Transformer Question


Old 02-14-11, 12:10 PM
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Transformer Question

Hi all, new to the site. I have a question regarding a transformer. I have a 120 VAC to 12VAC transformer. I want to take the 12 VAC, run it through a rectifier and choke/capacitor circuit to get ripple free 17 volts DC. I want to then run that 17 VDC through two separate 7812 regulators to get as clean a 12 VDC signal as possible.

One of the 12 volt outputs will be used to power the coils on 12 volt relays that will turn on/open various fans and valves I want to use the second 12 VDC with a voltage divider resistor circuit to run 2.4 volts to various sensors (temperature and humidity mostly), and then monitor it off data aquisition hardware attached to my computer.

Now here's the problem. The - of the 2.4 VDC is at -8.5 volts in reference to ground. The + of the 2.4 VDC is at -6.1 volts in reference to ground. My hardware needs the voltage to be within +2.4 to -0.3 volts from ground. How do I get the 2.4 VDC to be from ground, rather than -8.5? As a test, I quickly attached and then unattached the - of my signal voltage to ground, and that put the + at 2.4 volts above ground, but I am uncertain if that is the correct thing to do. Is it that simple, or am I going to fry things that way?

Please help if you can. This part of the project has me scratching my head.
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Old 02-14-11, 01:25 PM
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Yes. Just "ground" the lead you want to be "grounded". One and only one point in a circuit fed from from the secondary of a transformer can be grounded, or you can leave it ungrounded with results as you have discovered.

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