Help USB power

[TheBulk]

I am planning on running a 5V power source from a USB source to power my optical limit switches. Only problem is it seems that USB ports are supply and demand as I've got 5V coming off it but no current when the switch is wired in. Is there a way I can "demand" a current? or do I need to go a different route with a separate PSU?

[CarveOne]

I am planning on running a 5V power source from a USB source to power my optical limit switches. Only problem is it seems that USB ports are supply and demand as I've got 5V coming off it but no current when the switch is wired in. Is there a way I can "demand" a current? or do I need to go a different route with a separate PSU?

This link is to a Maxim article on charging batteries from the USB port but it gives the power limitations and pin connections you need.

Any load device will draw whatever amount of current it needs from the power source unless the power source just can't deliver it. At that point the output voltage starts dropping off or, if the load current is severe enough, a fuse is blown. You could switch the load on and off but the USB port still has to be able to supply enough current when the load is connected. (Switching the load on and off makes no sense in the context of limit switches though.)

If your limit switches are connected directly to ground, then a series current limiting resistor needs to be installed to limit the current to the 100 ma level (or lower) per load device so as to not overload the USB current output capability.

CarveOne

[zool]

You might consider either a wall wart, or draw 5-volts directly from the power supply in the computer. Both are more robust, and avoid all the risk of drawing too much power and making charcoal. Also you avoid all that resistor stuff ...

[wildwestpat]

1. The power on the USB socket is controlled (switched on and off) by the PC. Use the 'device manager' settings in the operating system. Make sure that the options to turn off the USB is not selected for the socket you are using to power the switches. Remember that the USB eject command can turn also off the USB power so you need to be careful if you are using your switches as well as using the eject command prior to unplugging memory devices from another USB socket on the PC or hub connected to another PC's USB socket. Select the correct one when using the eject command. The eject command should be used with memory devices to save corrupting them pulling them out mid write cycle. A lot of PC's are not set up correctly with the eject USB as a readily accessible user command.

2. The PC's USB is over current protected. (500ma is typical for a desk top or tower - Laptops can be a different ball game with their power economising tricks.

3. If you want to check all is well use a LED and current limiting resistor across either the switch or the USB so that the LED indicates power and or switch state.

Hope this helps - Regards - Pat





Hope this helps

[zool]

Wow Pat, what you added is really cool [and informative]!

Z.

[brtech]

I'd suggest you get a wall wart. They are only a couple of bucks + shipping.
Read this:
Universal Serial Bus - Wikipedia, the free encyclopedia
You really don't want to use USB as a power source, although you CAN make it work.

You can, of course, use your motor power source and a couple of resistors.

[ger21]

Just pull the 5V from a hard drive cable.