Need help with voltage issue

Thread Starter

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.
 

Thread Starter

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.
 
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