0
\$\begingroup\$

I have an analog voltage input of 0-5V DC, and need a corresponding output of (IN + 5V) 5-10V.

My first thought is to use the +5V regulator to "step-up" the voltage directly, connecting the input to the ground of the regulator and using the regulator's output as my final output.

I have a +15V power rail that I'm using to power the 7805, but when I try to hook it up, the 7805 pulls its ground (the input) up to almost 10V. Is it bad form to use voltage regulators in series? I'm modeling my input using a variable PSU for now, so could it be a problem with current not allowed into the PSU?

UPDATE: The problem is definitely that the PSU will not accept back-current, as suggested in the answer by Oli Glaser below. I would love to use a voltage follower op-amp to buffer this current, but I don't know if I have/can obtain any op-amps that can survive that much current (I can't remember exactly how much, but I know I measured sink current in excess of 100mA when the regulator was connected directly to ground). Would a voltage follower transistor design be more robust in that way?

\$\endgroup\$
10
  • 2
    \$\begingroup\$ Can you show us your circuit please? \$\endgroup\$
    – Andy aka
    Commented Apr 10, 2013 at 8:25
  • \$\begingroup\$ Sure -- what's the best method for me to draw it? \$\endgroup\$
    – VruNix
    Commented Apr 10, 2013 at 8:26
  • \$\begingroup\$ Any method that allows us to understand what you are describing \$\endgroup\$
    – Andy aka
    Commented Apr 10, 2013 at 8:36
  • \$\begingroup\$ Please draw your schematic in any tool of your choice (paper napkins and pencils have been used more than once on this site), and upload the image to any public image hosting site. Then post the link to the image here as a comment, and someone with editing rights will incorporate the image into your question pretty quickly. \$\endgroup\$
    – Anindo Ghosh
    Commented Apr 10, 2013 at 9:12
  • \$\begingroup\$ Your description does not make consistent sense. A FULL word description of what you are trying to do would be good. Vin range. Vout range. Why. Providing a circuit of what you are doing is a good idea BUT if we do not know what you are wanting the circuit to do the circuit diagram alone will not be enough. An LM317 will probably do what you want better. ||| I have a voltage source of xxx volts. I wish to obtain an output of yyy to zzz volts. I wish to vary the output by kkk. I ... \$\endgroup\$
    – Russell McMahon ♦
    Commented Apr 10, 2013 at 9:33

1 Answer 1

Reset to default
6
\$\begingroup\$

If I understand you correctly, you are trying to adjust the output voltage of your 7805 regulator to output 5V - 10V by adjusting the voltage at it's ground pin between 0V and 5V.
If you are doing this, the pin needs to be able to sink current in order to maintain regulation, so whatever the power source used, it needs to be capable of this. Many supplies will only source current, and if this is the case what you are seeing will occur.

A simple resistor divider with potentiometer can be used, or an opamp to drive the 7805 ground pin is maybe the best solution, preferably with a output including its negative rail if you want to go all the way down to 5V. You will find many example adjustable supply circuits on the web, and in the datasheets of the regulators also.

Here are a couple of example circuits:

Pot adjust, from this page with more info:

Pot adjust

Opamp adjust, from datasheet pg.24 (don't use a 741 if you can possibly avoid it, as they are obsolete):

Opamp adjust

\$\endgroup\$
11
  • 2
    \$\begingroup\$ Well done ! :-). Your interpretation makes good sense but escaped me. I can usually see what a question means and I'm surprised that I failed to in this case. Clearly, the smell of green = 6. \$\endgroup\$
    – Russell McMahon ♦
    Commented Apr 10, 2013 at 10:48
  • \$\begingroup\$ @Russell - :-) It puzzled me too for a while, but after a few reads through, I figured this was the most likely interpretation. Hopefully I was right, and the OP can confirm this or update the question as necessary. \$\endgroup\$
    – Oli Glaser
    Commented Apr 10, 2013 at 13:04
  • 1
    \$\begingroup\$ Now that sir is true engineering! You found a solution that makes sense in a situation when the requestor didn't or couldn't state the problem clearly. \$\endgroup\$
    – placeholder
    Commented Apr 10, 2013 at 15:11
  • \$\begingroup\$ +1, and I don't even need to say why! \$\endgroup\$
    – Anindo Ghosh
    Commented Apr 10, 2013 at 16:44
  • \$\begingroup\$ You've just about nailed it on the head, thank you. The problem was definitely the sink current on the PSU. I can't quite fix it in the way you explain, though. My control needs to be a voltage signal, not a physical control like a pot. I'll upload a better description (picture) of the problem once I get access to my library's scanner -- I don't have a camera. \$\endgroup\$
    – VruNix
    Commented Apr 11, 2013 at 3:01

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.