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Old 31-12-2011, 02:07 PM
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JohnH
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USB Laptop Voltages - and a dead CCD

I have a wierd problem with powering my AP kit. I have worked with this setup ok for a couple of years but recently changed my ccd and since then have had issues - finally resulting in me smoking my ST2000xm - not good!

So I will share this information for those who might have similar setups and/or similar experiance.

My original setup was to have my laptop indoors and connected to a USB hub at the scope using an active USB extension cable. The hub then connects to the mount (a G11G) via a Belkin USB to serial convertor, to the ST2k via a USB cable and to a Moonlite stepper controller with another USB cable. The ST2k connects to the autoguider port of the G11G using RJ11 6P6C cable. All the remote kit is supplied with power from a single, regulated power supply (Jaycar unit providing 13.8V).

This has worked ok so long as I did not use the Sbig AC power supply.

Recently however when I connected my ST2K the red light did not come on the fan did not run and the camera was not visible from CCDSoft. I disconnected and re-connected - smoke from the CCD.

What I have found is that the USB 0 V level on my laptop is +12V with respect to the 0V at the power supply. If I disconnect my laptop from the mains this drops to 0V. I have no idea what causes this.

The bottom line seems to be - even though I have the newer Gemini 1 which isolates the autoguide port from the electronics the use of the relay box is a good precaution. I now want to re-configure so that all equipment is isolated.

I believe the usb/serial convertor will provide isolation from the hub, the relay box from Sbig will isolate the gemini from the ST2k as well, there is still potential for problems at the hub to the ST2K. I think you can get special USB hubs that isolate - does anyone use those, where can I get them? Do they work?

Is there a better option?


John
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Old 31-12-2011, 03:42 PM
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multiweb (Marc)
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You need one of these. They have come down in price a lot.
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Old 31-12-2011, 05:29 PM
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Its most likely due to one of the power supplies is a Double insulated type.
You will have a differential voltage from the two supplies due to a different earth. The double insultaed has no earth, and thus when you reference it from a earthed point, you will see a voltage. I have seen voltaes as high as 200V.
You will need a proper earth before you connect the supplies together. This will then connect the 0v or earths together, and supply correct levels.
One easy way to tell if you have double insulated supply, is to check the AC cable for the earth pronge, if its not there its a Double insulated type. Another is to do a Ohms mesaurement between both earths. It should read very low ohms, like several ohms at most.

Theo
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Old 31-12-2011, 06:07 PM
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Hmm. The ?USB isolators all seem to only support LS/FS modes - that is 1.5 and 12Mbps not High Speed (480 Mbps). I am not sure if this will be an issue but I suspect it will be when using the AO unit as download speed is then critical.

I think I need to establish a common ground somehow...
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Old 01-01-2012, 01:44 PM
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I found a lower cost - the Keterex folks wanted AU$ 130 plus $ 40 for shipping. Considering I am not sure it will work I went for one of these:

http://electronics-shop.dk/galvanica...m?currency=EUR

Cost is about AU$ 50 inc shipping so if its no good I have not lost much. Will report back here.

John
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Old 01-01-2012, 02:30 PM
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Sorry to hear about your problems John, but thanks for sharing. I used to run all my USB peripherals through a single USB hub and had quite a bit of QHY9 trouble earlier this year. I now power the CCD solely from a 35AH battery and run it through a separate USB connection, so far with great success. Power supply used to be a double insulated type.

Will be interesting to hear how the USB isolation works for you.
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Old 01-01-2012, 04:57 PM
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A little more sleuthing with my AVO meter points the finger at my main PSU. This is (I should say was) a Jaycar model MP3096 13.8 DC regulated power supply - a transformer type not a switched mode unit. This was selected specifically to match the Gemini.

Unforunately it seems to be faulty.

The +ve terminal is 13.8V above the negative which you would hope BUT the -ve terminal is nearly double that (24.7V) below the case for some reason I cannot fathom it may always have been like that. This voltage is the same as I meased between my laptop's 0V (on the USB shell) and the 0V point on the camera power line - almost certainly the casue of the smoked PCB, I guess Iwas lucky not to do this earlier - the discharge path must be dependant on the device connection or power up order.

I am going to buy a new PS - obviously - so I am looking for a recommendation or two please for units to use with a Gemini (v1)...

Will still go for isolation as well if it works - belt and braces for me!



My other PS does not exhibit
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