Voltage reducer

[dave99]

I want to make a voltage reducer that takes out around 25% of the voltage at any time.
So if the input voltage is 5v the output will be 3.75, if its 4v the output will be 3v, and if its 3v the output will be 2.25v and so forth.
Any simple way of doing this, this is for a map sensor in my car. I thought of using zener diodes but they simply clamp the voltage and thats not what i want.

 

[Hero999]

This can be done using an LM317 which can be run from a 12V supply.

I still don't understand why you want to do this, please provide more information.

 

[Leftyretro]

I want to make a voltage reducer that takes out around 25% of the voltage at any time.
So if the input voltage is 5v the output will be 3.75, if its 4v the output will be 3v, and if its 3v the output will be 2.25v and so forth.
Any simple way of doing this, this is for a map sensor in my car. I thought of using zener diodes but they simply clamp the voltage and thats not what i want.

A simple resistance divider or even an adjustable pot could do that. We probably need to know more about the source and load impedenaces to actually give you a proper recommendation.

Lefty

 

[RadioRon]

Here is some info about MAP sensor.
https://autorepair.about.com/cs/troubleshooting/l/aa042603a_06.htm

I'm just guessing, but it may be best to assume it is a relatively high impedance output. I'd be tempted to just use a voltage divider, perhaps a 100K ohm linear pot.

Alternately, two large value resistors followed by a buffer amp might be enough.

 

[dave99]

Other than the info RadioRon posted i wouldnt be able to tell you much more about the 5v map sensor.

Leftyretro: wouldnt an adjustable pot simply clamp the voltage rather than reduce it by a certain amout at all voltages?
RadioRon: can you give me an example of the buffer amp approach? Not too familiar with this kind of stuff

 

[dknguyen]

Leftyretro: wouldnt an adjustable pot simply clamp the voltage rather than reduce it by a certain amout at all voltages?

Uhhh...no. It does exactly what you want. If you don't know what a resistive divider is wiki it. A pot is just a resistive divider where you can vary the ratio. I'm not sure why you think it would do the same thing since you mentioned zener diodes earlier (and assuming you knew they should be used with a resistor so that the current doesn't burn out the zener and also gives the extra voltage somewhere to drop across since the zener only drops the voltage it's clamping to and the rest of the voltage has to go somewhere), I'm not sure why you think two resistors would so the same thing as a zener + resistor.

It has crappy current capability though and the ratio is affected by your load impedance/resistance...which leads us to...

RadioRon: can you give me an example of the buffer amp approach? Not too familiar with this kind of stuff

THis is just a resistive divider that runs through a buffer which uses analog circuitry to "read" the voltage of the resistive divider using a amplifier with very high input resistance/impedance so that the ratio of the divider alone isn't screwd up and outputs the voltage getting around the current capability problem (for signal levels, not power levels). An op-amp wired as a buffer would do the job.

That's why we need to know the current you need- whether you just need the voltage as a signal or to power something. You can also wire adjustable linear regulators to do the same thing (and of course use very fancy complicated monitoring and power circuitry to do the same thing) for higher power (ie. higher current for a given voltage) capability. But they have minimum and maximum voltage limits.

 

[Hero999]

Google potential divider and non-inverting amplifier.