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We can make an enhancement to DAC drive fan controllers with the use of an external amplifier that closes the loop around the tachometer. Essentially linear control over the full range of 8 bit DAC codes from 0 to 255 is provided by this enhancement. This only requires a little signal conditioning and a suitable amplifier arrangement since the tachometer is already there. Figure a shows us that the tachometer pulses are conditioned such that all pulse width information is removed (differentiated) and only pulse rate information remains. Fan speed or frequency is represented by this pulses. Than, this pulse is used as the feedback to an integrating motor control amplifier. To provide pulses just shorter than the duration of the shortest tachometer pulse at full fan speed, the time constant of the differentiator is initially set. Usually, this will provide a very little gain. We can increase the gain by decreasing the value of either R2 or C1. We have to set the gain so the fan just achi...

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This is the schematic diagram of DC motor speed controller circuit. The circuit applies two oscillators/timers which are connected as a Pulse Width Modulator (PWM). The timer chip which applied in this circuit will be an nmos dual timer/oscillator NE556. This timer IC has two 555 timers in a single 14-pin IC package. One 555 (IC1:B) is set up as an astable oscillator. The output frequency of the trigger pulses is specified by: f = 1.44 / ((R3 + 2R4)C2), or about 410Hz. The time period for the high output is specified by T HIGH = 0.69(R3 + R4)C2 seconds. And, the low output by T LOW = 0.69R4C2 seconds. The 2nd 555 (IC1:A) is set up for Pulse Width Modulation. It will be build in monostable mode. It is triggered using the continuous pulse train from the first 555 timer. Nevertheless, by also applying a DC voltage to pin 3, the comparator reference levels are going to be modified from their nominal levels of one-third & two-thirds of the supply voltage. This has the effect of modu...

UC3844 is popular current mode controller which is commonly found on DC-to-DC converter circuits and switch mode power supplies. This motor speed controller is also based on UC3844 and it is specifically design to drive 20V – 24V DC motors. In this given configuration UC3844 produces (approx.) 50kHz to 240kHz PWM output and this range can be adjust by changing the value of C2 timing capacitor. As per the datasheet UC3844 is capable to produce PWM output frequency up to 1MHz. [ ]