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Old 14-05-2016, 07:35 AM
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barx1963 (Malcolm)
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USB powered dew heaters

This is a question for those with more technical "nouse" than I. I was looking a dew prevention options for my imaging rig and it occurred to me that I have a powered USB hub sitting there that can put out 5v through each usb port. Would it be possible to build a small USB device that you could simply plug into a powered hub and use that power to run a dew heating strap?
The internals would be very simple, all it needs to do is control the current in the strap itself so a very simple control program to do that is all that is needed. I envisage something as small as a memory stick with the cord for the strap coming out the back. I can run a DVD burner of this hub so i am sure it would have the capacity to run a simple dew strap.

Anyway this came about because it is proving difficult to source a mount hub pro which has more options but this way you could have at least a USB hub to run cameras etc and a basic anti dew system at a lowish cost.

Just to be clear, my knowledge of electronics is rudimentary at best so feel free to shoot down my idea.

Malcolm
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Old 14-05-2016, 07:52 AM
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DavidTrap (David)
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I don't think it's going to work with standard dew strips which are designed to work at 12V For example, a 4 inch dew strip from Dew-Not draws 0.61amp and produces 7.3 watts of heat at 12V.

According to my maths, that means it's around 20ohms of resistance.

If connected to a USB hub delivering 5V, it would draw only 0.25amp and produce only 1.25watts of heat.

Essentially, you've got a smaller voltage potential from the USB hub (5V) trying to "push" electrons through the same resistance. The current flow will be smaller and so the total energy converted to heat will be less.

To get the same "heat" delivery from 5V, you'd need a lower resistance and wind up drawing a much higher current. Theoretically, a USB hub that can deliver up to 2amps per socket could delivering up to 10watts, so if you want to use a voltage converter to raise the voltage to 12V, you could use standard strips, otherwise, you'll need to make custom dew strips to run at 5V.

I'm relying on the equations Volts = Amps x Resistance and Watts = Volts x amps from high school physics. Happy to stand corrected if I've got this wrong.

DT

Last edited by DavidTrap; 14-05-2016 at 07:53 AM. Reason: clarity
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Old 14-05-2016, 08:08 AM
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barx1963 (Malcolm)
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Thanks David.
My thought was to make the strap to operate on that voltage rather than the standard strap.
As I understand it the dew heater controller that I have controls the temperature of the strap by controlling the voltage. So if i set it at 10% it still uses 12v but rapidly switches it on and off so it is only actually running 10% of the time. So to my mind (given I have limited knowledge) it usually doesn't need to be going flat stick. I have never run my Kendrick above 50%.

Anyway I note my external DVD burner draws 1.5amp at 5 volts (it is written on the base!) so 1.5 x 5 = 7.5 watts so maybe possible to get that much heat.
Of course that uses one of those double usb leads to get enough power. But I have a 7 port hub and only need 3 ports to run the rig so doubleing up the leads would still allow 2 straps.

Anyway, probably a silly idea that has already been floated and shot down before.

Malcolm
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Old 14-05-2016, 09:23 AM
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jenchris (Jennifer)
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I think using p= IxV is correct and you'd have to hope you didn't blow the hub when drawing the required amps.
I made up a dew strip by stripping down a hair drier and slipping the required length of straightened resistance wire into some silicon sheathing.
I wrapped that round the end of the ota and that was it. I fed it from a 12v deep cycle battery. It only took an amp.
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Old 15-05-2016, 12:59 PM
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barx1963 (Malcolm)
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Just found this http://www.jaycar.com.au//diy-telescope-dew-heater
Which is not quite what I had in mind but maybe an idea.

Hmmm...

Malcolm
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Old 15-05-2016, 05:58 PM
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ZeroID (Brent)
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As far as I understand USB devices have limited current available due to the internal hardware. Front USB ports on desktop PCs for example seem unable to support some external devices whereas rear ports seem able to cope. Powered hubs still use the same or similar chips etc and the current capability I'd suggest was in most case insufficient to support any real heating, down in the milliamp range.
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Old 16-05-2016, 09:25 PM
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Windston (Dan)
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I know that most USB ports will put out 5v .5amps, so you will only have 2.5w of heating power max. However iPad chargers and the likes will often put out 10w. I have a phone charger for my android that pumps out 5v 4amps! (20Watts), its possible theoretically to hook on up to the adapter and then mains power like that after a modification to a cable of coarse.

Last edited by Windston; 16-05-2016 at 09:26 PM. Reason: missed a bit
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Old 20-09-2019, 12:19 AM
Fleegal (Tony)
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How did you go, I see this thread is quite old. I made usb dew heaters and they work very well
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Old 06-10-2019, 08:41 PM
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Astrofriend (Lars)
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Hi,
A year or two ago I built my own USB dew heater. It was a very narrow space where to place the heater band on the camera lens, I needed a special solution. It was aimed for my portabel equipment and it would be practical with USB 5 Volt as a power sourche.

Here is my project if you find it interesting:
http://www.astrofriend.eu/astronomy/...ting-band.html

Beware, the USB power backup battery can be very dangerous if handled in wrong way.

Lars
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