# What voltage would you expect in the following diagrams?

#### PeterPiper

Joined Aug 23, 2017
3
Figure 1 - What voltage if I measure between +15 and -15 and what polarity?

Figure 2 - What voltage if I measure between +15 and -15 and what polarity?

Joined Jan 15, 2015
6,312
30 volts in either case and polarity read will depend on on which line the meter probes are placed. Since you are measuring between two defined points the Ground symbol in the second image means nothing. It's a matter of point of reference for your metering device.

Ron

#### PeterPiper

Joined Aug 23, 2017
3
Is there a problem grounding the +15 on either schematic to obtain -30? Or would that even work?

#### MrChips

Joined Oct 2, 2009
24,992
If you ground either +15 or -15 in Figure 2, you are creating a short circuit. If the power supply is not short circuit protected then expect to see sparks fly.

#### Alec_t

Joined Sep 17, 2013
12,448
Is there a problem grounding the +15 on either schematic to obtain -30?
In Fig 1 you can do that providing the supply is truly isolated and you don't ground the original 0 rail. Just treat the +15 as your 'new 0' and ignore the original 0.

#### MrChips

Joined Oct 2, 2009
24,992
Look at another example.
How would you create -18V from two 9V batteries?