Another transformer questions-Dropping 12v to 9v

[theundeadelvis]

I've still had no luck finding the proper 9v power transformer for my SPL meter project, but the local Rat Shack has a couple 12v-12.6v ones that I thought might work. My question is how do I drop the secondary voltages on these to 9v? Thanks again!

[Yazoo]

I've been through this one myself. I think the only real way is to use a voltage regulator like the LM7809 chip. The standard layout uses diodes to convert to DC if the transformer outputs AC on the secondaries, and some capacitors to smooth the current. That's about as technical as I'm capable of! There is a circuit at General Guitar Gadgets for a regulated power supply which works fine.

I came across voltage dividers using resistors and using a Zener diode, but as I understand it, these are only useful for reference voltages. I'm sure someonehere will be able to explain better than I can, but the voltage regulator chip is the best way to go.

[theundeadelvis]

Now you have me wondering if I can use the 12v transformer as is. Right now the design call for 9v secondaries, but they go into a rectifier and then a 7808 voltage regulator. So I'm wondering if i feed it 12v if the regulator will still get it down to the need voltage for the rest of the circuit. I also need to figure out how to implement a 5A mains filter?

[R.G.]

Transformers can only work on AC. Effects and circuits we use in audio need DC. From whatever transformer you have, you need to rectify the secondary AC into DC, then filter it to provide clean, non-humming DC. You will probably also need to regulate the DC down to the exact voltage you need.

People who have read GEO, in this case "Power Supplies Basics", don't need to ask this kind of question; it's all spelled out in detail. I highly recommend it.

[MusicAudio]

The 7808 regulator should work just fine with 12V, just make sure that the DC is filtered well enough after your rectifier.

-mike

[dschwartz]

and just for caution, use a small heatsink for the regulator..
regulators converts the excess of voltage into heat (i believe)

[earthtonesaudio]

and just for caution, use a small heatsink for the regulator..
regulators converts the excess of voltage into heat (i believe)

Regulators get hot by dissipating power (P=I²*V).  So yes, it has to do with the voltage you drop through it, but more so the current the device requires.  If the SPL meter doesn't draw much current, then dropping a couple volts through the regulator won't need a heatsink.

[R.G.]

Power is not the square of the current times the voltage, it's the current times the voltage.

Resistor power is the square of the current times the resistance.

[dschwartz]

well we don´t know how much current it draws, so that´s why i said "for caution" ..
i always use a small heatsink on regulators, and by small heat sink i mean just even a nut and bolt or a small piece of metal the size of a quarter.

[earthtonesaudio]

Power is not the square of the current times the voltage, it's the current times the voltage.

Resistor power is the square of the current times the resistance.

Oh yeah... 

well we don´t know how much current it draws, so that´s why i said "for caution" ..
i always use a small heatsink on regulators, and by small heat sink i mean just even a nut and bolt or a small piece of metal the size of a quarter.

Yes, better safe than sorry.

[theundeadelvis]

Thanks for all the input guys! I was able to hunt down a suitable transformer (TR034). I did look through Geo and assumed the answer was in there somewhere, I just wasn't smart enough to find it. Thanks again everybody! I will be starting a new post on rectifier info, so tune in!

[R.G.]

I did look through Geo and assumed the answer was in there somewhere, I just wasn't smart enough to find it.

11/15/01 Power supplies basics
http://geofex.com/Article_Folders/Power-supplies/powersup.htm