Building Dual 18 V Power Supply Question

Hello All,
I have been a audio engineer for about 15 years and decided just to get into electronics about a year ago and could use some newbie advice.
I am trying to build a power supply that I saw online for the under the DIY Links, 'SSLtech's KPS-1' I belive it is.
My really dum question is this. How should I measure the rail voltages comming off of the +, -, and ground. Right now the multitester read 18.5 from the + rail and 16.9 from the - rail. Should these not be of equal power. Also I hear people talking about testing the power supply under load, I assume this means while its supplying power to the device, correct? Should I use my ground as reference when testing the + voltage or the - voltage. Again I know these are stupid questions but I could use some help.
Also one last thing I have noticed is that all power supplys use 2 pairs of small capacitors on the inputs and outputs of voltage regulators. much smaller then the smoothing cap. I know how to figure out the correct capacitor size needed for the first smoothing cap, immediatly past the rectification, but I cannot find no information on obtaining correct capacitor sizing for these regultors. Also I notice they vary between 100NF, .01 Uf, 10 UF, etc.. The only information that I can find is that they help keep the regulators stable. Again any knowledge on these would be appreciate. Thanks in advance for helping a very green newbie.

Todd

Thanks for the info. I am using 7818 and 7918 Regulators. I did double check the transformer and all connections seem solid.
I dug up some research that you can use a 100 ohm resister to test a power supply under load but have never done this. Any body have any info on this? Do I hook the resistors up in series from the output rails?
Again thanks for putting up with my lack of electronics knowledge. I am digging through every book and website I can, I just need a little proffesional guidence along the way.
After testing it again the postive output is stable at 18.5 but the negative output is running around 16.5, that concerns me, they should be running at the same even not under load correct?

Do you have enough voltage headroom going into the regulators? They need to have a certain minimum number of volts going into them in order to get a stable output. I think the positive regulators don't need as much headroom (I could be wrong though), which would explain why you're only having trouble with the negative rail.

Cheers,
--
Don

You guys are good! That was the problem! I only had a 36 Volt (18_0_18)Transfomer powering it. I switched to a (24-0-24) and now they are all running at the regulated power. I should of guessed that was the issue because I read you need at least 3-4 volts higher then what your regulator is running at, and it makes sense because I was only using 18 Volts on the transformer to power the 18 volt regulator. I also read you loose 1.7 volts in the recitfier diodes as well.
I would still like to learn more about the pair of capacitors on the input and output of the voltage regulators (usually 100NF, but I have seen people using different voltages). This is the only part of a power supply that I cant explain its functions, nor if I was scratch building one would I know what rating to use, unless all regulators use 100NF, or 0.1uf caps. I am assuming these function like the Ripple cap, but to a lesser degree?

Thanks again! You guys have made my day!

The 24-0-24 might be a little high, I usually wouldn't go over 20-0-20...

As long as you're not drawing too much current from the supply, you'll be fine, but if you try and draw too much with a 24-0-24 the regulatore will get warm...

As long as they don't get too hot, you'll be fine though.

Keith

And the DC voltage from the 24-0-24 transformer may be higher than the 35V max. Have you measured the input voltage on the regulators?

Best regards,

Mikkel C. Simonsen

> I notice they vary between 100NF, .01 Uf, 10 UF, etc..

Read the chip-makers' data-sheets.