached to can be exposed to line voltage.. Generally a bad idea. Also this is an RF receiver, so you're hard linking it to the mains hot and neutral lines with very littler buffering so noise will be a problem.

 

[Slowmo]

Maybe you can find a ready made switching power supply? Switching type power supplies will be small.

 

[ste5442]

If you are unsure about dealing with mains voltages then you are best using a small mains transformer to give you at least some protection should things go pear-shaped!
Small transformers are cheap and inherently safe.
One problem with the non-switching, transformerless power supplies is that you are limited in the amount of current you can draw from them.
Everyone has tried to build one of these at some point in the past but always run into the dissipation problem eg:

If you consider the circuit to be like (not really!) a linear regulator you can work out the dissipation. Suppose you require 100mA:

Dissipation in the reg (transformerless, non-switching supply) = P = IV = 100mA*(240VAC-5V) = 23.5Watts = Bloody HOT!

Using capacitive reactance reduces the dissipation somewhat but scares me a little for some reason!

Conclusion - just use a transformer ;-)

 

[KMoffett]

justin_t,

Not sure where you are in the world, but I find an unending supply for small 100-250vac to 5Vdc/~500mA switch mode power supplies at thrift stores like Goodwill, here in the USA. Usually for $0.99-2.99USD. They are chargers for cell phones. They usually have flip-out line-plug blades, and are much lighter weight than the normal wall warts for the same voltage/current rating. If you're not familiar with the dangers of line voltage circuits, you should just cut off the cell phone connector, and wire it to your receiver enclosure. If you take it apart and put it in your small enclosure, by the time you add a standard size line cord, you will find you haven't really saved any space...inside the box or outside.

I've even found some that use brand name IC's with standard circuit configurations, that allow me to change a resistor and tweak the output voltage to suit my needs. Being mindful of the maximum voltage ratings of the other components.

Ken

 

[justin_t]

im gonna give it a try, ive seen them work for the same type of application so its strengthened my confidence.
I have however heard of another option using an SR087 chip that as far as i understand uses a transistor to allow only voltages under a certain level to pass.
ive ordered parts for the transformerless option. ill let you know how i get on, good or otherwise...

 

[rjvh]

try your transformerless power supply on a net circuit that is protected with a earth leak protection on it at least if it goes wrong there is still some device that gives you a protection to some degree

Robert-Jan

 

[justin_t]

ok, im going to look into both options, while im waiting for the parts to come for the previously discussed power supply im going to look into the transformer option as i have the parts already.

When im putting together/testing the circuit is it ok to power up the 240vac to 9vac transformer under no load conditions to check that im getting the correct voltage out from it?

justin_t