Quick Power Supply Question

[kurtlives]

Building a little power supply for my pedals...

After the AC is rectified there is a 100mF filter cap, then a LM7809 regulator, then a 470mF filter cap.

Now I could add a 100R resistor in series with the V+ after all that. After the resistor I could also add a little (100nF) RF filter cap.

Do you think those two additions (resistor and RF cap) are worth it and make sense. Just making sure, think I know the answer.


Thanks...

[tempus]

What are you hoping they'll accomplish? The resistor, if nothing else will limit your current availablilty to 9v/100 = 90mA. How many pedals you need to power?

[kurtlives]

I guess 100R is a bit big, most resistors in pedal PSU I see are 47R.

I am using 4 200mA transformers. Going to have 8 DC output jacks.

[petemoore]

  Kind of boils down to what non-DC is there to filter [after a Vregulator and it's output cap], and what kind of 'spikes' does the circuit create, sometimes a little filtering right at the point where current draw rises or falls sharply [such as an LFO chip] will take out the last little bit of 'line spikes' between an otherwise smooth Vreg and the load circuit.

[head_spaz]

It depends on how much voltage drop you have between the transformer's output and the regulated output.
A resistor will help reduce ripple. But only if you place it in front to the regulator. And if that's what you aim to acheive, you should place it between your first cap from the rectifier and the second cap.
You do not want to use unnecessary resistance after the regulator unless you're specifically trying to regulate the current of that resistance, rather than voltage.

A dropping resistor in front of the regulator will also lower the supply's output impedance, but might be necessary if your transformer output is too high for the safe operation of the reg chip.

If your regulator is so noisy that you need more filtering afterwards, then your regulator is not doing its job.
It's better to use lots of capacitance in front of the reg, and very little afterwards. Often times circuits have too much capacitance after the reg, and that allows the reg to lose focus, and that makes it sluggish. The less interference, the better.

And in the situation where you might have a 9v transformer, and expect a 9v output after regulation, you're gonna need a LOW DROPOUT regulator.

[Darkness, Darkness]

When working on the power supply for my tube project, I came accross this power supply filter on the AMZ website. Maybe you'll find it helpfull : http://www.muzique.com/lab/filter.htm

[biggy boy]

I used this calculator to find the right values for an R C filter to help with the 120Hz ripple from the transformer.
The ripple feq is twice the power supply Freq, in our case 60 Hz x 2
I put it before the reg.

http://www.pentodepress.com/calculator/RC-ripple-filter.html

[biggy boy]

I used this calculator to find the right values for an R C filter to help with the 120Hz ripple from the transformer.
The ripple feq is twice the power supply Freq, in our case 60 Hz x 2
I put it before the reg.

http://www.pentodepress.com/calculator/RC-ripple-filter.html

A good example of it's application is found in the Real Mctube Schematic.

http://www.dogstar.dantimax.dk/tubestuf/graphics/mctube2l.gif