I have 2 USB-powered (i.e. 5V) accessories which I'd like to turn on and off with an arduino. It is best to connect them to a transistor and with an individual 5V power supply or have a relay? These each require 1A. Thanks!
I would just just a transistor, probably a mosfet. As long it's not a problem to switch the GND of the devices (so no shared GND).
So I cannot share the GND channel with each other and to the arduino? Sorry didn't quite get what you meant by the last sentence.
There are two ways to switch a load, on the high side (here 5V) or low-side (ground).
low-side switching is easier but the lack of common-ground this creates may have issues and thus high-side switching be preferred.
low side switching is done with NPN or n-channel, high-side with PNP or p-channel. With a relay you can do either, but you need to drive the relay itself, so its often simpler to just use a transistor.
I did read about the low side and high side control and the majority of cases the low side was preferred but in this case with more than 1 item hooked up you cant have a common ground and control the 2 separately....makes sense! Hope I understood you well!
Low side is proffered most of the time because it's easy to switch loads of different voltages. For example a 12V led strip or a 48V motor etc. High side you can't do that directly. You at least need a NPN as well to switch the gate. Also, a NPN / N-mosfet is easier to make, more used thus cheaper :p
But indeed, you can have a common ground because you switch the ground. But in a lot of cases that's not a problem at all. We have no idea what those usb devices are but you're probably just fine switching the low side.
The usb devices are basically phone chargers
But phone charges don't need 5V... All a phone charger does is making 5V and output it over USB...
If its USB chargeable I figured requires 5V input?
You didn’t said they where USB chargeable, you said they where chargers.
But again, what are the devices? USB chargeable doesn’t cut the mustard.
Haha sorry for the confusion. They are Qi wireless chargers which come with just a USB cable so planning on getting two USB power adaptors (one for each) and to control their on/off state, hook them up to the arduino. Hope its a bit better now? :)
Ahh, much better. No problem in controlling a Qi low side. Also, just get yourself 1 big USB power adapter. Way easier and probably cheaper. That way you can power both Qi's and the Arduino all from 1 source.
That's a good point but the issue is I need 9V for the arduino since I'm also powering an LED strip off it (the arduino).
galkno3: I need 9V for the arduino since I'm also powering an LED strip.
That part doesn't really make sens. The Arduino is happy with 5V so that's not the problem. The led strip I don't know. But I all pre-made ledstrips I know use 5V or 12V. So how much strip/what kind/what current? You could also use a step down module (like a USB car charger) to go from 12V or 9V to the 5V for the chargers. 2A+ models are pretty easy to get.
True, the LED strip (5m, RGB) needs 12V but works fine (slightly dimmer) with 9V (as suggested here: https://learn.adafruit.com/rgb-led-strips/usage). Actually using exactly the same setup shown there. Now that I need external power supplies for my chargers, I'm considering powering the LED strip also by an external power supply. The question would then be what's the most 'efficient'/clean way to power arduino (5V), 2 chargers (5V each) and then the LED strip (12V). Maybe like you suggested having a multi-USB 5V PSU and then a separate 12V charger?
There are as always multiple possibilities all with pros and cons. I would go for a 12V wall wart to power everything. That way there is just one plug => easy. And I would buy a car charger or a random DC-DC step down module to power the Qi's and the Arduino from that 12V. Or 2 if the Qi's need more power. But if it's 1A one is enough if you get yourself one of those 2A+ capable car chargers (bought my 2,1A charger at a dollar store for like €2,-) / step down modules (available for less then $1,- on eBay).
So you still suggest going for a low side? :)
Yep, no need for a high side so just go for a cheap n-mosfet.
Cheers, thanks! When would it be best to go for high pass instead? More powerful additions?
High pass, you mean high side? High pass is a thing for filters :p
But high side is useful if you can't disconnect the low side. For example when you have a common cathode RGB LED (or a 7-segemnt display) and want to be able to control each led.
Or the GND is already shared some how and you connot change it. In a car the chassis is used as ground for example which could make separating grounds impossible.
Or if you want to turn off power but don't loose GND because of data transmission. For example you can just connect GND and the data wires of a USB to an Arduino and program it as long as the Arduino has another source of power. etc etc