# Getting 9V and 400-500mA out of a 24V power supply

I recently bought this power supply to power a microstepping stepper motor controller (which requires an input voltage of 24V).

My project requires the ability to dynamically turn on and off an arduino (which is the heart of the embedded system I'm working on) using only code. I have literally everything working as of right now. The problem is that I have the arduino's power coming from a 9v power supply [through a relay that I control using my code], and the microstepping driver coming from the 24V power supply.

I want to power BOTH the microstepper driver and the arduino using the 24V power supply.

The power supply has two V+ pins and two V- pins, so I plan to just hook one pair directly into the microstepper driver. The goal is to have the other pair going into the arduino. However, it is quite obvious for anyone who has used an arduino before that 24V is NOT a good input voltage.

So, I'm trying to design a circuit that will step the 24V down to ideally 9V and around 400-500mA. As of now, I only have access to resistors for help. If there is a strong reason NOT to use just resistors, let me know! (I'm not an EE).

The resistors I have on hand are 10, 22, 47, 100, 220, 470, 1k, 2.2k, ..... , up to 1M. (ohms)

I have technically already designed the circuit I need, but after looking at it for 1/2 a second, you will realize that it it not what you would call.....good:

simulate this circuit – Schematic created using CircuitLab

This is basically three 20-ohm resistors in series, and I'm grabbing the voltage across the last "20 ohm" resistor. But, because a 20 ohm resistor will basically explode when subjected to that level of current, I have had to split each 20 ohm into five 100-ohm resistors.

Problems with my circuit that I am aware of:

• Requires FIFTEEN resistors. Eww.

• The resistors STILL get very, very hot. But they seem to be stable. But it's not good enough for my liking.

• I'm getting 8V out instead of 9V, I'd much prefer 9.

I have no idea if grabbing the voltage and current from the last set of resistors is even the best way to do this. I really have a very elementary understanding of circuit design, though I am trying to improve.

So, finally it's time for my question:

Can anyone show me an improved design using only resistors to step 24V down to 8-9V, as well as limit the current to 400-500mA? Again, I intend to directly take the output of this circuit and have it powering an Arduino.

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You don't want to do this only using resistors. – Ignacio Vazquez-Abrams Apr 6 '14 at 19:37
Can you elaborate? Is it simply inefficient? – Jake Apr 6 '14 at 19:38
It also won't work. The voltage will vary by how large the load is. – Ignacio Vazquez-Abrams Apr 6 '14 at 19:43
When using resistors to drop the voltage, the resulting voltage will depend on the current drawn. The current drawn by the Arduino will likely vary depending on what it is doing - whether it will vary enough to cause the voltage to vary excessively is unknown... – Peter Bennett Apr 6 '14 at 19:44
What are my alternatives? Preferably, what are my cheap alternatives? – Jake Apr 6 '14 at 19:44