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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    \$\begingroup\$ You don't want to do this only using resistors. \$\endgroup\$ – Ignacio Vazquez-Abrams Apr 6 '14 at 19:37
  • \$\begingroup\$ Can you elaborate? Is it simply inefficient? \$\endgroup\$ – Jake Apr 6 '14 at 19:38
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    \$\begingroup\$ It also won't work. The voltage will vary by how large the load is. \$\endgroup\$ – Ignacio Vazquez-Abrams Apr 6 '14 at 19:43
  • \$\begingroup\$ 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... \$\endgroup\$ – Peter Bennett Apr 6 '14 at 19:44
  • \$\begingroup\$ What are my alternatives? Preferably, what are my cheap alternatives? \$\endgroup\$ – Jake Apr 6 '14 at 19:44

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).

Strong reason. Wasted energy, fire hazard, and voltage will vary based on load. You are wasting (24V - 9V) * 0.5A = 7.5 Watts of energy in heat in those resistors! Horrible.

A small switching regulator module will work nicely. Heck, a common car usb charger, with a few resistor changes and maybe a new input capacitor (for greater than 24V), will do what you want. Most dollar store to 5/10 dollar chargers at a local corner store use a MC34063 regulator IC. Takes up to 40v input, adjustable output with a resistor pair, and can do 650~750mA without any other changes.

enter image description here

Even better, USE 5V NOT 9V! Giving the Arduino 9V will make it go through the regulator on board, wasting another 4V * 0.5A = 2W of energy in heat. Instead, use the 5v out of the usb charger to the usb connector of the Arduino (You would need to make a usb cable). Probably only need to change the input capacitor at that point.

  • \$\begingroup\$ @user2057387 updated. \$\endgroup\$ – Passerby Apr 6 '14 at 19:55
  • \$\begingroup\$ The relay I am using requires 6.75V minimum to flip, the nominal voltage is 9V. So I need 9V for that. However, I could definitely use a car charger as the arduino's power source when the relay gets switched. \$\endgroup\$ – Jake Apr 6 '14 at 22:42
  • \$\begingroup\$ Is it safe/recommended to use a USB charger and connect the Arduino to a PC via a second USB port? Are the two 5V sources compatible? \$\endgroup\$ – 101 Jun 20 '17 at 0:59
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    \$\begingroup\$ @101 normally you do not want to connect two different voltage sources directly together. Even the Uno R3 with all of its protection would not like that. Aside from making a custom usb cable that disconnects/cuts the USBVCC connection, you could use the regulator above and change it for 7V. Input the 7V to the VIN pin (or 7.3V at the Power Jack), and let the opamp/mosfet circuit block the usb power. This means you waste just 2V * 0.5A = 1W of power, but thats better than a dead board. \$\endgroup\$ – Passerby Jun 20 '17 at 5:14

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