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# Voltage Divider vs Series Resistor

#### bonbonbaron

Sep 11, 2015
16
Hi all! I'm new here. I'm sure you're going to see much more of me too.

So, question: if voltage drops across a resistor, then why wouldn't you use a series resistor to drop the voltage before a load instead of using a voltage divider?

Thanks!
Michael

#### dorke

Jun 20, 2015
2,342
What you are describing as "series resistor before a load" is basically a voltage divider.
But,
What is the load?
If it is a fixed value resistor, then it is a voltage divider with a known voltage on the load(the same as 2 resistors).
If however ,the load value is not fixed (like in many cases),the voltage on the load will not be fixed and will depend on the variable load value.

#### bonbonbaron

Sep 11, 2015
16
Ah. Thanks for the quick reply dorke. Good to know I wasn't just crazy for thinking they achieved the same thing in fixed loads.

So what advantage(s) do voltage dividers hold over series resistors in varying loads (like DC motors)?

#### dorke

Jun 20, 2015
2,342
Driving DC motors(or any high current/high wattage element) with resistive series (wasting) elemnts is not a good idea.
It shouldn't be done in both ways.

#### shlomo1234

Jun 20, 2016
10
For driving load with high impedence you have to use voltage divider, current low no voltage drop.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
For driving load with high impedence you have to use voltage divider, current low no voltage drop.

That's somewhat misleading.

#### AnalogKid

Jun 10, 2015
2,892
Since a series resistor is by definition in series with something, it is one half of a voltage divider. One way to address this is to separate the universe into two types of electrical energies - signals and power. Signals are low power and don't deliver significant energy to wherever they are going. Power is high power, from small audio amps to megawatts, and usually drives a load directly.

OK, those are very coarse definitions, so everyone calm down.

To adjust a signal, which normally comes from a relatively low output impedance source and goes to a relatively high input impedance destination, a two-element voltage divider can adjust the amplitude without being dependent on the actual value of the input impedance to complete the attenuation and set a specific amount of attenuation.

To adjust or limit power, such as limiting the amount of current through an LED or motor, a series impedance works directly with the load to set the power level. Yon can have a shunt element in parallel with the load to further adjust things, but that is less efficient, and expensive at high power levels. Also, the total load power usually is known, so its characteristics can be a reliable part of the attenuator design.

ak

#### BobK

Jan 5, 2010
7,682
Bottom line: Voltage dividers, meaning two resistors with the output take from their common connection, are not useful for delivering power at a reduced voltage.

A series resistor is sometimes useful, when the load current does not vary.

For other cases, you want an active circuit, i.e. a voltage regulator. A linear regulator is basically a voltage divider that is variable to keep the output constant to a changing load. A switching regulator is a more complicated and efficient means of reducing voltage without as much power loss. It can be thought of a switching the power on an off, with an inductor and capacitor to smooth out the result.

Bob

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