# Circuit Analysis Help - Need to know how to pick components

#### Erin_2006

Joined Sep 18, 2006
5
Hi All,

I'm a newbie here. This seems to be a very helpful site though

I found this circuit on microchip's website. (Sorry for the obvious questions but I just can't seem to figure this out =(

I want to build this circuit but before I do, I want to understand it. Particuarly, I'm concentrating on the voltage regulator circuit to bring AC voltage to 5V to power the MCU. (.25A Fuse, R2, R3, D1, D2, C2). However, they post an explanation but it's very diluted.

I just need to be able to understand the calculations so that I can configure the circuit to how I want it. But I'm not sure how they make the calculations to choose the particular values of each component. Any help would certainly be appreciated.

Erin

#### Papabravo

Joined Feb 24, 2006
20,374
It is an incredibly unsafe circuit to be fooling around with. Are you sure you want to risk death and disfigurement?

The zener diode limits the voltage across itself to about 5 Volts plus or minus maybe 10%.

The resistors limit the current

The other diode prevents conduction on the negative half cycle

The capacitor holds it's charge until the next positive cycle

#### Erin_2006

Joined Sep 18, 2006
5
Thanks for the fast response.

My follow-up question would be:

1. What will make it a safer circuit? (or maybe...why would microchip post something so dangerous?)
2. My main goal is to basically make a dimmer circuit. And I needed to start somewhere. But before I did that, I wanted to see if I can power my MCU with the wall power. Do you guys think I should go safe and build two circuits? One that powers the MCU and then use an output from the MCU to turn the triac on (which is part of a different circuit?)

I'm kinda scared to even take on this project now.....

#### Papabravo

Joined Feb 24, 2006
20,374
What will make it "safer" is transformer isolation. I can't tell you why microchip would publish such a circuit except to promote the use of their parts. Did you notice the very prominent warning at the beginning of the application note?

Let me refresh your memory. Yellow triangle, lightning bolt with arrowhead. Ring a bell -- so to speak.

I quote from that initial paragraph

Only persons experienced in electrical manufacture should use this document to complete the system
described. FAILURE TO FOLLOW PROPER SAFETY PRECAUTIONS COULD RESULT IN PERMANENT
INJURY OR DEATH DUE TO ELECTRIC SHOCK HAZARDS. To avoid risk of injury from electric shock, do not
build, or use, the system described in this document without implementing proper safety measures.