# Voltage Divider

Discussion in 'Electronic Design' started by Marco T., Jul 26, 2006.

1. ### Marco T.Guest

Hallo,
I have thought to use a voltage divider to have an input voltage range
of -10v to +10v and and output of -2,5v to +2,5v.

Following the schema: Input -> R1 ->output
|
R2
|
Ground

I can choose between:
R1 R2
7500 1500
10000 2000
12000 2400

What is the difference in choosing R1 R2 couples?

Many Thanks
Marco

2. ### Marco T.Guest

What is the difference between using Ground or a reference voltage?

Thanks Again
Marco

3. ### John LarkinGuest

No difference; they're all wrong.

John

Why?

5. ### Greg NeillGuest

Show your math for determining the resistance ratios.

6. ### Marco T.Guest

I have used a script into TI site.

http://www-k.ext.ti.com/srvs/cgi-bin/webcgi.exe?Company={5761bcd8-11f5-4e08-84e0-8167176a4ed9},kb=analog,case=obj(32620),new

I have re-used the script and it has showed different values.
R1 R2
12000 3000
30000 7500
Maybe I typed a wrong value.

Into the script there is the possibility to choose resistor scale 10, 100,
1000, ecc.

What is the difference, in this case between the 2 possibilities?

Many Thanks
Marco

7. ### Marco T.Guest

Input stage of voltage divider should be a strain gauge with signal
conditioning to obtain 0 10v in single ended, or -10 +10 volt in
differential.

Instead the output should be a voltage follower.

Which could be a good resistor configuration?

Many Thanks
Marco

8. ### Spehro PefhanyGuest

They're still wrong. There's not much data input validation on that
script. You will need to conform to American practice and use "."
rather than "," as the radix point. One exact possibility would be 39K
& 13K
example I gave above).

Best regards,
Spehro Pefhany

9. ### Greg NeillGuest

Do you have any additional specifications on the
strain gauge such as its output impedance or
allowable current range?

You want to choose resistor values such that
they won't represent too low an impedance
compared to that of the gauge in order to

10. ### mcGuest

I think you want the output to be 1/4 of the input, not 1/6. So: 7500:2500,
12000:4000, etc. (none of which are standard resistor values).
The amount of current that will flow into the input and the amount of
additional voltage drop caused by current flowing out the output.

What is connected to the input and to the output?

11. ### John LarkinGuest

You want the output to be 1/4 of the input.

So imagine a string of four equal resistors, +10 volts on one end and
ground on the other. Call the voltages at the connections A B and C

A B C
+10------r-------r-------r-------r------gnd

By symmetry, B os halfway from +10 to gnd, so Vb must be +5 volts.

Again, by symmetry, Vc must be 2.5 volts, which is what you wanted.

You can combine the left three resistors into 3R.

So R1 must be 3x R2.

John

12. ### John PerryGuest

The strain gauge specs don't matter; he has a signal conditioner. What
he needs is the output current limit of the signal conditioner, which
will be more than adequate for any of the total resistances that anyone
has mentioned, including Spehro's correct one.

Since he intends to put a voltage follower on it, the same applies to
the output. Any total resistance from ~2K to ~500K will be fine for
just about any combination of signal conditioner and voltage follower
I've ever heard of. Choose something convenient in the middle. Just
keep in mind John's demonstration of 3R -- R, and make sure 4R is great