# How to reduce voltage to meause it ?

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#### yemi11

##### Newbie level 4
i would like see the transient response of dc motor.

if i apply 24v to circuit, than take 2 wire from motor pins,

what should i do with this pins ?

1) i want ta connect them to the analog input of microcontroler and it can only measure 0-5 V..

2) the things that i will do to measurement wires shouldn't effect the motor circuit of course..

if i design a voltage divider with a "very very high" resistors is it works ?

or any other solution do you have ?

#### FvM

##### Super Moderator
Staff member
You won't use "very high" resistor values. Microcontroller ADC inputs work best with moderate source impedance, e.g. 1 - 5 kohms.

yemi11

### yemi11

Points: 2

#### KlausST

##### Super Moderator
Staff member
Hi,

I'd use a resistive voltage divider.

You say "transient" so I assume high frequency.
So don't use very high ohmic resistors.

There are many ways to calculate the resistors. I'd rather go to lower resistance. And I decide to dissipate less than 100mW.
With 24V this means about 6k Ohms mimimum. To the voltage you need some headroom. Maybe a 10k:1k resistive divider is OK.
This give a current of about 2mA at 20V, and a voltage range of 55V max. (11 × 5V).

Higher ohmic resistors have lower current, and thus lower power dissipation. But also lower frequency bandwidth.

Klaus

yemi11

### yemi11

Points: 2

#### Dept-74

##### Newbie level 4
I would use an op-amp mounted as a differential amplifier with a gain below 5/24.
But your power supply must deliver a wider range of voltages than 0v - 24v like -5v - 28v, or eventually use a rail-to-rail op-amp.

#### yemi11

##### Newbie level 4
Hi,

You say "transient" so I assume high frequency.
So don't use very high ohmic resistors.

Higher ohmic resistors have lower current, and thus lower power dissipation. But also lower frequency bandwidth.

Klaus

Could you please explain a little bit more about this ? i do not understand why ohmic resistor value is important what will be chance if it is 1M ohm

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