# Performance of method to decrease the output voltage

#### Kelvin Lee

Joined Oct 22, 2018
111

As my limited knowledge, I only know the voltage divider, regulator (like LM780x) or adding a resistor between the power source and output to lower the voltage pass through the device. What are the advantage and disadvantage to these three methods?

I think adding a resistor is the worst solution because it wastes the power from the resistor, am I right?

Best regards,

Kelvin.

#### Dodgydave

Joined Jun 22, 2012
10,031
Both are wasting power, the resistor worst. Any regulator will create wasted heat in dropping the voltage while taking high current, best to use buck regulators like LM2596.

#### Kelvin Lee

Joined Oct 22, 2018
111
Both are wasting power, the resistor worst. Any regulator will create wasted heat in dropping the voltage while taking high current, best to use buck regulators like LM2596.
Dear Dodgydave,

Do you mean both are the resistor and regulator? How about the voltage divider? Is it a better way?

#### Dodgydave

Joined Jun 22, 2012
10,031
Best way is a buck regulator...

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#### Alec_t

Joined Sep 17, 2013
12,150
Any linear regulator, such as the LM78xx, wastes energy as heat. A switch-mode regulator is much more efficient.

Joined Mar 10, 2018
4,057
A simple method, but inefficient, is use a series zener diode to drop
V by a fixed amount.

Are you trying to drop input V or output V ?

Regards, Dana.

#### BobTPH

Joined Jun 5, 2013
3,490
A voltage divider can only be used to reduce voltage to a load if that load is constant. And ib this case, it consists of just one resistor, the second resistor is the load.

And that method us exactly as efficient as any linear regulator.

The efficiency is simple, it is just:

Vout / Vin

To get efficiency better than that, you need a switching DC to DC converter. This is a more complex circuit that uses an inductor to reduce voltage without the power loss of a resistive element.

Bob

#### MrSoftware

Joined Oct 29, 2013
2,015
Each method has pluses and minuses.

A series resistor - simple and cheap, not the most energy efficient and only good for steady loads or where precision isn't require. Commonly used for things like limiting current to an LED, because the load is constant, the heat generated is typically negligible, it's cheap and easy and current through an LED does not need to be precise as long as it's not too much.

A linear regulator - can keep output voltage very stable and clean over varying loads, such as circuits where different parts will be turning on and off, or where the supply voltage may change over time such as when a battery is the source which loses voltage as it discharges, but you need really stable voltage. Typically very small or no extra capacitance is required which can be a plus in some situations. If the input to output voltage change is big then you can still lose lots of energy to heat, just like a resistor.

A boost/buck or switching regulator - can keep voltage stable over varying loads and supply voltages, and can be significantly more energy efficient than either a linear regulator or resistor. They are more complicated and expensive to implement, typically require more parts such as an inductor and multiple capacitors, and the output usually has some ripple (noise) on it which is perfectly fine for some applications, but problematic for others. The high frequency switching can also produce a lot of EMF, both from the inductor and from the traces on the board, which may or may not be an issue depending on the use case.

It is not uncommon to see a combination of all of these in the same circuit. For example if your main voltage supply is 24V, but you also have a processor on board that runs at 3.3V, it would not be uncommon to see a switching regulator used to bring the 24V down to 5V or 4V due to it's efficiency, then use a linear regulator to bring the 5V or 4V down to 3.3V due to the cleaner output. Here you leverage the benefits of both. Maybe throw in a resistor for a green "power on" LED.

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#### nsaspook

Joined Aug 27, 2009
8,513
It is not uncommon to see a combination of all of these in the same circuit. For example if your main voltage supply is 24V, but you also have a processor on board that runs at 3.3V, it would not be uncommon to see a switching regulator used to bring the 24V down to 5V or 4V due to it's efficiency, then use a linear regulator to bring the 5V or 4V down to 3.3V due to the cleaner output. Here you leverage the benefits of both. Maybe throw in a resistor for a green "power on" LED.
Exactly. In this design there are three board regulators. Switchers 24vdc to 5vdc, 24vdc to 12vdc. The linear regulator 12vdc(from the switcher) to 10vdc.