power supply

[axeman010]

Hi

I have searched through the site and can't find the answers to some specific questions on building power supplies, so here goes....

Background:

I have a mixture of negative and positive ground pedals and a Neovibe and I want to build a supply to power them all.

I am sure that I have read here somewhere that you cant run  +ve & -ve  ground from the same supply just by wire the plugs differently. If this is the case please could somebody explain why???

Assuming that to be the case I have come up with the following design:



My main concern is about joining the two secondary coils of the transformer to provide the 15V AC that I need for the Neovibe, which also means that two bridge rectifiers will also be connected – can this be done?

Any comments and help would be much appreciated.

Axeman.

[axeman010]

Hi

I noticed that one of the rectifiers was wrong, so have corrected the schematic.



As before this is just an idea and if somebody could validate or advise of any errors that would be great.

Axeman.

[davent]

Hello,
The 7809 and 7909 I believe need at least 2v more then then their 9v rating in order for them to regulate. With a 7.5 secondary they're only going to see 10.5v so the regulators would drop out of regulation. There might be lower dropout regulators (LM2940) that could do the trick? Will it all work as described, that I'm not sure of.

Take care,
dave

[dschwartz]

those 1000uF  caps before the rectifiers are wrong.. they should go after the rectifier...
and yes.. you should use a 9v center tap xformer. and just use 1 rectifier bridge.. center tap to ground

i think you can use 18V AC for the neovibe, since it has a regulator inside..just check the voltage ratings of the neovibe caps

[kurtlives]

For the +ground side of the power supply the caps before and after the regulator need switching around polarity wise. You seem to have them both wired for - ground.

[dschwartz]

oh i haven´t notice that both regulators are 7809..

you should use a 7809 for the positive supply and a 7909 for the negative supply.

Kurt his caps are ok.. the idea is wrong

look..
you need something like this:
http://www.eidusa.com/Electronics_Kits_P9_N9_Volt_Reg.htm

or like this
http://www.electronics-project-design.com/DualPowerSupply.html

[kurtlives]

The -ground supply should have the regulator on the bottom rail.

[davent]

The 7809 is fine for both the positive and negative supplies, that's taken straight from the the National Semiconductor LM340/LM78XX datasheet.
Page 13 or 14 on the datasheets I've seen. 

http://www.national.com/mpf/LM/LM340.html

Had a chance to look at the Neo-vibe/Univibe document and it shows 14-20Vac or 18-24Vdc for power, so a higher transformer secondary, 18Vct, as mentioned earlier would cover the +/-9v and the Univibe needs.

dave

[axeman010]

Hi Guys

Thank you for all the helpful feedback. It looks like the best solution to my problem is a bi-polar power supply and run the Neovibe from the 18V DC supply.

Please could somebody explain why the for a negative ground pedal this kind negative regulated supply is required. I would really like to understand this one.

Many Thanks

Axeman

[R.G.]

You can't do it that way.

The AC lines feeding a bridge rectifier are NOT isolated. They are alternately connected to first the minus, then the plus side of the output through forward biased diodes.

You CAN do the +/-9V that way, and you don't even need one of the bridges.It has the same effect if you merely make the CT be the center of the output DC voltages. It is anyway, as you have it drawn, but always through two forward biased diodes. You would have to break the connection of the two windings as a CT to do two isolated 9V outputs, then connect the two 9V's together at the output side only to use two 7809s; a 7809 and a 7909 would let you use the CT version.

But you can't do the 18Vac that way. That's because the Univibe/neovibe also has a FWB in it, and the AC sides are alternately connected to signal ground through diodes in that FWB too. So the signal ground between effects will short the transformer through all those diodes.

[head_spaz]

R.G. is 100% correct.
The example shown above is a recipe for making smoke.
Better to use a single bridge, like this :


You could also build dual supplies, separated, using two indivdual bridges,
but you cannot use the dual bridges with a common center-tap.

[head_spaz]

BTW... if you use separate / dual bridges.... you'll lose your VAC tap,
because the transformer secondaries are separated, without a common.