# Building an AC to DC power supply

I'm building an AC to DC power supply for a class and my question mainly revolves around the input and output of the whole thing. My source of confusion is where to include my DC output in a circuit such as

I would assume that it would be the leftmost node of that full bridge rectifier, and the + of the ac input is the top node of the rectifier, whilst the - component is the bottom node.

## Answers and Replies

gneill
Mentor
I'm building an AC to DC power supply for a class and my question mainly revolves around the input and output of the whole thing. My source of confusion is where to include my DC output in a circuit such as

[snip image]

I would assume that it would be the leftmost node of that full bridge rectifier, and the + of the ac input is the top node of the rectifier, whilst the - component is the bottom node.

Surely the DC output is at the load resistor. It is after all a DC supply for a load, right?

Yeah, that makes a lot more sense haha. I suppose I just tricked myself into thinking that the output must always be at the end of the circuit; this is my first attempt at transferring anything on paper to reality.

... this is my first attempt at transferring anything on paper to reality.

Please be careful -- working with AC line power is hazardous. It is very easy to accidentally touch live wires with tools and fingers.

I'm rather surprised that a teacher would approve this project for a novice.

Follow up question: would the ground be a connection from my PCB to the chassis of the box?

berkeman
Mentor
I'm building an AC to DC power supply for a class and my question mainly revolves around the input and output of the whole thing. My source of confusion is where to include my DC output in a circuit such as

I would assume that it would be the leftmost node of that full bridge rectifier, and the + of the ac input is the top node of the rectifier, whilst the - component is the bottom node.

Follow up question: would the ground be a connection from my PCB to the chassis of the box?

What class is this for? The grounding scheme for safety should be part of the instruction in the class. Ans your diagram does not show the ON/OFF switch and input fuse arrangement. Why not?

Introduction to circuits and signals; I'm also unsure as to why the switch isn't included in the diagram, but I guess I can just put it before the load resistor right? We haven't really received any instruction regarding safety or soldering, our instructor only provided us with the diagram and the parts and said "build it".

berkeman
Mentor
Introduction to circuits and signals; I'm also unsure as to why the switch isn't included in the diagram, but I guess I can just put it before the load resistor right? We haven't really received any instruction regarding safety or soldering, our instructor only provided us with the diagram and the parts and said "build it".

That's crazy! Are you planning on actually plugging it into the wall AC Mains?

First of all, the fuse at the output on the right where the load resistor is located generally would not be there. Instead, the fuse should be over at the left in series with the Hot leg of the 120Vrms AC Mains input to the transformer. And there needs to be a switch in at least the Hot leg of the input to the transformer. And Earth ground needs to be connected to the metal chassis of your power supply. And, the connector lug you use to Earch your metal chassis has to be of a particular type. And......

What is your instructor trying to do? Fry a bunch of students?

Yeah there doesn't appear to be an rationale for the fuse being where it is, so I may just move the fuse and switch before the full bridge rectifier. I'm a little unclear on the grounding still, my project box doesn't have any metal on it and no additional components were provided in my kit.. and soldering a wire to the plastic box doesn't make much sense to me, but it seems to be my only option. This is my first engineering class and my first time soldering etc, so I don't have any perspective.

Yeah we are plugging this into AC Mains

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gneill
Mentor
Yeah there doesn't appear to be an rationale for the fuse being where it is, so I may just move the fuse and switch before the full bridge rectifier. I'm a little unclear on the grounding still, my project box doesn't have any metal on it and no additional components were provided in my kit.. and soldering a wire to the plastic box doesn't make much sense to me, but it seems to be my only option. This is my first engineering class and my first time soldering etc, so I don't have any perspective.

You may end up changing your username from fashizzle to flash-sizzle!

There must be a fuse on the hot lead of the mains plug. There should be a switch on the the mains supply, too.

You can also have a fuse on the load circuit as depicted in your schematic. It'll blow if you accidentally short circuit the output.

So I'm a little confused by some of the jargon because I've never wired anything before, hot lead means the positive of the AC from which the current "originates" before progressing from one circuit element to another? So in this case the hot lead would be one of the wires coming from my AC input jack that is connected to the transformer ( I assume 3 wires come from the jack since there are 3 prongs, one positive, one negative, and one I'm not sure).

gneill
Mentor
So I'm a little confused by some of the jargon because I've never wired anything before, hot lead means the positive of the AC from which the current "originates" before progressing from one circuit element to another? So in this case the hot lead would be one of the wires coming from my AC input jack that is connected to the transformer ( I assume 3 wires come from the jack since there are 3 prongs, one positive, one negative, and one I'm not sure).

One lead is "hot", one is "neutral", and the third is "ground". There are lots of references for AC wiring on the web, so a little research may help.

You may note that most 2-prong power plugs have one prong that is slightly wider than the other. This is to assure that the plug is inserted in a particular way, so that the hot and neutral leads always make the same connections. There is no ambiguity with a three-prong plug -- it can only be inserted with one orientation.

Have your instructor verify your wiring for the primary circuit of the transformer before you plug it in.

berkeman
Mentor
So I'm a little confused by some of the jargon because I've never wired anything before, hot lead means the positive of the AC from which the current "originates" before progressing from one circuit element to another? So in this case the hot lead would be one of the wires coming from my AC input jack that is connected to the transformer ( I assume 3 wires come from the jack since there are 3 prongs, one positive, one negative, and one I'm not sure).

Here are some links you should be reading before starting to work with AC Mains voltages:

http://www.penguintutor.com/electronics/electrical-safety

http://books.google.com/books?id=ju...=electronics projects ac mains safety&f=false

And your instructor really should be spending some time talking about electrical safety before having you build something involving anything above SELV (safety extra low voltage, around 60V, depending on the standard). Also, check the transformer to be sure it is rated "Double Insulation". If it is not, you should not be using a non-conducting enclosure.

AlephZero
Science Advisor
Homework Helper
You can also have a fuse on the load circuit as depicted in your schematic. It'll blow if you accidentally short circuit the output.

Most power supply regulator ICs have current limiting protection, so a fuse as shown is not really "protecting" anything. A fuse in low voltage circut before the IC isn't really protecting anything that needs to be protected either. The risk of the 4700 or 0.33 capacitors going short circuit is negligible assuming you construct the circuit correctly

The primary function of the fuse at the mains side of the transformer to protect the AC wiring of the building, not to protect your circuit from self-destruction.

I entirely agree with Berkeman. Designing and building something like this is a crazy "first practical project", unless you are properly supervised. There are some mistakes that you only get to make once in your life