# How to power a device using a USB port instead of AA batteries?

I have a device that uses 3 AA batteries and I would like to power it from a USB port instead.

As I understand I will have to use resistors but I have no idea on which ones.

I have found the following calculator: Voltage Divider Calculator

While it does its work, the wiring shown in diagram is quite confusing:

I guess this is some logical schema rather than real wiring schema.

Question:

What resistor values do I need to get ~3.6V from a 5V USB port, and how should I wire them ?

Is there a better way than using resistors to get 3.6V from 5V?

• What is the device? Can it use Alkaline batteries? – Bruce Abbott Sep 19 at 21:18
• Surely 3 AA batteries will normally give you a nominal 4.5V and quite likely you can run your device straight from the USB's 5V without any resistors. But we would need more information. What is the device? How much power does it require? There is a previous question here which may help and a detailed article on how to power things from a USB port by Texas Instruments. Failing that google "USB powered circuit" for lots more information. – Peter Jennings Sep 19 at 22:04
• It is a motorized toy and can use Alkaline batteries. – Aybe Sep 20 at 20:20

Resistors will not be the best option, you need to know the source current of the device and there will be large voltage swings if the device changes it's current. Resistors may also not be possible if the device draws large amounts of current.

Three alkaline batteries is lower than 4.5V (could be as low as 3.6V when their dead). Since the device running from 3 alkaline batteries should be able to accept 4.5V, a voltage regulator like the might be a good way to ensure that the device is getting the appropriate voltage, however you might need to select one with a lower dropout than 0.5V which might be hard if your device needs more than 100mA.

Another option would be to find out if the device already has a linear regulator or switching regulator on the input. If it does, then you might be able to run 5V straight into the device.

• A fresh Alkaline is just over 1.6V so a device that runs on 3 of them should accept at least 4.8V, and may work fine straight off USB. – Bruce Abbott Sep 19 at 23:35
• Or just add a 1N4004 in series. – Spehro Pefhany Sep 20 at 20:57
• I second the @BruceAbbott opinion. Fresh no-load AA alkaline cells can have up to 1.65V initial voltage, making 4.95V total. The OP should do NOTHING and just connect his "device" straight to USB VBUS. The 0.05V excess shouldn't break any "motorized toy". Mentioning "low 3.6V" voltage is inconsequential in this situation. However, a "motorized toy" might have high current draw, could be a spike of several amps, and USB port might shut itself down. So the answer is faulty on two counts. – Ale..chenski Sep 30 at 23:42
• @Ale..chenski Yes, as you've found, my answers are always faulty – Voltage Spike Oct 1 at 2:47