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### Author Topic: Voltage Divider Help  (Read 1141 times)

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#### StAnkys

• Jr. Member
• Posts: 15
##### Voltage Divider Help
« on: June 03, 2009, 07:39:03 PM »
I am trying to use a variable resistor (FSR = Force Sensing Resistor) to drive a signal circuit as the input to a speed controller.

The input to the speed controller comes from "Vout".

Data:
Vin = 24V
R1 = infinite when no force is applied to the FSR, down to 50kOhms under full load
RL = 1.5kOhms.  This is the internal resistance of the speed controller, and cannot be changed.

How could I achieve a 0-5 volt value for Vout, where 0V when no load applied to FSR, and 5V when full load applied to FSR?  By my calculations, it's impossible...but hey, I'm a mechanical engineer not an EE, so maybe someone here can help me to it :-)

#### TrickyNekro

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##### Re: Voltage Divider Help
« Reply #1 on: June 04, 2009, 04:50:34 AM »
Well... without looking a lot in your calcs....
Voltage divider is giving you only a fraction or the Vin... For example say Vout = 1/10 Vin....
This means that if Vin is 10 volts then you get a Vout at 1V.....
Simple....

Now.... The 1.5K load is probably the current is needs to operate.... So...
Say you have 5V and 1.5K load then... The voltage divider needs to allow a max of ~3,5mA at Vout....
If you can not achieve this then..... Use a op-amp with 1X gain.....

Also note that thanks god, all these stages are quite lineal.... Of course you have some offsets but hey....
Get a scope and calc them....
Also you need a rail-rail op-amp.... AND STAY THE HELL AWAY FROM CMOS OP-AMPS....... I totally order you to do so....
Use only analog op-amps.... CMOS op-amps tend to have hell of problems and you are gonna literally don't know what hit you....
For sure.... tested.......

Well..... that's all for now, I guess.....
Need anything.... Post it....

Best Regards, Lefteris
Greece
For whom the interrupts toll...

P.S. I've been inactive for almost a year... Don't give promises but I'll try to complete my tutorials. I'll let you know when..

Cheers!

#### Soeren

• Supreme Robot
• Posts: 4,672
##### Re: Voltage Divider Help
« Reply #2 on: June 04, 2009, 03:04:03 PM »
Hi,

[...]  AND STAY THE HELL AWAY FROM CMOS OP-AMPS....... I totally order you to do so....
Use only analog op-amps.... [...]
So, CMOS op-amps have suddenly gone digital?
Regards,
SÃ¸ren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Engineering is based on numbers - not adjectives

#### TrickyNekro

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##### Re: Voltage Divider Help
« Reply #3 on: June 05, 2009, 03:31:36 AM »