I am working on a simple audio mixer project. The op-amps I am using (LM741) take +9V and -9V inputs. I am currently getting this by wiring two 9V batteries together, connecting + on one to – of the other, and grounding the pair. This seems to work, but I would now like to switch to a power supply. I can find a bunch of 9V power supplies, but they all only have the one polarity. Is there an easy way to get +9V from a center-negative power supply? I figure I probably need an inverter circuit or something, but I don't really understand what I'm doing.
Electronic – +/- 9V DC Power Supply
power supply
Related Solutions
Are you sure your routers take 1.5 amps at 120v? It seems far more likely to me that they use 1.5 amps at 12v or less, which means your AC power strips only have to supply say 0.2amps per router.
The condition to consider for this to happen is a load beyond the rating of the supplies in question. Most severely, a short circuit. A short circuit need not be permanent and can even be a part of normal use: A motor, when starting, or a capacitor, when being charged from 0 V, looks like a short circuit, too.
To explain how a negative voltage can appear on one supply's output, let's use a diagram with some arrows. Supply A's output voltage shall be called VA, supply B's output VB, and the voltage across the load shall be called VL.
With KVL:
\$V_A + V_B = V_L\$
This is true for normal operation, when we want to use, for example, two supplies with VA = VB = 12 V for a load that needs VL = 24 V:
\$12V + 12V = 24V\$
For a short circuit, VL = 0 V, and KVL still applies. Thus, \$V_A + V_B = 0\$ or
\$V_A = -V_B\$
If we want to avoid negative voltages on any of the two supplies, the only possible solution is
\$V_A = V_B = 0\$
However, because of \$V = R \cdot I\$, and because any real power supply has a non-zero internal resistance, this also means that no current must flow. Now, a power supply will try to deliver current. Since the current or power limit of the two supplies will not be exactly equal, the "stronger" supply will be able to maintain a small, positive output voltage, and the only way to solve the KVL equation is by back-feeding the other supply, with a negative voltage across its output.
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Best Answer
Assuming at least one of the power supplies is isolated (which will be true if they are good power supplies, but maybe not if they are cheap), you can make a stack of two supplies, so you have outputs at 0 V, 9 V and 18 V. Then, connect your op-amp circuit like this:
Another way to think about this is to understand that ground (0 V) is a relative choice; you can change your choice, as long as you're consistent within one circuit.
One other note-- I should explain what I mean by "isolated." A cheap power supply will have its ground pin connected to the neutral wire of its power cord. Most of the time, that's fine. But if you want to stack supplies, you need an isolated supply, i.e. one where no output pin is connected internally to the power cord.
If you do try to stack non-isolated supplies, you'll be connecting the positive output to the neutral wire, which is the same as shorting out the supply, so its internal fuse will blow. (If it's a really cheap supply, this will destroy it.)
One other solution
If you can't lay your hands on an isolated supply, you could use a switched capacitor voltage converter. This is a chip that charges up a capacitor, and then quickly shifts the pins so what was previously the positive lead is connected to ground, and the old ground becomes a negative output. It does this charge/switch routine at 10-100 kHz, so to humans, it looks like a negative supply.