Help with -15V converter

[Trzalica]

Hi guys,

I have (actually trying to make) a PSU for pre amp that have +/- 24V. Since this pre doesn't need a lot of power I am going to use it for the small RIAA pre as well. For the RIAA part I need +/- 15V. For the positive branch I used R-7815-0.5 (with some capacitors as well). Now I need to do the same trick for the negative branch but I can't find a simmilar DC/DC converter. Friend of my mentioned 7915 but I can't find it.

[agdr]

Your friend was probably referring to the LM7915, a linear negative 15V regulator

http://www.national.com/profile/snip.cgi/openDS=LM7915

or the smaller 100mA version LM79L15

LM79L15 - Series 3-Terminal Negative Regulators

which are the complements to the LM7815/LM78L15 linear positive regulators.

I'm not clear on what you are trying to do. What is the input voltage (from your transformer or DC wall brick to the regulator) and output current requirement on your first +/-24VDC preamp, and what are the same for your second +/-15VDC preamp. Is your goal to drop the +/-24VDC from the first down to +/-15VDC for the second? If so there are (adjustable) LM317/LM337 dual rail boards like this all over the place

MINIATURE DC POWER SUPPLY KIT BASED ON LM317H LM337H! | eBay

You could just leave off the diodes and feed the +/-24VDC to the regulators directly. You might not want the board in this link though... "[the regulators] are second hand". Or easy enough to wire those LM7815/LM7915 up on a small perfboard.

[Trzalica]

Thank you agdr,

I was trying to find a matching Recom product and they do not have any, that is why I was stuck. I'll order LM7815/LM7915 pair and this will resolve my problems.

[Trzalica]

Wiped, duplicated B-(

[simon7000]

If the output floats on a DC-DC convertor all you do is ground the + output and it is then a negative voltage supply.

[agdr]

Except that the Recom he linked is non-isolated. Couldn't feed the positive and negative rail converters from the same voltage source (and hook positive V- to Negative V+) without using an isolation transformer somewhere. Check out the first application example on page 1-5 in the data sheet

http://www.recom-international.com/p...R-78xx-0.5.pdf

They are following the Recom with a second isolated 1:1 converter to make dual rails. That is another way to do it - just use an isolated dual rail converter to start with, like these

http://www.murata-ps.com/data/power/ncl/kdc_ndtd.pdf

NDTD2415C Murata Power Solutions DC/DC Converters & Regulators

Feed it with 24VDC and get +/-15VDC out. But given that +/-24VDC already exists that may not be very cost effective at $20 for 100mA per rail (if 100mA is even enough). There is also a minimum load of 25mA per rail. The linear regs might be the easiest way to go. If using that dual isolated DC-DC converter the "24VDC" input is actually 18-36VDC so he could likely feed it off the input to that +/-24VDC regulator rather than attaching it to the positive +24V rail, keeping things balanced.

[Trzalica]

Actually, on the original solution there wasn't any type/brand of converter so I picked a Recon one, and was, incorectly of course, under impression that it will work for both positive and negative branches. I also ordered two of them and THEN friend warned me about my mistake.

[agdr]

Easy mistake to make! Ebay here they come.

Actually that dual rail isolated converter would help with heat dissipation. With the linear regulators you will get a watt or two of heat dissipation depending upon your current draw. The converter is around 80% effecient. But a lot of folks don't like DC-DC converters for preamps out of noise concerns. The murata units are rated at 60khz - 620khz so shouldn't interfere with audio, but for a pre-amp linear is a solid way to go.

[jaiaditya4u]

since u have both +/-24 volts u must be having gound as well.... u can use lm137..it is a 3-terminal negative voltage regulatoranc can be used with -24v and ground..!! its has a great output voltage limit from -1.2v to -37volts...so for u it will surly work..

[agdr]

Well look what I found. Turns out you CAN use those non-isolated Recom's to produce dual rails, essentially doing what simon7000 said, but with a bit of a twist. Unfortunately the negative rail converter configuration winds up limited to 18VDC maximum input, so it wouldn't solve your problem, and the maximum current output drops to 200mA. But here is the info anyway just for reference...

In the top block of the data sheet for your Recom it says "see application note for use as an inverter (alternative to LM79xx)", but that app note is MIA. Can't find it on Recom's site or even via Google. The Recom app note that does come up for the part says nothing about the negative rail issue.

Turns out Cutter has an excellent write-up on that R-78XX-05 series on their website

Cutter Electronics

and at the bottom is a link to the missing negative rail app note!

http://www.cutter.com.au/prodimages/...tion-Notes.pdf

The two diagrams on the second page of the note are the ones. The diagram at the top of the page shows how to hook up your R-7815-05 as a negative regulator (7915 replacement). The at the bottom of the page shows the two R-7815-05s hooked up off the single supply to produce the +/- 15V rails.

Two 10uF caps are added on the negative rail converter as shown, and the input voltage and output current gets de-rated on that rail as per the chart.