transformer for a stabilised 9V power source?

[yeeshkul]

I bought this transformer:
prim: 230V
sec: 9V/500mA (13V sec unloaded)
4.5VA.

I want to build a stabilised  9V power source. I have a strange feeling that i should have bought 230/12V  transformer
sec: (9V -1.4 drop on rectifier)*1.4 (peak) = 10.64V, which is not enough to run 7809 ... i am i right?

[earthtonesaudio]

Depends on a few things.
First, rectifying and smoothing will boost the voltage above the RMS value.
Second, the transformer is rated 9V at 500mA, but some higher voltage at lower currents.  You'll still get regulation as long as the total current draw of the regulator + pedal is small enough.

[yeeshkul]

Thanks man, i will just give it a try. The 12V piece of the same VA value is a bit weaker anyway.

[yeeshkul]

by the way, can i test the power source by connecting a large pot & a small resistor (i don't wanna short it) at the output - setting a current and measuring the voltage? just to be sure about what it can take

[Sir H C]

Look for a low dropout regulator, those need a lot less headroom to regulate, usually less than 1 volt.  Still you are cutting it close if you have low voltage (~90V in).

[yeeshkul]

Guys i am i right when i count (9 - diode drop)*1.414 or shall i count (9*1.414 - diode drop)

EDIT: oh, mistake! Sure it must be 9*1.414 - 1.1 max. voltage drop on the bridge i bought = 11.6V. That should be enough. I am sorry for all the panic 

[R.G.]

Power Supplies Basics, Geofex.

[darron]

schottkys could give you slightly less drop? i would have rathered the 12v

[Cliff Schecht]

Power Supplies Basics, Geofex.

Shameless plug

[R.G.]

Shameless plug

I agree. 

[yeeshkul]

I did read that time ago. Or doesn't it look i just made a mistake in my counting? - see the EDIT. If not then tell me what's wrong. If i really can get 11.6V at the 7809 input then it should be working allright up to 500mA draw. If not then i can get 12V trafo of the same size(=VA value) and it will be working for sure up to 300mA. The trafo i have actually can do the same thing as i suppose it may give more voltage (at the 7809 input) at less current draw. I have my PCB ready, so you can imagine i am not that happy with the idea of buying a bigger (let's say 6VA) transformer 

Actually, how does the 7809 behave when it gets less than 2-2.5V difference between input and output? Does it turn the output voltage down to 0 immediately?