Bipolar power supply related question

[JebemMajke]

I have this transformer with a 34,9VDC output.

I was wondering if I could use this diagram



To get +24v and -24v out of it?

I have 7824 and 7924 ready and also I have a plenty of heatsinks.

An additional question would be if this works, will I be able to add 7818 and 7809 after 7824? With heatsinks, ofc.

[GGBB]

No. That design requires an AC supply.

[JebemMajke]

Perhaps this is a stupid question, but isn't the output of a transformer AC, until it is connected to a rectifier bridge, which changes it to DC?

[JebemMajke]

And to answer my question, yes it does provide ac until it goes through a rectifier bridge. And I just measured it, the result is 23,8VAC

Going back to my question

Will this be enough to supply +24VDC and -24VDC?

[Nasse]

Long time ago I did this kind of rectifier and regulators for small circuit, worked ok. But perhaps you should Look regulator datasheets for min/max input for wanted output. Was it 35 volts max for 8 and 12 voit regulators but perhaps too much heat If The voltage is near max

[GGBB]

And to answer my question, yes it does provide ac until it goes through a rectifier bridge. And I just measured it, the result is 23,8VAC

Going back to my question

Will this be enough to supply +24VDC and -24VDC?

Probably, but it depends on exactly what type of VAC that is. It's probably RMS. If it's RMS or average, yes. If it's peak or peak-to-peak, no. It's unlikely to be peak-to-peak since it's used in a 34.9VDC adapter. If you can find a part number somewhere on the transformer itself (not the adapter) then you can probably find a data sheet online for it.

[mth5044]

Perhaps this is a stupid question, but isn't the output of a transformer AC, until it is connected to a rectifier bridge, which changes it to DC?

Most of the time, but in your OP you stated DC. I imagine GGBB thought you may have been talking about a wall transformer / wall wart or something? Like he said, part number will help, but if you're measuring 23.8VAC, you should get 33.7VDC when rectified, which is below the stated input max of 35V for the 7824.

Don't know why it wouldn't work. The 1000u seems like overkill. The datasheet states the two lower input and output caps should be 0.33u and 0.1u. 

[PRR]

> 23,8VAC

Raw 33V DC (+/- if wired that way).

Some of the popular regulators are 35V max. You are very close to the edge. I have run stuff that way. Do not know if I have been lucky (good parts, no line surges).

[JebemMajke]

Thnx

The reason for that kind of power supply is I want to have multi JFET preamps, all run at 24 v, and i found one +24 and - 24 eq that i want to place after these preamps, before the poweramp section.



And maybe I ll add one of those TDA poweramps or 3886 after that, run with unregulated 33Vdc

[mth5044]

I think they are using 24V in that as it may be the power supply found in the rest of the project, or they needed a lot of headroom. Looks like a gyrator setup with transistors, similar to the one found at GEOFEX (5th pic down, but the whole article is worth a read). I've made a similar schematic work fine at +/-9VDC for a synth, so unless you NEED the huge headroom, I would think you could run it off +/-15V without breaking a sweat?

Edit: GEOFEX Link May have helped: http://www.geofex.com/article_folders/eqs/paramet.htm

[Rob Strand]

Don't know why it wouldn't work. The 1000u seems like overkill. The datasheet states the two lower input and output caps should be 0.33u and 0.1u. 

BTW the datasheet requires the 0.33u and 0.1u so it doesn't oscillate. 
The datasheet assumes DC is at the input of the regulator.
The 1000u is definitely required as it is part of converting the AC to DC.
So the 1000uF serves a completely different purpose to the 0.33u and 0.1u.

[JebemMajke]

I think they are using 24V in that as it may be the power supply found in the rest of the project, or they needed a lot of headroom. Looks like a gyrator setup with transistors, similar to the one found at GEOFEX (5th pic down, but the whole article is worth a read). I've made a similar schematic work fine at +/-9VDC for a synth, so unless you NEED the huge headroom, I would think you could run it off +/-15V without breaking a sweat?

Edit: GEOFEX Link May have helped: http://www.geofex.com/article_folders/eqs/paramet.htm

Lately, i've been fixing many cry baby pedals and some eq's. And they all suffer from the same problem, there is always some distortion to them.

So my goal was making a pristinely clean eq.

But if 9v or 18v one will be clean as well, i am all ears

As for JFET based preamps, I always pushed mine ( Dr boogie and similar ) with 18v and all of them sounded more open, natural. My guess is 24v will be either better or just as good.

[mth5044]

Don't know why it wouldn't work. The 1000u seems like overkill. The datasheet states the two lower input and output caps should be 0.33u and 0.1u. 

BTW the datasheet requires the 0.33u and 0.1u so it doesn't oscillate. 
The datasheet assumes DC is at the input of the regulator.
The 1000u is definitely required as it is part of converting the AC to DC.
So the 1000uF serves a completely different purpose to the 0.33u and 0.1u.

Perhaps my wording was misleading; I wasn't saying replace the the 1000u with the 0.33, just saying the 1000u value seems high where possibly a 470u could suffice. The 0.33 and 0.1 was a separate comment.

[R.G.]

@ JebemMajke
It would help you with power supplies a lot to go read

http://www.geofex.com/Article_Folders/Power-supplies/powersup.htm
Where a lot of this is presented.

[Rob Strand]

erhaps my wording was misleading; I wasn't saying replace the the 1000u with the 0.33, just saying the 1000u value seems high where possibly a 470u could suffice. The 0.33 and 0.1 was a separate comment.

Sorry for the misunderstanding.

[mth5044]

erhaps my wording was misleading; I wasn't saying replace the the 1000u with the 0.33, just saying the 1000u value seems high where possibly a 470u could suffice. The 0.33 and 0.1 was a separate comment.

Sorry for the misunderstand.

The fault is on me! I wasn't clear.

[JebemMajke]

@ JebemMajke
It would help you with power supplies a lot to go read

http://www.geofex.com/Article_Folders/Power-supplies/powersup.htm
Where a lot of this is presented.

Thank you so much.

It was very helpful. I should study more about this.

I always wondered about the diodes. I saw people using 6 or even 8.

What is the benefit of more or fewer diodes?

So +24 and -24 are not possible from this one

But +18 and -18 are. Ok, at least that.

[antonis]

I always wondered about the diodes. I saw people using 6 or even 8.
What is the benefit of more or fewer diodes?

If you refer on series diodes, other than raise total maximun reverse voltage, none..
(same for parallel and forward current sharing..)