Control an LM317T with a PWM signal
Set a regulator's output voltage under microprocessor control.
Aruna Prabath Rubasinghe, University of Moratuwa, Moratuwa, Sri Lanka; Edited by Martin Rowe and Fran Granville -- EDN, February 3, 2011
The LM317T from National
Semiconductor is a popular adjustable-voltage
regulator that provides output voltages
of 1.25 to 37V with maximum 1.5A
current. You can adjust the output voltage
with a potentiometer. The circuit in
Figure 1 replaces the potentiometer
with an analog voltage that you can
control from a PWM (pulse-width-modulation)
signal. You control this
signal with a microcontroller or any
other digital circuit. You can use the
same microcontroller to dynamically
monitor the output and adjust the
LM317T.Using an RC lowpass filter and an op amp, you can convert the PWM signal to a dc level that can adjust the LM317T’s voltage output. Varying the pulse width of the input signal lets you generate an analog voltage of 0 to 5V at the output of the lowpass filter. The op amp multiplies the voltage to achieve the desired voltage range.
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You can improve the circuit by replacing the RC lowpass filter with an active filter and then feeding a feedback signal from the circuit’s output into the microcontroller for dynamic adjustments.
Talkback
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guys please learn to think outside the box.You should know that LM317T has input voltage upto 37v. You can use more than 12V of course.So if you want to get the range mentioned in example of this circuit,you have to apply higher input voltage.That is common sense.He doesn't have to explain everything.He gave us the idea.we need the a brain to use it.
paul - 2011-1-3 21:48:47 PST -
hi
it says like LM741 i belive what he meant is op amp with null offset adjustment.its obvious you need to use rail to rail op amp.therefor i belive what he meant is rail to rail op amp with null adjustment.LM741 is used tell about null adjustment feature.
APR - 2011-1-3 20:24:05 PST -
To use LM317T instead of the Darlington transistor is not a new but is not a bad idea, but an implementation and claims about achieved parameters definitely are not right by the following reasons:
1. Suggested OP Amp LM741 does not have rail to rail input and output and could not work well with input signal at least from 0 to 2.5V.
2. LM317T is not low drop voltage regulator and required at least 2.5V headroom at 1.5A. So with 12V power supply as shown on Fig.1, claimed the maximum output voltage could not be higher that 9.5V
To make this solution useful and matching to the author claims needs to replace Op Amp LM741, to increase input voltage to 15V and to connect resistor R4 instead of Vadj terminal of LM317T to the output.
Vladimir Doubovis - 2011-28-2 23:16:09 PST -
Why use a pwm signal too vary output of an LM317T?
Are you guys for real? Because it allows DIGITAL control of the 317!! AND active modulation of output voltage. So basically you get a modulated digital power converting 1.5A dc/dc converter for cheap that relies on radio shack available cheap parts. with an arduino or pic microcontroller ic you then have an agile infinitely variable (within the range and resolution limits) 1.25 to 37 VDC 1.5A microcomputer controlled PWM modulated power supply that is programmable using a couple dollars and maybe 10 components. That's why
ndrew Lamb - 2011-19-2 01:16:16 PST -
I guess he want's to show everybody that he has the tater's to cough up something like this.
stomper b. quick - 2011-17-2 14:04:18 PST
