transformer current help

[stretch2011]

If a step down 120v to 12.6v transformer is rated at 4.4 VA what's it's current handling capabilities? [/u]

[John_P_WI]

4.4va / 12.6v = 0.35a, therefore approximately 0.35 amps.

[stretch2011]

Well that was simple! So if I want 2 amps I'd need 2amps×12.6v=25.2VA

[John_P_WI]

Yes, and round your numbers up.

[stretch2011]

Ok I appreciate it. Thinking about making a regulated Filtered 9v supply. A simple,easy project. I cent believe how expensive 12v 2amp transformers are. $12+!!!!

[eniam rognab]

VA is fancy talk for volt amps, in other words IV. learn your Watts law P=IV

I=P/V, V=P/I

[stretch2011]

Neal I know my power laws! Just havnt really ran into VA before. As a musician I only have to worry about vd right?

[eniam rognab]

more power to you!

watch out for that ED too

[stretch2011]

Hahaha. Anyone have some good cheap resources on these transformers? Its sad I can buy an entire wall wart using a transformer for $5 but the iron itself is over 2x that.

[xtian]

Check the bottom of this page for pedal supply transformers:

https://taweber.powweb.com/store/magnetic.htm

Weber also has voltage regulators...someone will post a link?

[R.G.]

Well that was simple! So if I want 2 amps I'd need 2amps×12.6v=25.2VA

Yep.
**IF** what you wanted was 2A of 12.Vac, not 2A of DC.

Rectifiers and filters being the beasts they are, the current into the rectifier/filter cap setup has an RMS/heating value that's larger than the DC being pulled out of the caps. And it depends on how big the caps are compared to the DC out, or the ripple voltage, depending on how you look at that, and the rectifier circuit.

Cutting to the quick answer, if you run a transformer winding into a full wave bridge and a big filter cap, allowing only about 5% ripple voltage on the cap, your transformer winding is heated by an RMS current between 1.6 and 1.8 times the DC going out of the cap.

So if you have a 2A rated secondary, you only get 2A/(1.6 to 1.8 ) = 1.11 to 1.25A dc. That's for equal heating in the transformer windings, which is what really drives transformer power rating.

Or, the other way, if you want 2A of DC, you need 3.2 to 3.6A of current ability in the transformer secondary.

[stretch2011]

I was wondering how much current was soaked up/heat energy lost. I found a 3a 12.6v transformer on Amazon for 6.xx+shipping with a refund on shipping if I buy multiples. Thays way more in the realm of reasonable.

I see why people prefer the switching mode power supply over transformers and linear voltage regulators. But since they are a lot more work and I like iron I'll go the old fashioned route!

For a filtered pedal power supply I figured 2a of dc would be plenty.

[sluckey]

I bought 4 each 12.6V@3A transformers from RadioShack back in February for about $1.25 each. This was during their bankruptcy/going out of business days. I don't know if there are any left in the few remaining stores or if they have reverted to the original price of $12.

[stretch2011]

lucky!