Power Regulation Circuit

[talktomrgibson]

hi all, so i've successfully made a 9v regulated circuit using a 7809 regulator and an input of 12v and 1.5A

my question is, how do i know how many regulated 9v circuits i can run off this input voltage?  i have a friend that is running three, but i'm wondering what determines the maximum? is it all based off the amount of Amps the pedals draw? so once i get a draw of 1.5A i have maxed out the number of 9v outputs?

for example if i have 3 9v circuits available but one pedal takes 500 milliamps and one pedal takes 1amp, it makes the third redundant because i've used the 1.5 amps available at the original 12v wallwart.....??

thanks for any light you can shed on it for me
Nic

[Seljer]

Look at the datasheets . Those regulators are rated usually rated for 1A or 1.5A but only if you have heatsink on it!

The 7809 works by taking a higher voltage, and dropping it down by converting all the excess power directly into heat.

You can do some back of the envelope math to figure out how much power you're dropping over the regulator:
If the input voltage is 12V and the output voltage is 9V thats a 3V drop. 3 volts x 1amps = 3watts of heat that have to be dissipated in the regulator. You can then go looking in the datasheet for thermal conductivity specifications (its in degrees celsius per watt). The TO220 without a heatsink can typically soak up about 1.5watts without overheating. So without a heatsink, you're probably on good for up to 500mA from your circuit.

To figure out how much current capacity you need you need its probably best to measure actual consumption (e.g.: if the official Boss adapter says 500mA it doesn't mean that the circuit it powers will actually draw all of those 500mA). Most pedals use only a couple of 10mA at the most with larger circuits with lots of digital stuff and clocks going up to a couple of 100mA.


And yep, you're still limited by the 1.5A of the primary power supply.