9V power supply questions

[Lizard King]

I'm going to build a power supply.  I've reviewed various schematics and they don't seem all that difficult.  (Yes, I've worked with AC voltage and safety aware.)

Two question:

1)  What amperage do I need for 10 -15 effects?  I always figured 100mA each which would work out to 1.5A - which seems to be the max on many of the voltage regulator chips.  Then I read an article on small bear that said 200mA was sufficient for 10-12 pedals.  I'd just as soon go with the lower current but don't want to starve my pedals.

2) If I use a resettable fuse do I get a fuse that matches my max current?  If I go with 1.5A do I get a 1.5A resettable fuse? 

Thanks.

[R.G.]

1)  What amperage do I need for 10 -15 effects?  I always figured 100mA each which would work out to 1.5A - which seems to be the max on many of the voltage regulator chips.  Then I read an article on small bear that said 200mA was sufficient for 10-12 pedals.  I'd just as soon go with the lower current but don't want to starve my pedals.

It depends on what the pedals are. Some pedals use as little as 1ma; 20ma is a decent guess for a simple transistor distortion with an LED. Some digital pedals can use 250-500ma all by themselves. There are compilations of pedal power use on the net that I've seen, but measuring your pedal current drain seems like a good way to go.

For perspective, I know of a commercial power supply that can supply 1.7A of 9V. It (accidentally) was running nearly 200 pedals in one test I found out about. This tells you not much except that pedals vary in how much current they need.

2) If I use a resettable fuse do I get a fuse that matches my max current?  If I go with 1.5A do I get a 1.5A resettable fuse? 

That depends. Most regulator ICs are internally current limiting and will refuse to supply more than their maximum current. What is the fuse for?

[Lizard King]

That depends. Most regulator ICs are internally current limiting and will refuse to supply more than their maximum current. What is the fuse for?

Thanks.  That answers my questions very well.  I pretty much have the information I need now!

[PRR]

> I always figured 100mA each

Battery powered pedals, 100mA drain you would not last one night. And you can do a lot of muck-up with just a few mA. So that class of pedal hardly counts.

A few boxes suck a battery fast. That may be dozens of mA.

And there's stuff that is really for wall-power, any battery clip is just a token or emergency only. These may be 20mA to 2,000mA.... if they don't tell, and you can't test, it's down to wild guesses.

If it gets warm, it is "significant" power. 9V at 100+mA is 1 Watt which is perceptable warmth in something as small as a pedal.

[smallbearelec]

2) If I use a resettable fuse do I get a fuse that matches my max current?  If I go with 1.5A do I get a 1.5A resettable fuse?

As R. G. notes, this depends on how you design. IC regulators will limit the output current under short-circuit conditions, but they will not stop the transformer/regulator/filter from pumping current and possibly overheating the transformer. I built my Small Warts like wall warts, without fusing in the primary circuit, so I needed a way to protect the transformer. A resettable fuse does this nicely. Fuse the primary circuit instead? Yes, If your proposed transformer draws sufficient current. Small ones may not draw enough--even when the output is shorted--to pop even a really tiny fuse.