Power supply question?

[cebreez]

I just got most of the parts to build a new pedal board power supply. I ordered 9V regulators rated at 1A and I've been told that as a matter of practice the transformer should have a output voltage of at least 3V higher than the voltage your supplying. Now for the dumb question... The transformer I have is an 18V 2A secondary. Should my voltage regulators also be rated at 2A? I do plan on having at least 9 outputs if that matters.

[bcdon]

For a 1 amp regulator, you'll only get about 110mA for each of the 9 outputs. I'd go with a 2A regulator so you can have more margin.
BTW, where'd you get the design for this?

[jim p]

You will have way more raw voltage (rectified voltage before the regulator) then you need with 18 VAC. 18 VAC will be 25.2 volts peak (1.414 X 18) loose 0.7 for each diode in the rectifier so if you use a bridge that is 23.8 VDC unloaded. With 9 VDC as the output that is approx 14 volts across the regulator, if you have 1 amp as the load that is 14 watts dropped by the regulator. What 9 volt regulator IC do you have? You could add a power resistor in series with the regulator to reduce the voltage across it and its power dissipation. The limiting factor is the power dissipation which may depend on a heat sink used with the regulator and its rating. If the regulator is a 7809 which is a fixed voltage LN317 they do need a 3 volt difference to stay in regulation. As far as the transformer 2 amps is its rating which when used for a DC supply will be lowered by the form factor of approx 1.4 for a capacitor input filter. So that would give you 2/1.4 for 1.4 amps DC as the max load for the transformer. The Hammond transformer catalog gives you voltage and current formulas for different rectifier setups and filter configurations on page11 I think.

http://www.hammondmfg.com/pdf/5C08.pdf

[cebreez]

Thanks jimp, that works. It is in fact a 7809. bcdon I really don't remember where I got the schematic but it is for a regulated supply 9v and 12v. I've also been looking at the ultraclean 9v supply by Rick Barker. The one I'm building now is an ultra simple design just for testing. Transformer, 4 1N4007 rectifier diodes, 2 capacitors, and a 7809 regulator IC.

[jim p]

If that is a TO220 package as I stated above you will have approx 14 watts across it without a heat sink you can only do approx 2 watts by the data sheet (65C/W). With a heat sink the TO220 package is 5C/W junction to case so 14 X 5 for 70C plus room temp of 25C – 30C for 100C for maybe 25C-50C left. So you need a heat sink that is 50C/14 watts for 3.6C/watt. You could add a series power resistor on the input side of the regulator to cut the power dissipated by the 7809
http://www.fairchildsemi.com/ds/KA%2FKA7809E.pdf

[cebreez]

According to the specs the heat sink I ended up buying was 25.9 C / W

[jim p]

Well if you have a load as high as 1 amp and your raw supply into the 7809 is approx 23 volts as calculated above that is (power + current X voltage) (23 – 9) X 1 for 14 watts. With a say 26C/watt heat sink and the added 5C/watt case to junction of the device then add in a little interface loss say 2C/watt for 33C/watt minus 30c for the room temp equal to (125C – 30C) / 33C/watt for 2.8 watts of power dissipation. If you added a 10 ohm 15-20 watt resistor in series on the raw side that would just about work out by dropping 10 volts before the regulator.
Here is a good posting on heat sinks http://sound.westhost.com/heatsinks.htm

[cebreez]

Awesome! Would it be considered good or bad practice to use the one transformer tied to multiples of the rectifier,capacitor,regulator circuit?

[jim p]

Not sure what you are thinking about with multiple rectifier, filter circuits and regulators. Are you just trying to split up the load across more then one regulator? If it is to just split up the maximum power across more the one regulator you may need more filter capacitance but not necessarily more then the one rectifier.

[cebreez]

That is exactly what I was thinking.