Downsides to using voltage divider for converting 24 to 5V.

Thread Starter

Bod

Joined Sep 18, 2016
301
At the moment, I am using one of these to step my voltage down. It had been modified to use a resistor rather than the potentiometer so I can get a constant output.
They came in a pack of 2. One was my test board and is now broken. The one I am using now has just stopped working. I also don't want to use them because they have been designed to pull up to the maximum output voltage rather than dropping to 0 if the pot broke. This is a big issue if you have something sensitive like a micro controller on the output, which I did and it released its magic smoke.
I have some LED strips which are 24V. On them they just use a voltage divider to get a 5V output.

What are the downsides to using a voltage divider to take 24V down to 5V? Obviously things like over-current protection, reverse polarity protection would be on a prebuilt module but since I'm putting the voltage divider straight on the board that's not a worry.
 

AlbertHall

Joined Jun 4, 2014
10,056
What are the downsides to using a voltage divider to take 24V down to 5V?
It would be entirely impractical to use a voltage divider if you want 3A output (3A * (24V - 5V) = 57W. The divider resistor(s) would need to dissipate 57W+.
If you want 1mA output then it becomes practical, but the output voltage will vary with the load current.
With a switched mode board, the dissipation (= power wasted as heat) will be minimal and the output is regulated so will not vary significantly with the load current.
If you are worried about the pot, then either replace the pot with fixed resistors or buy a fixed 5V output board.
 

Thread Starter

Bod

Joined Sep 18, 2016
301
If you are worried about the pot, then either replace the pot with fixed resistors or buy a fixed 5V output board.
Which I have and it was working until last night. Powering it up this morning I got nothing on the output.

It would be entirely impractical to use a voltage divider if you want 3A output (3A * (24V - 5V) = 57W. The divider resistor(s) would need to dissipate 57W+.
If you want 1mA output then it becomes practical, but the output voltage will vary with the load current.
3A is way more than enough. I can't test how much my board draws since the power supply is now dead however the current draw of the ESP32 (the micro controller) is about 27-44mA. So if I were to make a guess with a bit of a margin I would say probably 50mA possibly 60 is the most it will draw.
 

AlbertHall

Joined Jun 4, 2014
10,056
Dropping 24V at 60mA is 1.15W.
Now the next thing is how much is your load current going to change and what are the limits for the output voltage?
 

Thread Starter

Bod

Joined Sep 18, 2016
301
Dropping 24V at 60mA is 1.15W.
Now the next thing is how much is your load current going to change and what are the limits for the output voltage?
The load will vary almost none. I believe that to be the case since the micro-controller will always be on (when the board is powered) and that is where most of the current is being used. I want to say probably just about 10mA of variation depending on conditions.
As for the output voltage I can really only a have a range between 4.5V and 5.1V but 4.5 is pushing it very low for the micro-controller. Over 5.1 is definitely a no-go.
 

OBW0549

Joined Mar 2, 2015
3,454
Voltage dividers are used to scale down signals or reference voltages; they aren't used to supply power to devices, because they waste a lot of power and provide absolutely no regulation. Using a voltage divider to power a microcontroller is just plain dumb.

Do it the right way, and use a voltage regulator IC instead.
 

crutschow

Joined Mar 14, 2008
25,126
The good, old LM7805 +5V linear series regulator in a TO-220 case should work for you.
It will get rather hot (about 180°F at 60mA) so you may want to put it on a small heatsink.
 

Thread Starter

Bod

Joined Sep 18, 2016
301
Voltage dividers are used to scale down signals or reference voltages; they aren't used to supply power to devices, because they waste a lot of power and provide absolutely no regulation. Using a voltage divider to power a microcontroller is just plain dumb.

Do it the right way, and use a voltage regulator IC instead.
Fair enough. I have a completely different method to get it to work but I will purchase a step-down converter in the near future.
The good, old LM7805 +5V linear series regulator in a TO-220 case should work for you.
It will get rather hot (about 180°F at 60mA) so you may want to put it on a small heatsink.
It's funny that. That was what I had built onto my first board revision. It was a bare bones L7805 converter. I calculated it to be dissipating about 1W. I didn't even bother to calculate the efficiency since I know it was appalling.

My method now is to just run an extension cord to my power supply. That way I can plug in the power supply for my lights and also a 5V wall wart adapter to power the board and that means I still only need 1 plug on the other end.

Thanks anyway,
Bod
 
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