Power Supply queries

[smccusker]

hi everyone,
I'm finally getting around to knocking out a basic regulated power supply (7809 based),and i have a few questions. i've been reading through some old threads, but i just want to double check before i start mucking around with line voltages.
Firstly, regarding a suitable transformer, i plan to only run 3-6 pedals off this supply (only tuner, distortion, delay at the moment), and i was wondering what an appropriate secondary current rating would be? I figured the more the better, but i have already have an enclosure that i think would be suitable, and the larger transformers are just a fraction too physically large for the inside, so i was wondering if a smaller transformer, say 150ma, would be cutting it too fine? and would a 12v secondary give the 9v reg enough headroom to operate smoothly?
Secondly, regarding fusing, the transformer's ive been looking at are "thermally fused", is this likely to be a one shot kind of deal? it didn't say anything else and the staff at my local dick smith are fairly usless. if it is a resetting fuse, does that mean that i only have to worry about fusing to prevent fires and not damage to the transformer? and if so, would a 500ma slow blow be appropriate?

I have the feeling i'm probably cutting it all a bit too fine, but hey, i will be the first to admit that im cheap and want to minimise costs as much as possible.

well i think that about covers it, thanks!
-sam

[oldrocker]

According to smallbears website info the fuse is a resettable 50ma hold.  I think a 150ma 12v transformer will work but others with more knowledge on the subject could shed more light I'm sure.  The one I'm using is a 500ma 12v trans.  There are grounding safety concerns if you're mounting a transformer in a metal enclosure.  I was told to use extreme caution. 
I used a LM317 regulator instead of the 78L09 so I could use a trimpot to adjust the voltage from 7 to 12 volts.   I wanted a variable power supply to simulate dying batteries and use it with other electronic projects not stompbox related that used voltages other than 9 volts  I'm going to make another on that is strictly for the pedalboard at 9 volts.

[smccusker]

Thanks for the reply,
i shouldve mentioned that i'm not buying from smallbear, just from my local electronics store. i would post a link to the products i had in mind but the website appears to be down, and anyway really the only information the website gave was a lovely colour photograph. With regards to mounting the transformer, should i put some kind of insulation between the transformer and the enclosure? or were you just talking about the possibility of a line lead shorting out accedently?

[the_random_hero]

Thanks for the reply,
i shouldve mentioned that i'm not buying from smallbear, just from my local electronics store. i would post a link to the products i had in mind but the website appears to be down, and anyway really the only information the website gave was a lovely colour photograph. With regards to mounting the transformer, should i put some kind of insulation between the transformer and the enclosure? or were you just talking about the possibility of a line lead shorting out accedently?

You could use something like a toroidal that has a physically smaller profile, while reducing any potential EMI problems that may arise. They generally have an output current that's a fair bit bigger for their size than the standard type of transformer. I think 150mA would be cutting it a bit fine for some effects - my Octobooster draws around 90mA by itself, not including the LED.
What I did was use a plug-in 12VAC 500mA transformer that I connect to my power supply through a 2.5mm socket/pluf (so I don't ever get confused between that socket and the one for daisy-chaining). 12VAC is plenty enough for running a 7809 - 12VAC rectified becomes approx. 16.9VDC, after rectifier losses you get around 16VDC. The 78xx series only really requires a few volts headroom to work properly.

[smallbearelec]

Hi--

Even though I sell a transformer that would be adequate for this job, I have to discourage you from trying to roll your own in the way you describe; it's just too easy to create a lethal shock or fire hazard. A far safer, easier way for a beginner to get a good result is to start with a wall wart that puts out 9 volts DC at 200 ma. or so. These things already have overcurrent protection built in, though they are noisy. Then build a metal box that contains a better filter cap and a 7809 regulator, and suitable jacks for input and output. Other people here can direct you to suitable tested builds.

Regards
Steve Daniels