+ / - variable Power supply perf layout

[RLBJR65]

Sometimes it's much easier to perf. Thanks Dragonfly, ROG and Smallbear.
Perf layout http://aronnelson.com/gallery/album08/VariablePS_Perf
Schematic http://aronnelson.com/gallery/album08/VariablePS?full=1

OK, I have built this and it is working great Little noisy but OK for testing.

Couple of Qs though.
I noticed that other similar projects call for a transformer with say a 25 - 0 - 25 voltage rating. Why? What are the advantages? Is it more stable that way? I just used a 40V transfomer no CT

This schem listed 2 - 220uF caps in paralell for each side I have seen these caps listed anywhere from 1uF to 4700uF what in particular determines the choice of caps?

Also instead of using a dual ganged pot this schem specifically called for 2 seperate pots? I guess so you could use pos. side only or neg. side only.

The way it's layed out I get + 1.22 to + 24.9 volts from pos. out to "ground" - 1.25 to - 24.8 volts from neg. out to "ground" and 30 volts from pos. out to neg. out.

Any help answering my Q's would be greatly appreciated.

How does the layout look? Is it easy enough to understand?

Thanks.
Richard

[RLBJR65]

Bump

[Nasse]

Are bridge diodes oriented correctly in the circuit diagram or no Nice circuit anyway if it works

I believe dual pots have too much mismatch, much easier to dial it just to the desired value with multimeter

[CS Jones]

Hey Richard,
I figured Niftydog would have jumped on this. He's written a lot in the past about power supplies and the relative nature of ground etc. Search his posts for "ground" and "power supply" etc. He and R.G. have written up some great info.

Here's a mixed bag of scattershot, probably useless or redundant information from the top of my head.


The bridge rectifier you're using lets you get a little more efficiency out of the supply and you can build a PS with a transformer whose secondary is just a single wind with no CT. The bridge setup lets you charge the top part of the cap with 2 pumps per cycle. This helps when the load (whatever is hanging off the end of the supply lines) starts to increase and more electrons are pulled away and put into use. This is where the whole "sag" deal comes in with heavy loading PP amps and their supply demands. As the cap is charged and your load kicks in the voltage gets pulled down while waitng for another shot of juice from the transformer's pulsing action. However much pulldown happens is the "sag". This is as against a "stiff" supply which will have enough capacitance to better fill the voltage falloff as the load kicks in. Your supply is electronically regulated and the loads are so small that it doesn't really apply. It would be real tough to load down a, say, 250 mA available supply line with a typical pedal setup (assuming you weren't using it to supply the current for boatload of switching relays or something). As far as I know no one wants power supply fluctuations in their pedal effects.

I don't know why they have used the two filter caps like they have in that setup.

General rules of thumb when selecting cap values would be...
Voltage rating  - 1 1/2 times what your transformers putting out per side. You've got up to + and - 25v available I think. 50v cap values would be plenty. Doesn't hurt to up this if you can get the higher ones for the same price and space isn't an issue.

Capacitance - 1uf per mA of current drawn under load for decent filtering. Again, increase it even more if the cost is the same and you want even better filtering. Check out the values used for the circuits you've already built to see what values were chosen on those. I overfilter everything. The stiff vs. sag thing is for the amp guys to worry about.
Again, we're using electronic regulation anyway. Check the spec sheet on the 317 for the input current requirement which allows it to provide a stable, non varying output and what the load limits are.

The 2 separate pots will allow you to power up the lines assymetrically if you wanted. Some famous pedals are set up like that for whatever reason. Klon, Korg PD-01, Maxon OD820 among the ones I've seen. Or you could just charge down the negative side say, and isolate it out to run you PNP effects while running your plus side effects at different voltages. It just makes the supply a little more versatile.

Nasse, is right. Your diodes aren't oriented correctly.

[toneman]

Retrace all your wiring with schematic.
Look at each connection/junction and determine how many things should B tied together.
Use a yellow felt pen to highlite the schematic as U verify each junction.
good luck
T

[RLBJR65]

Thanks guys!

CS great explanation! I think I understand how it works now without the ct. 1uF per mA, didn't know that, so if the notes on the schem are correct and the PS is good for 850mA it could use more filtering if maxed out. The schem. I based this on was hand drawn old and stained, the more I look at it the caps may have been marked 2200 with no uF designation That should be plenty of filtering!
BTW I used 63v 220uf and 50v for the rest of the caps so I'm good there but should note that on the layout too.

I do have the W02 bridge in correctly, just forgot to mark it on the layout
As I understand it the 1N4004's are protection diodes for the regulators (not part of the bridge) added in case of short circuit or reverse current spike, see bottom pg. 8 top of pg.9 http://cache.national.com/ds/LM/LM117.pdf probably not really needed but better safe than sorry I suppose.

I'm going to use it as a bench supply for testing, just need to get some gauges and jacks.
Oh yeah and bigger filter caps.

Think the layout is easy enough to undertand?

Thanks again Richard

[RLBJR65]

Duh, My apologies guys, I see that you were referring to the way the rectifiers were drawn on my schem. 
I’ll revise and post corrections ASAP.

Thanks again!
Richard

Edit: Schematic corrected and posted! Rev 1.1 12/16/2005
http://aronnelson.com/gallery/album08/VariablePS?full=1