Actually depending on your hotside cooling system, you may lose CPU cooling as you take the TEC voltage above 80% of Vmax.
TEC power consumption goes up roughly as the square of the voltage, while the TEC has already reached about 90% of it's cooling capacity at 80% of Vmax.
The added temperature differential you can get across the TEC by taking the voltage above 80% of Vmax, is generally offset by the increase in hotside temperature due to the extra heat the hotside cooling system must remove.
Putting a resistor in series isn't an efficient way of reducing the voltage to the TEC, but at least the power dissipation in the resistor doesn't have to be cooled by the TEC's hotside cooling system. An additional heatsink in some existing airflow path can do the job.
A better way to go, would be to put a high efficiency adjustable DC to DC converter between the PSU and the TEC and adjust it to the voltage that gives optimum cooling, but I'm not going to explain how to do that here.
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