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This application brief
from National Semiconductor inspired
this design. Their Boomer line of power amps is nice in that they've
got bidirectional drivers built in but I wanted something with more power,
so I used an LM2877,
a dual 4W power amp(8W total) instead. Yes, it's discontinued, but I
had a few handy, and a nearly equivalent part is still available as the
LM1877.
This is how the circuit works: There are actually two amplifiers, one inverting, one noninverting, both with a gain of 2. As vref changes, the output of one amp increases and the other decreases, and connecting something across the two outputs lets you drive power through the load, in this case a peltier effect device, in both directions. The + input on the lower op-amp is actually connected to a potentiometer in the circuit I built, because it turned out to need some fine-tuning to get both amplifiers to work in the same range, as in, when one amplifier is maximized the other is minimized and vice versa. The 10 ohm resistors are current limiters, and also convenient for measuring how much current is being driven: 100ma of current means exactly 1v on the meter. This seems to work pretty good so far, driving 100ma through the peltier device for 30 seconds and chilling the target by several degrees. Tests at higher power will come later. |