# Reducing variable voltage

#### Jmoses

Jan 19, 2018
5
I am not well versed in the correct terminology for electronics, but I'll try to explain it as best as possible. Basically I want to have a variable voltage to be reduced to a component but still retain its variability. Say I had a base voltage of 12 variable to 24v. I have a component that I want to read 0v when base voltage is at 12v. Then as base voltage increases to say 18v I want the component to see 6v. What would I need between the power supply and the component in order to achieve this?

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,769
You'll have to subtract 12 V.
A simple and not tooo precise way is using a 12 V zener diode in series with the variable power supply.
Another method is using an operational amplifier to subtract the 12 V.

What exactly do you want to achieve?
What is the current drawn from the variable power supply?

#### BobK

Jan 5, 2010
7,682
Is the output simply a signal, i.e. drawing negligible power, or it is providing power. If the latter, how much current?

I would use an op amp to subtract 12V then, if more than 20mA is needed, use that voltage as a reference in a regulator circuit.

Bob

#### Jmoses

Jan 19, 2018
5
I am currently brainstorming so I don't have specific number, but it would be in the ballpark of 20 or 30mA. I have sensor that needs to see 12v less than what the actual voltage is for it to function properly. Also, I have a bad internet connection right now so replies might be a little slow.

#### Jmoses

Jan 19, 2018
5
I'm not 100 percent on the current, but I would think it would just be signal.

Last edited:

#### Ratch

Mar 10, 2013
1,099
I am not well versed in the correct terminology for electronics, but I'll try to explain it as best as possible. Basically I want to have a variable voltage to be reduced to a component but still retain its variability. Say I had a base voltage of 12 variable to 24v. I have a component that I want to read 0v when base voltage is at 12v. Then as base voltage increases to say 18v I want the component to see 6v. What would I need between the power supply and the component in order to achieve this?
It would be nice to know for sure whether you are referring to AC or DC.

Ratch

Jan 19, 2018
5
Dc

#### BobK

Jan 5, 2010
7,682
20 to 30 mA is right on the edge of what typical op amps can output. I would look for one that has a little more power, like 50mA if you really need 30. If the max need is actually below 20, a very common op amp like LM324 or LM358 would work. You probably want a single supply one, which both of the above are.

Bob

#### Jmoses

Jan 19, 2018
5
Thanks for all the answers, I have a pretty good idea of what to look for now.

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