Dimmer From 10V to 5V

[LukeKnepp]

I need a 10k potentiometer to dim my application from 10V to 5V with a 10V input.

So far, this works in LTspice, but not in real life...

It only dims from 10V to about 2.5V...

Do I just need a larger resistor in series with the potentiometer?

 

[JimB]

So far, this works in LTspice, but not in real life...

In that case, tell us about "real life".
In what way does real life not work?
What are you using Vout+ for?


JimB

 

[LukeKnepp]

I have a fan that runs on 5V low and 10V high. I want to make it adjustable between those two voltages.

 

[JimB]

Your simulation completely ignores the current drawn by the fan motor.
A simple resistive voltage divider is not a good solution for adjusting the speed of a motor.
What is the power/current rating of the motor?

JimB

 

[LukeKnepp]

The motor runs at about 250 mA.
Should I put in a current or resistor load?

 

[alec_t]

Have you considered a PWM motor speed controller such as one of these?

 

[LukeKnepp]

I want to try and design my own, but I may have to consider that.

 

[rjenkinsgb]

As a simple modification for a fairly low current load, you could add an NPN darlington transistor as an emitter follower after your potentiometer circuit, to give it higher current capability.

The pot wiper would connect to the base, +10V to the collector and the emitter connects to motor positive.
Also connect a diode such as a 1N4000 series (4002, 4004, 4007 etc) across the motor output from the circuit, with cathode (band) to the transistor emitter and anode to 0V. That protects the transistor from any inductive spikes (back EMF) that the motor may produce).

The output voltage will be around 1.2V - 1.5V lower than the pot voltage, so you may need to compensate with the supply voltage.

The transistor will get warm (or hot) and needs a heatsink.
A TIP120 is readily available NPN darlington which should be suitable.

That pretty much gives you the same circuit as used in many simple model train speed controllers - eg.

http://www.sumidacrossing.org/LayoutElectricity/ModelTrainPower/DCTrainControl/files/kato-pack2.jpg

That has an AC power supply included and a reversing switch in the motor connections - but the actual "speed control" part is a potentiometer and darlington transistor...

 

[crutschow]

Below is a circuit, similar to what rjenkinsgb suggested, except it uses a Sziklai pair instead of a Darlington to reduce the output drop voltage at the maximum pot setting from two base-emitter drops to one.
The simulation shows a maximum output of 9.29V with a TIP32 transistor.

 

[LukeKnepp]

This is working great!!!!