I am building a bench supply. I have a 36 volt 9 amp supply that I was going to use as a voltage source. But it doesn't have negative voltage. I came across this circuit for creating negative voltage:
So that's good. I can use fixed voltage regulators for many voltages but it would be nice to have an adjustable voltage as well. So I am looking at this from a data sheet for the LM79xx:
Here is the link to the data sheet:http://www.hep.upenn.edu/SNO/daq/parts/lm7915.pdf
and this is the part I am having some difficulty with:
Look at the bottom of the page. There is a set of values for R2 at different voltages. Using these values I need to find R1 but the numbers I am getting are impossible. So I suspect I don't grok what Vset is and the only place you will find Vset in the data sheet is right there. I am using 36 volts for the supply, that is what I thought Vset would be, and I want a negative 15 volts out. How is this done? I keep coming up with a negative value for R1 and unless things have changed a lot I don't think you can get a negative resistance potentiometer.
The circuit is a voltage divider where the voltage on R2 is a set voltage (whatever the regulator's fixed output). So if you're using the -5V supply, Vset is -5V.
Run the number with Vset at -5 volts and that is what you want Vout to be and you get zero for your resistance of R1. Run the numbers you will see what I mean.
joseph_m:
Run the number with Vset at -5 volts and that is what you want Vout to be and you get zero for your resistance of R1. Run the numbers you will see what I mean.
Huh? Why would you be building a variable output circuit when the fixed regulator already has the voltage you want. Of course you get 0ohm, because you wouldn't need a divider to -5V out of a -5V supply.
joseph_m:
[quote author=Runaway Pancake link=topic=103413.msg775646#msg775646 date=1335572725]
Sorry, joseph_m, but that circuit, as drawn, is dead in the water.
Well that's helpful. Want to elaborate on that? It came right out of the data sheet.
[/quote]
The first circuit. Where did you get it?
The first circuit will (might) output +15V with respect to its GND on the +15V side, OR -15V wrt GND on the -15V side, but there isn't any way that +15V will be 30V above -15V output, which is also necessary for a bipolar supply.
Really ?