Digitally selected outputs for LM317

[Guest Zeppelin]

I was thinking of using digitally selected outputs for the LM317 regulator. IC's Datasheet has the circuit attached which uses discreet transistors; though for a large number of outputs this is not practical :(
Is there anyway to get around this without using discreet transistors?

Thank you
Zeppelin

[audioguru]

Hi Zeppelin, the resistors need something to drive one with 10mA, so a Cmos output won.t be strong enough.
Think about driving the transistors with a binary group of signals, the 4 transistors would give 16 voltages, 5 transistors would give 32 and 6 transistors would give 64 voltages etc.

[gogo2520]

Hello Zeppelin
  I built a power supply like that, it uses a logic switch to control the voltage. I got a PDF file of the project.
                      check it out
                        gogo

power.pdf

[windoze killa]

You could use something like a UNL2003 darligton driver chip. It has 7 outputs. Would give you more combinations than you could ever need.

[audioguru]

You could use something like a UNL2003 darligton driver chip. It has 7 outputs.

Darlington transistors don't saturate so have a pretty high voltage drop when turned on. A transistor array IC would be better because a transistor can saturate with a very low output voltage. Then the current in the voltage-adjustment resistors is predictable.

[windoze killa]

We current use ULN2003 chips for a very similar application and they work very well.

[audioguru]

We current use ULN2003 chips for a very similar application and they work very well.

What about temperature compensation?
The turned-on voltage would drop with increasing temperature, wouldn't it?

[windoze killa]

To be honest we haven't noticed it or paid much attention to it. Being in R&D we tend to work on the premis that if it works, leave it alone or if it aint broke, don't fix it.

We came up with the design and it seems to work well. When we have trouble we will investigate more

[audioguru]

We came up with the design and it seems to work well.

Good. I just didn't want somebody to make it with a darlington array then discover that it has a problem with temperature change when the output voltage is set low. An ordinary saturating transistor array won't have a temperature problem.

[audioguru]

Five binary bits will give you 32 different output levels. A bit's output is a transistor with a voltage-setting resistor in series with its collector.

[audioguru]

You are making your own D to A converter. Look it up in Google.

[audioguru]

The LM317 is the summing amplifier.
The D to A converter just gives it some resistors for its ADJ terminal to ground.