HELP PLEASE: Variable_Voltage Source

[Nillsy]

Hello Everyone, 

I'm new to the forums...So hopefully somebody may help me out with this.

I'm doing a variable voltage source (DC/0-24V) with some friends as a final project. So far we have been able to do the simple design in Multisim. 

I'm using the LM317AH voltage regulator. As you probably already know, the output voltage goes from 1.25 to 30 V. In order to eliminate the 1.25V. We decided to use a Negative voltage source, with a Zener Diode. The Negative V. source is using a LM137H regulator, and we are having some issues with the simulation. We are not quite sure about what is wrong. 

I'll attach the project as we have it...Thanks 

[NetoNI]

Hello 

Could you please give me a more detailed explanation of what is wrong with the simulation?

Regards. 

Ernesto. 

[Nillsy]

Hello Ernesto

We are task to do a Variable voltage source as a final project. And we are having trouble with one the requirements of the source:

It needs to go from 0 to 24 V. We are using the LM317 voltage regulator wich provides a minimun of 1.25 volts with its adjust resistence at zero.

Somehow we need to eliminate those 1.25 volts in the circuit. So far we have been able to design the source with the 1.25 volts. We aren't quite sure how to do this. Is there any way you could help us out with this?

We ask our professor for some advice and he tolds us about using a Zener as a "negative voltage regulator" by desinging a negative source in order to solve this issue, yet we are not completely sure how is use it? 

I'll attach, both of the design we have been able to do: the varibable source (1.25 to 24V) and the "negative source"  that we have came up so far.

Thanks in advance for your time and help.

Regards,

José

[johnsold]

Jose,

I do not have Multisim so I cannot see your circuits.

Why not use a simple -5 V regulator like 7905 to create the negative voltage. Then put a fixed resistor in series with the adjustment potentiometer so that the lowest voltage at the LM317 adjust terminal is -1.25 V?  You could trim it to adjust for the actual reference voltage (1.25 V +/- 0.05 V) of the device used.

Lynn

[Nillsy]

Hello Lynn, 

I have a question about your suggestion. In order to use the -5V regulator (7905) means I have to make another source right? or is it possible to do it in the same circuit with LM317?.

In case we are making another source, we are not sure how to connect it to the main circuit (LM317)

I'll attach screen captures of the desing.

Thanks.

José