Getting appropriate 5VDC power from a 24VAC transformer

You’re looking for this:

http://www.murata-ps.com/data/meters/dms-78xxsr.pdf

I bought mine from Digi-Key. The problem that you’ve got is that your 24VAC is actually RMS. Peak voltage is actually ~34V. Furthermore, if your AC wall-wart is like mine, you’re actually closer to 40V peak. The neat thing about that regulator (besides everything) is this:

[quote]Operation at 40Vdc: Operating with inputs up to 40Vdc is permissible if, for inputs
between 36 and 40Vdc, the maximum load current is reduced to 0.35A for 7805SR-C
and 7803SR-C, and to 0.3A for 7812SR-C. Under no circumstances should the input
voltage be allowed to exceed 45Vdc.[/quote]

This regulator, plus a rectifier and a filtering cap, and you’re good to go.

You may have hit the nail on the head…I’m using this one which is very similar:
http://au.mouser.com/Search/ProductDetail.aspx?R=OKI-78SR-5%2F1.5-W36-Cvirtualkey58010000virtualkey580-OKI78SR5%2F1.5W36C

Output current is 1.5 Amp where as the max of my adaptor is 1amp. Looks like I bought the wrong version of the regulator - does this make sense (would it be trying to draw 1.5 amps and therefore blowing the fuse?

That’s the exact same part. It’s an awesome little device.

Remember that your adapter doesn’t “push” 1A through the circuit, it CAN (safely) supply “up to” 1A of current. It could probably supply somewhat more than that for a short period of time before the smoke was released.

The regulator CAN (safely) supply “up to” 1.5A of current. Because it’s a switching regulator and not a linear one, it’s very efficient (up to 95% for this one, depending on input voltage and a few other things). That means it won’t draw much more from the input that it supplies at the output. How much it draws on the input is nearly equivalent to how much is drawn from it’s output by the circuit you connect there.

It would only try to draw 1.5A if your 5V circuit tried to draw 1.5A.

Hmmm back to the drawing board as to why the circuit is tripping fuses then :’(

Im just driving fez cobra + wifi module so shouldnt be getting anywhere near 1 amp. Any ideas what might be causing it? The fez powers up ok for maybe a second, then the fuse trips, so the 5v is working ok. Is it likely to be a short somewhere, wouldn’t have thought the fez would get the 5v if a short was in the circuit?

Any tips for getting to the bottom of it?

I’ll triple check the bread board and try again in the next few days (need to get some more fuses…). It is very frustrating!!!

“Tripping fuse” - where is the fuse place on the circuit ? Can you share the schematic ?

You should break down the jig for testing. Does you power source (assuming wall wart) run continous ? You can check the output voltage (this will higher than the rated voltage (24v ? ) with no load.

If the wall wart looks ok, disconnect the FEZ and test the 78SR and the rest of the circuit for continous operation ? What is the output voltage from the 78SR ?

And, if worse comes to worst, you could always wire in your ammeter…

your problem is your fuse. dont use fast blow, use slow blow.

The first filter cap causes a big inrush current to charge it up, this blows the fuse.

the other transformer doesnt blow the fuse because it can only supply 300mA while charging the cap…