Powering a PIC

[lerameur]

Hello,

I was wandering how to power a chip from 120Vac main. Using an isolated 120v/120v transformer. what would be the best way to obtain 5v.
I found this circuit:
http://avr.15.forumer.com/a/microcon..._post1191.html
its almost too simple, what about safety issue ?

thanks

k

[dhouston]

Go with this http://www.sparkfun.com/products/8269

[lerameur]

Hi,

I am making a module that already contains an isolation transformer 120v/120v, therefore I do not want to add a second one. I have to use the 120VAC (rectified to dc) being supplied. Maybe use a voltage divider ?

K

[gadelhas]

Hi;

Please see this post;
http://www.picbasic.co.uk/forum/show...ransformerless

Or search for transformless power supply

[dhouston]

I am making a module that already contains an isolation transformer 120v/120v, therefore I do not want to add a second one. I have to use the 120VAC (rectified to dc) being supplied. Maybe use a voltage divider ?

Since you are designing for 120V, I assume your market is the USA. By the time you meet UL requirements and power supply efficiency standards (which can only be met by switch mode supplies), you'll appreciate my suggestion.

[lerameur]

Dave,
where can i read up on these UL requirements? (www.ul.com) but do you have a pdf document with these norms.. i could not find it. So basically you are telling I need a switching supply ? no transformer ?

k

[mister_e]

my 2 cents.

When you design something, you must use a whatever kind of PSU that meet at least the minimum current needed by your target application.

Then the safety. Anything in your design will be controlled by the end user? If so, can the enclosure be enough insulation to avoid shock hazards? If so... does it really worth to sacrifice safety against few bucks more PSU design assuming your enclosure is big enough to fit a Transformer or already built PSU (Switching or else)?

In some situation, transformerless PSU are great, while can be dangerous in some other situations. Transformers are not that expensive, but have input voltage & Frequency range, while many Switching PSU will work in any country without any modifications but the AC prong.

If you look for a WallWart, some wall-wart are shipped with replaceable AC prongs. They are somehow interesting for worldwide use/target.

[dhouston]

where can i read up on these UL requirements? (www.ul.com) but do you have a pdf document with these norms.. i could not find it. So basically you are telling I need a switching supply ? no transformer ?

Sorry, it's been a few years since I investigated this. The biggest issue is UL wants to have ongoing access to your operation to verify assembly operations. It's just not practical for a small business - you're better off using approved external modules like the $5 wall transformer I referenced. While I'm not familiar with the internals of the Spark Fun PS, most of these have internal transformers (at a high frequency) and provide isolation from the power line.

You could design a switch mode supply into your device and choose an offshore assembler that can meet UL requirements but, again, this really isn't practical (or cost effective) for a small business.

California already has power supply efficiency standards that can only be met by use of a switch mode supply. Other jurisdictions are sure to follow.

Why does your design have an isolation transformer? Are you doing power line communication?

[mister_e]

If you feel lucky, and if there's room... wind some extra turn on the secondary side of the isolation transformer...

[lerameur]

I need 200 watt at 120VAC. I already have a big transformer that I am using. So I want to use a bit of the power out of that to supply my PIC. I would also like this to be safe and regulated because I would like to eventually commercialize this.

K

[dhouston]

Many chipmakers offer SMPS chips.

• http://www.fairchildsemi.com/applica...er_supply.html
  http://www.st.com/internet/analog/class/1454.jsp
  http://www.microchip.com/stellent/id...redirects=SMPS

But, if you commercialize your product, it would be prudent to get UL approval. Otherwise, one fire plus one attorney could bankrupt you.

[Ioannis]

Keeping in mind all the safety matters, look also here:

http://www.picbasic.co.uk/forum/show...former+elektor

Ioannis