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### Topic: Voltmeter Design (analogue) (Read 3352 times)previous topic - next topic

#### cjdelphi

##### Jun 10, 2013, 08:51 am
Where on earth do I start? lol

Since I don't know the values of the resistors, how can I determine say R1's value?  I'm confused where do i start? at the bottom with the lowest voltage and work
my way up? or calculate them all?

#### pito

#1
##### Jun 10, 2013, 09:09 amLast Edit: Jun 10, 2013, 09:12 am by pito Reason: 1
Hint: if you measure 0.1V with 100uA meter FCD, it has RO=0.1V/0.0001=1kohm resistance itself. R1 must be such it creates a volatage divider 10:1 .. so the resistor R1 will be probably 9k (you have to calculate it). And so on upwards..
Have fun!

#### AWOL

#2
##### Jun 10, 2013, 09:10 am
Ohm's Law.
If 100uA is the full scale deflection of the meter, you know the resistance of the meter movement itself, so all you need to do is plug in the voltage, and out pops the value of the resistor.

#### cjdelphi

#3
##### Jun 10, 2013, 09:34 am
That simple?...

I was worried about the total resistance throwing off the accuracy off on a 100v input so then...

R1?'s value...

1v / 0.001
= 1000ohm resistor?

10v / 0.001
= 10000 ohm resistor.

Like that?

But surely the total resistance will effect a 100v input reading?

#### pito

#4
##### Jun 10, 2013, 09:40 am
Quote
1v / 0.001
= 1000ohm resistor?

10v / 0.001
= 10000 ohm resistor.

Like that?

No, certainly not. Follow the algorithm I described above..

#### cjdelphi

#5
##### Jun 10, 2013, 09:49 am

Hint: if you measure 0.1V with 100uA meter FCD, it has RO=0.1V/0.0001=1kohm resistance itself. R1 must be such it creates a volatage divider 10:1

0.1V/0.0001 (0.1ma)
= 1000 ohms

fair enough.

R1 =  I've been staring at this for so long I've gone completely blank.

Brain freeze!, 10:1 so I multiply 1k * 10 then? lol - no seriously i've gone so blank I think i may have to call a doctor :O

#### pito

#6
##### Jun 10, 2013, 09:56 am
Study "voltage divider" or "resistive divider". It is your homework, isn't it?

#### AWOL

#7
##### Jun 10, 2013, 10:02 am
Quote
Study "voltage divider" or "resistive divider".

In a voltmeter, they're called "multipliers".

Two resistors in series, with an applied voltage of 10 volts give a current of 100uA.
One of the resistors is 1000 ohms. Solve for the other series resistor.

#### cjdelphi

#8
##### Jun 10, 2013, 10:08 am

Study "voltage divider" or "resistive divider". It is your homework, isn't it?

Certainly is... but it's a public holiday today or i'd be asking a professional who would give me the answer to R1 .

Volts/TR = amps
amps * resistance = voltage at given point

would it help if i said it's not my homework ? and I need it to build a multimeter?

#### cjdelphi

#9
##### Jun 10, 2013, 10:11 am

Quote
Study "voltage divider" or "resistive divider".

In a voltmeter, they're called "multipliers".

Two resistors in series, with an applied voltage of 10 volts give a current of 100uA.
One of the resistors is 1000 ohms. Solve for the other series resistor.

ok then... thanks!

#### pito

#10
##### Jun 10, 2013, 10:13 am
Quote
or i'd be asking a professional who would give me the answer to R1 .

A school work expects the pupil will study something - books, papers, wikipedia, etc. You might not have a privilege to ask experts all the time in the future
Enjoy holidays!

#### cjdelphi

#11
##### Jun 10, 2013, 10:24 am
I do study... not sure what brought you to that conclusion, this is a simple case of "logic fail" for me....   so i'm taking a break and then i'll approach it again!

#### AWOL

#12
##### Jun 10, 2013, 10:29 am
Kirchoff's Law tells you that the current flowing in the (very simple) circuit is the same at all points.
You know what the applied voltage is, you know what you want the current to be, and you know the value of one of the series resistances.
It isn't logic that is failing you, it is simple arithmetic.

#### jackrae

#13
##### Jun 10, 2013, 11:04 am
Cribbing is not an education - unless copying-without-thinking is how it's done these days.
If we give you the answers you learn nothing.
From your existing knowledge and the information others have already given (read it!),  prepare your own answers as per the test sheet you have, writing down all your assumptions and your calculations.
Then we will advise, if necessary, where, and why, you are wrong.

#### cjdelphi

#14
##### Jun 10, 2013, 11:31 am
Cribbing my ass...

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