## AC to DC question

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Was planning on building a circuit that is going to run off the wall voltage and was going to use a transformer and a bridge rectifier to step it down and full wave rectify it. My question is, after using a ripple capacitor (and voltage regulator to step it down further), will I get a smooth enough wave to power a avr without it resetting or browning out?

I understand the wave has a ripple in it still.

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will I get a smooth enough wave to power a avr without it resetting or browning out?

Maybe.

It depends upon many factors, including the current load being drawn by the load, (micro, leds, relays, motors, RF, etc.), the size of the regulator's input and output caps, the specific regulator used, the capacity of the transformer...

It is possible to calculate it all out to design in the safety margin desired, but for many projects it isn't worth the effort to do so.

What you have described is essentially what is inside many power supplies, routinely used to power electronics without difficulties.

JC

Well I would just use a wall wart or what ever, but my project requires 120VAC untransformed and rectified as well.

AC wall warts are also available, and save you the hassel of wiring 120 VAC into your box.

JC

Yeah but you see I need to wire 120VAC into my box also.

This topic is covered by a number of websites. At a pinch, use a 2200uF cap. If using a 7805 or similar regulator, you will need at least 7 VDC as an input.

Yeah cause the 2V drop or whatever right? Yeah I'm planning on using a 120V to 12V transformer

Most regulators need a minimum voltage drop in order to regulate. A 12V ac transformer with rectify to 1.4142 * 12 = 16.9V. Let's assume your circuit draws 250mA at 5V, so the voltage across the regulator is 16.9-5 = 11.9V, times this by the current to get the wattage to be wasted 11.9 * 0.25 = 3W. If I guess the C/W rating is around 70C/W for the bare 7805 in free air, the 7805 will reach 3 * 70 = 210C. It will shutdown at this temperature and probably burn the part number into your finger. You'll need a heatsink in this instance. Substitute in your current and have a read of the 7805 datasheet to confirm my guess of Tja(temperture junction to ambient). All this and more is convered on the interwebs. Just Goggle '7805 howto' as a start

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but my project requires 120VAC untransformed and rectified as well.

You don't say whether it is half wave or full wave and whether it will be filtered or not.
Just be a bit careful with that. Is there a common between the 5 Volt line & the HV line (170VDC if full wave).
If there is you should use a 1:1 isolation transformer.
If the HV is totally isolated from the +5VDC than a nice way to go is to get a switchmode wallwart, crack it open and then then you can pick up 170V from the rectifier circuit and +5 Volt from the SMPS as normal.Light & efficient!

Charles Darwin, Lord Kelvin & Murphy are always lurking about!
Lee -.-
Riddle me this...How did the serpent move around before the fall?

ltkenbo wrote:
Was planning on building a circuit that is going to run off the wall voltage and was going to use a transformer and a bridge rectifier to step it down and full wave rectify it.

Quote:
Well I would just use a wall wart or what ever, but my project requires 120VAC untransformed and rectified as well.

This tells me that there will be two supplies.
a) +5 V after bridge rectifier etc. etc.
b)
Quote:
120VAC untransformed and rectified as well
but it gives no idea of type of rectifier or filtering ?

Charles Darwin, Lord Kelvin & Murphy are always lurking about!
Lee -.-
Riddle me this...How did the serpent move around before the fall?

Ok dude, not like it really matters, but I made it pretty clear in my post where I already provided this information. Read my original post at the top of the page. This time it should be quite clear at least about rectifying lol. I'm not filtering anything other than the ripple capacitors mentioned above also.

ltkenbo wrote:
Transformer and a BRIDGE RECTIFIER to step it down and FULL WAVE RECTIFY IT.

Well, I am just trying to help here as your technical knowledge is very obviously very meager and we aim to provide quality advice.
if you look at your second & third post, you provided additional information, which is contrary to your very first and last post. But it appears that you do not understand the implication of what you wrote in those posts.

I gather that "dude" means

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A word that americans use to address each other. Particularly stoners, surfers and skaters.

My name is clearly in all my posts. The conclusion to be drawn, is that you are a discourteous american stoner, who can't read & discuss things rationally and you have little knowledge about electronics and no ability to learn. I suggest you take up surfing and/or skating. :lol:

Charles Darwin, Lord Kelvin & Murphy are always lurking about!
Lee -.-
Riddle me this...How did the serpent move around before the fall?

Nah dude, DocJC pretty much summed it up for me. Thanks! 8)

@LDEVRIES

Quote:
My name is clearly in all my posts. The conclusion to be drawn, is that you are a discourteous american stoner, who can't read & discuss things rationally and you have little knowledge about electronics and no ability to learn. I suggest you take up surfing and/or skating.

Well said, but careful. I live in NewYork and try to be as polite as possible despite the jerk that is giving me hell.

@OP
Your original post was simple but then your posts contradict your requirements. The fact that you sent an un-warranted nasty shot at LDEVRIES
for nothing shows how ignorant you are.

When you plug whatever you ugly-rig up I hope you have a blast and then learn to grow up. :(

Jim

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