# Increasing input signal by 5V (Using 7805 voltage regulator)

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?

• Can you show us your circuit please? Commented Apr 10, 2013 at 8:25
• Sure -- what's the best method for me to draw it? Commented Apr 10, 2013 at 8:26
• Any method that allows us to understand what you are describing Commented Apr 10, 2013 at 8:36
• 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. Commented Apr 10, 2013 at 9:12
• 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 ... Commented Apr 10, 2013 at 9:33

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: