18V AC Regulator

[lumstar]

I'm building a 18V+/- AC power supply and am looking for + and - 18v regulators like the LM7818 and LM7919. Are these suitable for AC or are they DC only?

[mshilarious]

Originally Posted by lumstar

I'm building a 18V+/- AC power supply and am looking for + and - 18v regulators like the LM7818 and LM7919. Are these suitable for AC or are they DC only?

DC. What are you building the supply for?

[lumstar]

For my mixer. Seems it will be a 2 month wait and over $50 to bring one in from Behringer. For some reason they went with an AC input and have the rectifiers on the mixer itself. Weird. I might just construct a DC power supply and bypass the on-board rectifiers since I can't seem to find any AC regulators suitable.

[mshilarious]

Originally Posted by lumstar

For my mixer. Seems it will be a 2 month wait and over $50 to bring one in from Behringer. For some reason they went with an AC input and have the rectifiers on the mixer itself. Weird. I might just construct a DC power supply and bypass the on-board rectifiers since I can't seem to find any AC regulators suitable.

Is this like a wall wart type supply or something? The reason why they would do an 18VAC supply is they could then use a voltage multiplier to generate 48V for phantom, and at the same time since the supply to the mixer is low-voltage it doesn't have to be UL listed.

Does it really need to be regulated AC then? I'd be surprised if the Behri supply was doing that. AC voltage regulators often use a switching mechanism on a multitap transformer. I bet a regular ol' 18VAC transformer would do the trick.

[dementedchord]

Originally Posted by mshilarious

I bet a regular ol' 18VAC transformer would do the trick.

i'm with ya there... the likelyhood that behr would do an AC regulated supply is well nil...

[lumstar]

Actually that makes perfect sense to me. Except the power jack has 3 pins +18V, -18V and ground. How will I get a -18v from the transformer?

[dementedchord]

Originally Posted by lumstar

Actually that makes perfect sense to me. Except the power jack has 3 pins +18V, -18V and ground. How will I get a -18v from the transformer?

you'll need one that is dual 18V and is center tapped... the center tapp is the ground ...

[mshilarious]

Originally Posted by lumstar

Actually that makes perfect sense to me. Except the power jack has 3 pins +18V, -18V and ground. How will I get a -18v from the transformer?

OK, this is no longer making sense. You can't have negative AC. This sounds like a DC supply to me IF it is actually labeled -18V. AC would usually be labeled with two pins as "~" and ground.

If somehow they really just mean two AC legs plus ground, then do what dementedchord said.

[kubeek]

Originally Posted by mshilarious

OK, this is no longer making sense. You can't have negative AC. This sounds like a DC supply to me IF it is actually labeled -18V. AC would usually be labeled with two pins as "~" and ground.

If somehow they really just mean two AC legs plus ground, then do what dementedchord said.

You can´t have negative AC amplitude, but you sure can have AC with opposite phase.

My behringer PSU realy is a center-tapped 2x18v transformer.

[mshilarious]

Originally Posted by kubeek

You can´t have negative AC amplitude, but you sure can have AC with opposite phase.

My behringer PSU realy is a center-tapped 2x18v transformer.

Yes, but polarity would be a function of the rectifier circuit, not the incoming AC. That's why I don't understand the negative voltage label.

[dgatwood]

Originally Posted by mshilarious

Yes, but polarity would be a function of the rectifier circuit, not the incoming AC. That's why I don't understand the negative voltage label.

Dude, it's Behringer. They probably pretty much the design from somebody else anyway, so asking for correct labeling might be too much to ask....

[kubeek]

Originally Posted by mshilarious

Yes, but polarity would be a function of the rectifier circuit, not the incoming AC. That's why I don't understand the negative voltage label.

No.
Imagine two separate secondary windings A and B on a transformer. Now if you connect the start of A with start of B, the voltage between the ends of the windings will be zero, beacuse they are in phase, so the voltages substract.
Now if you connect start of A with end of B, the voltage between the ends will be twice the voltage of one winding, because they are 180° out of phase and they add up.

Is that more clear?

Cheers

[mshilarious]

Originally Posted by kubeek

Is that more clear?

Cheers

Clear as mud. I am talking about this particular Behri PSU and trying to figure out what is up with it. You are now talking about a dual secondary, not just a center tap. Even so, that dual secondary cannot be labeled negative or positive. I mean you can take a dual secondary and wire it as a center tap, but what exactly is your point? How does this help the OP?

[kubeek]

I was trying to say that polarity is not only a function of a rectifier, but that AC voltagel has polarity too.

I don´t think the outputs are actually labeled + and -, but rather 2x18. I will have a look later at my behringer supply, but don´t think even behringer would write that.

[dgatwood]

Originally Posted by kubeek

I was trying to say that polarity is not only a function of a rectifier, but that AC voltagel has polarity too.

No, it doesn't. Not until it has been rectified (unless you use... say a -110VDC feed as your zero reference or something similarly nuts). Polarity is defined as the direction of current flow, which means it has to always be on the + or - side of zero volts. AC, therefore, by definition, has no polarity because it keeps swinging to opposite sides of zero. It has phase (defined as a delay to the arrival time of the waves), but not polarity. What you are describing as polarity is really phase.

[kubeek]

I think this is slipping to another flamewar about polarity vs phase. Let´s get back to behringer.

[lumstar]

Sorry - let me clarify. This was a mistake in my post. I was confused because of a 7815 (+15v dc regulator) and 7915 (-15v dc reg) on the board itself, completely separate from the external power supply (but part of the onboard power circuitry).

The end result is that the external power supply is a centre-tapped 36V (at 23W btw) transformer as you both suggested. I've ordered the 161H34 which will run 2*17v @1.4A (24w) which will be fine to power the 7815 and 7915.

Thanks for the suggestions guys!

PS: Labelling on the board is, as kubeek assumed, not particularly helpful.