In short
How to power a 9V (max) board with a 12V power supply?
In detail
I have a board with a 78M05 regulator and when I power it with 12V it "shuts" down after a few seconds. Regulator can work with 9V max. There is not much space to use some step-down module which would be ok. Idea is to use a few resistors as voltage dividers. Now comes the problems...
If I use two resistors 10K and 20K I should roughly get 8V between them and GND, which I get, but, when I connect them to a board terminal I get nothing. Board is not powered. When I check it on terminals, I have only 5V as an input and there should be 8V.
Use a 7808 linear regulator? But that will have to dissipate heat too. A small adjustable buck converter
board is probably the ideal solution, set it for about 7V out, enough for the on-board 78M05 to function.
A resistor divider can't be used for power, as it relies on having zero load current - its for measurement
or sensing.
who_took_my_nick:
There is not much space to use some step-down module which would be ok.
If you do not have sufficient space to use a switchmode "buck" converter, then you certainly do not have enough space to mount a linear regulator such as a 78xx with the necessary heat sink.
who_took_my_nick:
Idea is to use a few resistors as voltage dividers. Now comes the problems...
If I use two resistors 10K and 20K I should roughly get 8V between them and GND, which I get, but when I connect them to a board terminal I get nothing.
As you would of course expect.
A voltage divider only delivers a proportion of the input voltage when you do not draw any current from it.
I do not know what sort of "9V (max) board" you are using, but Arduinos require a 5 V supply. If you want to use them with a 12 V supply you need a 5 V switchmode converter to power them via the "5V" pin.
This is not an Arduino board. The 78M05 regulator is already on board. This board needs 12V for other things so on input screw terminals 12V is a must. The only solution should be to "bypass" 78M05 and its input pin by unsoldering it and connecting it with a step-down module output pad. It will look like a Frankenstein but I see no other solution.
Forget looking on Ebay for a "module", or wherever you are looking. There are plenty of stepdown devices if you look at DIGIKEY or other commercial sources. Surely one will fit your space requirements.
An LM7805 regulator depending on package is goo0d for an amp with proper heat sinking and an input voltage up to 35 volts
When you create a voltage divider you are placing a load across the divider. So what happens to the resistance that load is across? Your load is in parallel with your 20K so how do you figure that will play out? This is why voltage divider circuits are not a good idea. Additionally there is another problem, what happens to the current in the circuit? Think about that? Even if your load was an impossible 0 ohms in parallel with your 20K what happens to the current?
You have 12 volts and want 9 use either a buck converter or use a LM7809 regulator.
In the past, when using an LM7805 in a circuit with a similar situation (12V was needed elsewhere), I've found that the max current it could deliver (without heatsink) was around 70-100mA. Got a nice boost in available current (about 250mA) by using a 3.3V/5W zener connected in series with the 12V power input.
