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Simplified PSU

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kybert

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Hi,

Trying to simplfy a psu (shown in the image). The supply is 240VAC @ 50Hz (UK Mains) and the output is 5vdc @ aprox. 100mA.

I thought of using some sort of line attenuator / zenner arrangement that will replace the expencive and bulky transformer, to directly derive about 24Vdc from the bridge diodes for the input of the VReg.

Is this possible? the 5V output must be provided by a vreg becuase it needs to be stable for a uProcessor is attached to it.


Can anyone come up with some design ideas / circuits?


Joe
 

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The transformer is there for two reasons:

It is an efficient way of converting the mains voltage to the low voltage required for the rectifier and regulator.
Using an "attenuator" and zener as you suggest would dissipate about 24 watts as heat, (240 volts x 100mA = 24 watts), the thing would get very hot.

SAFETY, the transformer provides reliable isolation from the 240volt mains.

Use the transformer, it may be more expensive, but it is a lot safer.

JimB
 
Sure there are some shortcuts but there are also some risks involved. One way is to use an LR8 regulator with a bypass transistor. The transistor will dissipate more than 30W of heat at 5Volts and 100mA so it needs a good heatsink (maybe as bulky as the transformer). And a circuit without a transformer will always carry mains at the output so you better isolate whatever you are powering.

Ante :roll:
 
kybert said:
Hi,

Trying to simplfy a psu (shown in the image). The supply is 240VAC @ 50Hz (UK Mains) and the output is 5vdc @ aprox. 100mA.

I thought of using some sort of line attenuator / zenner arrangement that will replace the expencive and bulky transformer, to directly derive about 24Vdc from the bridge diodes for the input of the VReg.

Is this possible? the 5V output must be provided by a vreg becuase it needs to be stable for a uProcessor is attached to it.


Can anyone come up with some design ideas / circuits?

DON'T DO IT!.

The transformer is a cheap, compact, safe, and efficient way of doing this, there are ways without a transformer. Voltage droppers are very inefficient, and don't provide isolation - so your entire circuit would be live to mains!.

You don't need 24V to feed a 5V regulator, a 9V transformer would be plenty.

To make the PSU smaller and lighter you could go to a switchmode PSU, at the expense of much greater complexity - plus a special transformer!.
 
OK, so the transformer stays.

You see attenuators in electronic light switch dimmers and stuff, but i couldn't figure out how they worked.
 
kybert said:
OK, so the transformer stays.

You see attenuators in electronic light switch dimmers and stuff, but i couldn't figure out how they worked.

Only for very low current demands, and only if it doesn't matter if the entire circuit is live.

There are always alternatives, but if you need to ask you shouldn't be attempting anything potentially fatal.
 
I remember an electronics book from when I was a kid and it said NEVER mess with mains unless you know what you are doing.

It kinda scared me off even thinking about using anything except batteries until pretty much uni when I played with a couple of kW.

If you are gonna mess with main you really should know what the dangers are. There arnt many but a transformer is needed
 
I SYNC THEREFORE I AM :wink:
 
Transformerless power supplies do exist.
In fact, they implemented it in a plasma globe circuit I own. I was wondered when the box states it accepts both 230VAC and 12VDC supplies.
So I opened up the casing and was surprised that they actually left out the transformer :shock: That explains it. These Chinese manufacturers really dangerous.
 
there are many ways to make transformerless power supplys some simple some complex but as some one allready said if you have to ask then you dont know enough to be messing with mains, however i do belive microchip has a pdf on the subject of transformerless supplys aimed for use with pic's involving a cap a diode a zener and a resistor i think be warned these circuits do indeed work but the GND is normaly directly connected the N terminal so if anything fails in the live part the entire circiut would be 240vac LIVE , but if ya kill yourself dont moan to me ;)
 
I was shoked of my voltage multiplaying curcuits (mostly form my shocker in the projects page)but never form mains.(once i got 2kV thru me but it was low curent but painful anyway)

Once during class my frend played whith the mains plug (220V here) a bit and he got a shock.He told me that it was wery painful and his finder was a bit red.

Use an trasformer in your PSU becose its the simplest and the safest method.
you cod also use an 5V regulator on an 8-30V wallwart

I never buyed an tranformer i rip them out of electronic stuf (have an 15V 13 A transformer and that is almost 200W)
 
:twisted: i got shocked 250v 20amp i kept having spasms all day:twisted:
 
yea I copped a shock from an arc welder when I was welding in 60 degree's, my body was full of sweat and my welding glove arced. There was a small hole in my middle finger and another one in my index finger on my right arm. The quack said it was lucky I'm a left hander or I woz 1 dead baby, the quack couldn't work out why the arc went up my arm then exited at nearly the same point. Needless to say I'm 1 lucky bloke. Sure this ain't electronic related but a shock from mains power would be just as bad if not fatal.
 
bryan1 said:
yea I copped a shock from an arc welder when I was welding in 60 degree's, my body was full of sweat and my welding glove arced. There was a small hole in my middle finger and another one in my index finger on my right arm. The quack said it was lucky I'm a left hander or I woz 1 dead baby, the quack couldn't work out why the arc went up my arm then exited at nearly the same point. Needless to say I'm 1 lucky bloke. Sure this ain't electronic related but a shock from mains power would be just as bad if not fatal.
There is one way, how do "drop" main voltage to desired value. Use capacitor in series with your circuit. Beacause capacitor sets it resistance depending on frequency, and has NO power loss. Xc=1/(2*3.14*f*C), where Xc = Resistance. This alows you to power your circuit efectively.
 
bryan1 said:
yea I copped a shock from an arc welder when I was welding in 60 degree's, my body was full of sweat and my welding glove arced. There was a small hole in my middle finger and another one in my index finger on my right arm. The quack said it was lucky I'm a left hander or I woz 1 dead baby, the quack couldn't work out why the arc went up my arm then exited at nearly the same point. Needless to say I'm 1 lucky bloke. Sure this ain't electronic related but a shock from mains power would be just as bad if not fatal.

Arc welders are usually low voltage, but VERY high current, unless it was some very unusual (absolutely huge!) welder I doubt it could give you enough of a shock to be dangerous.

BTW, a smallish charged up capacitor (47-100uF 350V) will 'blow' two holes in your fingers quite easily (don't ask how I know?, or how many times I know?).
 
welders are at about 20V (you can feal it if your hand are wet)

I toched an 10uF cap charged to 300V and it wasent wery plesent

I also had an voltage multiplyer that had an 1 uF cap on the end and it got charged to 2kV wen that arcs it makes an prety loud bang

you shod see how it fryed an pencill track on an sheet of paper.First it fryed the cuductive path making yelow sparks and then it started arching violete (it proboly the carbon in the air coloring it) arcs.
 
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