Q: split +/- 9v from +30vdc

[km-r]

can i use this configuration:


30VDC ----- 7818 --------------------------------- +9v FX
                    |       |                         
                    |       |                             
                    |        |----- 7809 ----------------- GND FX
                    |                    |
                    |                    |             
0VDC ---------|----------------|------------------- -9 VDC FX


has anyone tried this?

[Cliff Schecht]

It's usually a bad idea to run linear regulators in series, it absolutely KILLS your efficiency. This may not sound like a bad idea if you're running everything off of wall power, but your linears are going to get really hot by trying to drop so much voltage. For example, 30 V - 18 V = 12 V. Multiply this by 100 mA of current (12 V x .1 A) and you get 1.2 Watts dissipated just in your linear, which is pushing the limit of even the TO-220 packages without a heatsink of some sort.

Aside from this, your schematic has a fundamental flaw in it. To get a regulated negative voltage, you need to provide an unregulated negative voltage. You don't get negative voltage from a negative linear regulator by running on a positive rail, you just get a lot of smoke .

I'll draw you up a better way to go about doing this in a second.

[Cliff Schecht]

So for a 12 V AC input (NOT DC!!), you get a regulated plus and minus 9 V output. Simple enough, just make sure your power supply can provide enough current for your application. Also when this method is used, it's a good idea to keep the power ground and signal ground completely separate and only connect them at one point.

Edit: Really, a 9 V AC adapter will work just fine - the rectified DC voltage just needs to be over 2.1 V to turn on the regulator.

[km-r]

very neat! thanks dude! thats exactly what i was looking for. i was gonna use a 30VDC switching psu but nevermind... ill get an AC transformer for that...

but half-wave rectifying the transformer will need big caps? will this be not much of a noise problem especially in high gain circuits?
ill overspec the transformer voltage up to 12vac.

[Cliff Schecht]

With proper filtering and a grounding scheme like I described, you should have very good (and low noise) results.