I am trying to design a PIC-programmable pwer supply. I'd like to select by
software any voltage between 1.5V and 15V. PCB space is VERY small.
So I decided to use a Dallas DS1804 8-pin programmable potentiometer managed
by a PIC 12C509 or similar to drive a LM317L (100mA is enough). The problem
here is that the DS 1804 can't stand more than 5V on ANY pin, including the
potentiometer pins. So, I can't use it to directly drive the LM317L.... It
follows that I have to use a transistor or FET to convert levels. While I
have no problems with digital circuits, I have little experience with Analog
ones, so I don't exactly how to do this.
I've done something similar once, but I can't find the schematics right now,
but if you want me to, I will do a little digging in my archives.
Its a little dificult for me to see the details in your ascii drawing, so
maybe you could post a better drawing, and I might be able to help you
At 09:50 AM 99.07.22 -0300, you wrote:
>Hi,
>
>I am trying to design a PIC-programmable pwer supply. I'd like to select by
>software any voltage between 1.5V and 15V. PCB space is VERY small.
> ( ... stuff deleted ...)
I've done something similar - not exactly a programmable supply, but a PIC
controlled voltage regulator.
What I did was build a simple voltage regulator using an op-amp and a
series pass transistor. The reference voltage for this type of circuits is
normally taken from a Zener diode or a reference IC, and a pot is used to
sample to output voltage and feed it back to the op amp. (This is basically
how the 317 works, and you are talking about having the pic control the
resistor in the sampling circuit.)
What I did was have a D/A (MAX522) driven by the PIC (you could use a PWM
technique instead, I suppose) and used that to provide a variable reference
to the regulator.
Space: the D/A is an 8-pin dip, as is the op amp. The series pass
trasnistor is whatever you need to carry the load. A couple of resistors
and capacitors and away you go.
If you wish to pursue this idea, send me E-mail, and I'll e-mail you a
diagram.
Larry
Larry Bradley
Orleans (Ottawa), Ontario, CANADA
>I am trying to design a PIC-programmable pwer supply. I'd like to select by
>software any voltage between 1.5V and 15V. PCB space is VERY small.
Hi Andres, what is the resolution required? 0.05V per step? 0.50V?
>So I decided to use a Dallas DS1804 8-pin programmable potentiometer managed
>by a PIC 12C509 or similar to drive a LM317L (100mA is enough). The problem
>here is that the DS 1804 can't stand more than 5V on ANY pin, including the
>potentiometer pins. So, I can't use it to directly drive the LM317L.... It
>follows that I have to use a transistor or FET to convert levels. While I
>have no problems with digital circuits, I have little experience with Analog
>ones, so I don't exactly how to do this.
Unless u'd want to vary the output voltage in small steps my preference is
to "pre-program" several output voltages you can select from through an
array of resistors from ADJ pin of the LM317 and selectively connecting
them to ground through a transistor. (It's shown in the applications
section of the datasheet)
Set R1=240R and R2=2.7K for 15V as the default and maximum output.
Q1 turns on to parallel R3=9.1K with R2 = 12V
Q2 turns on to parallel R4=4.7K with R2 = 10V
Q3 turns on to parallel R5=1.0K with R2 = 5V
Q4 turns on to parallel R6=390R with R2 = 3V
That would give you a useful range, now if you were to pick your resistor
values carefully, you could parallel R2,R3,R4 = ~8.8V
In all, u could have 2^4 = 16 presettable voltages.
Weather the output voltages are useful to you is something you'll have to
figure out by playing with the resistor values. (Something i don't want to
do right now... *grin*)
Lastly, u might need to configure a soft start for the LM317 or use an
output pin from the PIC12C508 to enable the output.
If everything was SMD, it wouldn't take up much board space. Hope this helps.
>
> >I am trying to design a PIC-programmable pwer supply. I'd like to select by
> >software any voltage between 1.5V and 15V. PCB space is VERY small.
>
> Hi Andres, what is the resolution required? 0.05V per step? 0.50V?
>
>
> >So I decided to use a Dallas DS1804 8-pin programmable potentiometer managed
> >by a PIC 12C509 or similar to drive a LM317L (100mA is enough). The problem
> >here is that the DS 1804 can't stand more than 5V on ANY pin, including the
> >potentiometer pins. So, I can't use it to directly drive the LM317L.... It
> >follows that I have to use a transistor or FET to convert levels. While I
> >have no problems with digital circuits, I have little experience with Analog
> >ones, so I don't exactly how to do this.
Circuit deleted for brevity.
One last thing. I use 7406/7407 Open collector hex inverters/buffers for
selecting the resistor for the LM317 circuit. Compact and can stand input
voltages up to 30V. Just a touch easier to deal with than raw transistors.
Be aware that a LM317 cannot do 0V because the output is always 1.25V above
the input. The first time I saw the circuit, in Steve Ciarcia's Intelligent
Serial EPROM programmer in Byte Magazine, he used a transistor with a negative
bias to pull the line to 0V. I've found for PICs MCLR/Vpp that it's sufficient
to limit current from the LM317 output pin through a 100 ohm resistor then
use another 7406/7 buffer to pull the output of the resistor to near 0V.
This is circuit I have on my current home grown 16C84 programmer. Works like
a champ.
smart.com.ar
