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Old 09-06-2013
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You can use an LM431 shunt regulator driving a power transistor to drive several fans

View figure 18 on page 8

The problem with using the 7V trick is that it causes current to flow from the +12V rail to the +5V rail. The circuit generating the +5V rail is only designed to move current in one direction; it is not designed to sink current. If the load from your 7V fans exceeds the total load on the +5V rail, the circuit will be trying to sink current, and will be unable to. Depending on the circuit topology the unit may shut down; or become unstable and malfunction; or fail catastrophically. One or two fans is usually safe enough, but if you seriously want to run a lot of fans at 7V, use a voltage regulator.
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