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BASIC4MCU | 하드웨어 | OPAMP | PWM Control

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작성자 키트 작성일2017-08-25 11:23 조회1,866회 댓글0건

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Quick jumps: LM324 PWM Controller – Nomad's 555 PWM Controller – Simple 555 PWM Controller – LM324 PWM Controller MkII

Yet more Variable Electronics...

PWM Control

Duty cycle waveforms

Pulse-width modulation control works by switching the power supplied to the motor on and off very rapidly. The DC voltage is converted to a square-wave signal, alternating between fully on (nearly 12V) and zero, giving the motor a series of power "kicks".

If the switching frequency is high enough, the motor runs at a steady speed due to its fly-wheel momentum.

By adjusting the duty cycle of the signal (modulating the width of the pulse, hence the 'PWM') ie, the time fraction it is "on", the average power can be varied, and hence the motor speed.


Advantages are,

  1. The output transistor is either on or off, not partly on as with normal regulation, so less power is wasted as heat and smaller heat-sinks can be used.
  2. With a suitable circuit there is little voltage loss across the output transistor, so the top end of the control range gets nearer to the supply voltage than linear regulator circuits.
  3. The full-power pulsing action will run fans at a much lower speed than an equivalent steady voltage.

Disadvantages:

  1. Without adding extra circuitry, any fan speed signal is lost, as the fan electronics' power supply is no longer continuous.
  2. The 12V "kicks" may be audible if the fan is not well-mounted, especially at low revs. A clicking or growling vibration at PWM frequency can be amplified by case panels. A way of overcoming this by "blunting" the square-wave pulse is described in Application Note #58 from Telcom. (a 58k pdf file, right-click to download). I've tried this, it works, but some of advantage #3 is lost.
  3. Some authorities claim the pulsed power puts more stress on the fan bearings and windings, shortening its life.

How It Works


PWM waveforms

An oscillator is used to generate a triangle or sawtooth waveform (green line). At low frequencies the motor speed tends to be jerky, at high frequencies the motor's inductance becomes significant and power is lost. Frequencies of 30-200Hz are commonly used.

A potentiometer is used to set a steady reference voltage (blue line).

A comparator compares the sawtooth voltage with the reference voltage. When the sawtooth voltage rises above the reference voltage, a power transistor is switched on. As it falls below the reference, it is switched off. This gives a square wave output to the fan motor.

If the potentiometer is adjusted to give a high reference voltage (raising the blue line), the sawtooth never reaches it, so output is zero. With a low reference, the comparator is always on, giving full power.


A Practical PWM Circuit

PWM circuit with op-amps
LM324 pinout

LM324 pin connections (top view)

This uses the LM324, a 14-pin DIL IC containing four individual op-amps and running off a single-rail power supply. The sawtooth is generated with two of them (U1A and U1B), configured as a Schmitt Trigger and Miller Integrator, and a third (U1C) is used as a comparator to compare the sawtooth with the reference voltage and switch the power transistor.

Rather than have the fourth op-amp sat there doing nothing, it's used as a voltage follower to buffer the reference potential divider. The high input and low output impedance of this draws very little current from the PD, so high value thermistors can be used in the thermal version of this controller.


Here's the very neat saw-tooth wave coming from the output pin on U1B. Frequency is about 130Hz with the components as shown, with the amplitude swinging between 3.5V and 9.5V on a 12V supply.

Sawtooth from U1B pin 7

The reference voltage system is designed to apply a level ranging from 3V to 7.5V to the comparator. At 3V the fan is getting power all the time, at 7.5V about 30% of the time (when the sawtooth wave goes over 7.5V). Below is the pulse power applied to the fan at the minimum setting, which was just enough to keep my test fan spinning.

Fan Power on minimum setting


Construction


324PWM stripboard
Parts List
U1LM324 quad op-amp, 14-pin DIL socket
Q1IRF530 n-channel mosfet (see below)
R1, R2, R6100k (all resistors 0.25W 5% or better)
R356k
R4, R547k
R768k
R8(see below)
VR1100k 16mm lin PCB pot
C133n mylar film or ceramic disc
C2220u 16V radial electrolytic
C3100n ceramic disc
C44u7 16V radial electrolytic (kick-start option)

Construction Guide – Component side view

 
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1                        
2            Q1(g/b) R8         
3 Fan–          Q1(d/c)           
40V   R5  C3    Q1(s/e)   j7 C2(–)  R7  
512VFan+j2   R4C3          C2(+) R6   
6                        
7   R2 R1  U1#1(br) U1#14j4j5     (br) R7 VR1(a)
8j1   R5   U1#2(br) U1#13 j5          
9   R2R3   U1#3(br) U1#12    C4(+)    VR1(w)
10  j2     U1#4(br) U1#11

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