Question about USB hub and mAh

[Neko]

Due to the cable dragging and all... I'm thinking on getting a DC Hub from DeepSkyDad to power my QHY8L through it's DC201 , and also a powered hub. 

Been trying to research about it, but have a doubt.  I have this little Logitech 4port USB2.0 hub, that can be powered by 5v and requires 2.4A. 

DDS DC hub has 2 x 5v usb ports at 3A, and 5x12V at max 10A.

Now, the qhy8l uses max 4.5Ah when TEC goes 100% which is not always since when it reach the temperature, depending on the ambient temp will go pretty

low to maintain it. 

Then to this hub would connect the DC201 to 12v socket, then the hub I could get a 3.5mmx1.3mm power socket to USB to power the Logitech HUB and will

have spare 4x12v and 1x5vUSB ports . 

Question is... will it work?  and also

Read that USB 2.0 can draw from a single device a max of 0.500mAh... That is the port in my laptop, but when conneting to a hub is the external power that 

provides the rest of mAh , right?  since for now would connect 2 devices USB 2.0, lets assume both uses their max draw of 0.500mA, a total of 1Ah would be

used from the USB 2.0 hub  ...  I'm sorry if all this sounds too dumb!! but basically trying to figure out if I will be able to run it this way or I should need a different 

USB hub 

Thanks in advance!!

 

Carles

Edited February 13, 2021 by Neko

[drjolo]

So as I understand:

• you will have one 12V PSU that will power DC hub
• one 12V DC hub output you will use to power QHY8L 
• one 5V 3A DC hub output you will use to power USB2.0 hub

That should work, but you will have one potential ground loop, because ground (voltage minus) will be delivered to the camera with two cables - 12DC power cable and USB cable. It may, but does not need to cause troubles, depending on cable and connection quality, and also on how camera deals with such scenario. The ground loop in general happens, when the same signal (ground in this case) is connected between the devices with two different cables, that may have different voltage drop due to different current in these cables (like one power cable and one data cable).

It would not happen if this DC hub has 12V to 5V converters with galvanic isolation, but it is very doubtful. Manufacturer would tell about it for sure.  

[Neko]

Thank you for your answer

I might have misunderstood your answer but, seems you are referring to 2 individual DC hubs

But I mean  only one DC hub but it has.  5* 12v  connections AND 2 USB ports delivering 5v. So it is the same device.  And the USB hub would get powered by this DC hub through 5v USB port instead of the 12dc port. 

 

Does that still make possible the ground loop you're talking about ?

 

 

 

[drjolo]

Yes, because you have two paths for the ground signal:

• first:  power supply cable -> DC hub -> 5V power cable ->  USB hub -> USB cable -> camera
• second: power supply cable -> DC hub -> 12V power cable -> camera

The 5V voltage is of course separated from the 12V voltage, but they share the same minus (ground), unless the DC converter inside the hub is isolated, but as I mentioned it is not probable. 

But in your setup it will be easy to detect. If any problems with camera signal or connection occurs, you may just power the USB hub with separate 5V supply. If the problem will disappear, then it was a ground loop. 

[Neko]

38 minutes ago, drjolo said:

Yes, because you have two paths for the ground signal:

• first:  power supply cable -> DC hub -> 5V power cable ->  USB hub -> USB cable -> camera
• second: power supply cable -> DC hub -> 12V power cable -> camera

The 5V voltage is of course separated from the 12V voltage, but they share the same minus (ground), unless the DC converter inside the hub is isolated, but as I mentioned it is not probable. 

But in your setup it will be easy to detect. If any problems with camera signal or connection occurs, you may just power the USB hub with separate 5V supply. If the problem will disappear, then it was a ground loop. 

Oh, right.. .now I get it. So the same device (in this case the qhy8l) would have two different voltages 12 and 5 but from the same origin..  I assume this would produce image interferences or this sort of problems? in such case, isn't a wa to isolate that on the cable itself?  It seems the best option would be to get 12v USB hub then.. 

 

any alternative comes to mind for all this ? Since I don't have the DC hub either yet. 

[drjolo]

It may, but it does need to interfere - depends on camera behavior. In my estimation most probably it will not cause any problems. I just wanted to point out the root cause of some problems, because many astrophotographers are not aware of it and trying to replace cables, USB hubs, power supplies, and the ground loop is still present in the system.