Votage Divider for virtual Earth with NI PS-15?

[Bife]

Hello,

I have an NI PS-15 24V DC power supply that I would like to use for the NI DAQCard based data aquisition and control system for my servo-hydraulic test machine that I have built (with a lot of help and advice as its not my field at all!)

However, I need -12V, 0V and +12V inputs to power the amplifiers etc.

I thought I could just use two large equal resistances as a voltage divider accross the 24V DC outputs of the PS-15, giving me +-12V and a virtual earth at 0V.

However, when I did this with two 1MΩ 5% resistances and measured the voltages accross them (before connecting to anything else), I get 11.5V accross each one, but 24V across the both of them, which just doesn't add up to me!

I tried measuring Volts between each DC output and the earthed casing, but this gave me 0V for both outputs.

Can anyone explain all this to me please, or a better way to get the virtual earth of 0V between the two DC outputs (which then become +12V & -12V relative to this GND)?

[chuggins1434]

How much current are you requiring?  I only ask because a standard computer power supply has a -12V supply, very little in the way of current, but it is there.

Enjoy!

Chad

[Bife]

Very little, I think under 100 mA

But I used an ATX power supply as the first solution, but these are very noisy & it transmitted through to my signals.

So now I have a  ‘Triple Output DC Power supply’ (Escort EPS-3250 http://www.microtekinstruments.com/showProduct.asp?pNo=176&cNo=198 ), which got rid of the noise problem, but now I'd like to use this NI PS-15 as it's small and unused, I am worried someone will turn up the voltage on the Escort etc.

I just dont understand how its 24V across both resistors, and only 11.5 across each 1M Ohm resistor - where is the missing volt?

Attached is the scheme of my system