# Need help with voltage issue

#### Thewebexpert1

Joined Oct 4, 2022
2
My source voltage is 22V. I need to connect this source to a 5V circuit. I need a resistor in between, but I've forgotten how to calculate this. Can anyone help please? Thanks!

#### MrChips

Joined Oct 2, 2009
27,196
It is not as easy as that.
You need to specify the load current I @5V.
Then the resistor value is R = (22V - 5V) / I = 17 / I ohms
and the resistor will need to dissipate 17 x I watts.

It would be better to use a buck switching regulator.

#### Papabravo

Joined Feb 24, 2006
19,346
My source voltage is 22V. I need to connect this source to a 5V circuit. I need a resistor in between, but I've forgotten how to calculate this. Can anyone help please? Thanks!
I agree with @MrChips and would hasten to add that almost any other solution would be superior. We need to stamp out the notion that using a resistor for this purpose is a valid idea.

#### Tonyr1084

Joined Sep 24, 2015
7,109
The intent of a resistor is to resist current, not voltage.

You can use two resistors in series to form a voltage drop. However, the voltage drop will also need to be calculated against the load current. It gets a little complicated. And it's not a good idea for a power source. There's a huge amount of wasted energy turned into the form of heat. So I'm in agreement that using a resistor to drop the (um) voltage isn't the correct approach.

Using a buck converter is the way to go.
Here's one from Amazon:
https://www.amazon.com/dp/B08HS19W8Z/ref=redir_mobile_desktop
As you can see, a good circuit is quite complex.
Heres how they work:
https://en.wikipedia.org/wiki/Buck_converter

#### BobTPH

Joined Jun 5, 2013
5,810
What is the load? A resistor will only work if the load draws a constant current. And if it is more than 100 mA or so, you want a different method.

#### Ian0

Joined Aug 7, 2020
6,329
Don’t forget the good old 7805!
If the current is low, or the efficiency is irrelevant, then a linear regulator is a valid solution.

#### Thewebexpert1

Joined Oct 4, 2022
2
Thanks, everyone. I had no idea of the current. The circuit I'm trying to power is usually powered by a button cell, but can take an input that's higher without frying it. I've run it off 3 AA batteries just fine - but my new source is 22V (and again, I don't know at what current). My documentation, unfortunately, is scarce.

I didn't know there was any such thing as this buck converter, but that sounds like the ideal way to do this. Thanks again!

#### Ian0

Joined Aug 7, 2020
6,329
Thanks, everyone. I had no idea of the current. The circuit I'm trying to power is usually powered by a button cell, but can take an input that's higher without frying it. I've run it off 3 AA batteries just fine - but my new source is 22V (and again, I don't know at what current). My documentation, unfortunately, is scarce.

I didn't know there was any such thing as this buck converter, but that sounds like the ideal way to do this. Thanks again!
If it normally runs off a button cell, it must take a very low current. A linear regulator would be better. I'd suggest LP2951

#### Jon Chandler

Joined Jun 12, 2008
391
You might consider changing your power supply – it will be simpler if you don't need the 22 volts for some reason.

A AC–USB power supply or old cell phone charger will work great. If you don't have any hiding in a drawer, you can probably pick one up for a couple bucks at a thrift store.