I have a very rudimentary command of DC electronics, so I am hoping someone can help me out with answers to some questions.

I need to build a portable power supply box using a 12V deep cycle battery as the power source, to supply power to a telescope system and a laptop computer.

The telescope setup requires power for a 12V GoTo computer which is part of the telescope electronics (can be connected directly to a 12 battery, and for the following ancillary components: 12V dew heater strips (total of about 35 watts needed), 12V focuser (plugs into the GoTo computer), filter wheel (driven by USB computer connection), videocam (driven by USB computer connection), CCD camera (driven by USB connection), CCD camera guider (driven by USB connection), and te laptop computer (its transformer is a 19.V/6.7amp 120VAC to DC transformer). And last, a DC computer fan to ventilate the box for cooling.

How can I isolate the surge frrom the dew heaters from the other electronics without having to use a separate 12V battery?

Dell has a 12V cigarette lighter adapter for the laptop, but how does a 12V adapter recharge the battery when the OEM transformer is a 19.5V/6.7amp (130Watt) transformer? Due to the power demands on the laptop from all the USB connections, would I be better off running the laptop from a 12VDC to 120AC inverter and using the OEM transformer, or use a 12V to 19.5V converter, or just use the 12V adapter from Dell?

I will need a USB hub to have enough connections. How do I power it?

Of course, I will have a main power switch with indicator light, and switches for each of the power box's other connections. How are the indictor lamps powered? Are there dedicated 12V switches and indicator lamps? How can I dim the switch indicator lamps?

Of course, I will have no components connected to the power box when I recharge the 12V deep cycle battery. Do I need to worry about disconnecting the battery from the rest of the circuit when recharging it?

Are there other things I have not mentioned that I need to be concerned with in building this power box?

Please accept my thanks in advance for any help you can give me!!

Mike

 

...would I be better off running the laptop from a 12VDC to 120AC inverter and using the OEM transformer, or use a 12V to 19.5V converter, or just use the 12V adapter from Dell?

IMO, use the 12V adapter. That will allow you to have just two voltages to worry about - the 12V and the 5V for all the USB stuff.

Another approach to your problem would be to get a large automotive-type inverter and pull the PSU from an old computer. That would give you gobs of 5V current and 12V also, although you can pull that directly from your battery.

I will need a USB hub to have enough connections. How do I power it?

It's 5V and the combined current of all the devices. I'd consider the worst case scenario of all devices on at full, and then design for at least 25% more than that. You need to estimate what that is for folks to help you.

Of course, I will have a main power switch with indicator light, and switches for each of the power box's other connections. How are the indictor lamps powered? Are there dedicated 12V switches and indicator lamps? How can I dim the switch indicator lamps?

Most folks use LEDs for this, and just use a current-limiting resistor. There are switches with built-in lights and you could probably find them in an auto supply store.

If you want to dim them, things get more complicated. One strategy might be to have all your panel illumination supplied from the same source, which you could then regulate just like your dash lights in your car. I imagine you'd want to dim them all together, right? (turning them off would be easy!)

Do I need to worry about disconnecting the battery from the rest of the circuit when recharging it?

Yes, that would be wise unless experts here can help you come up with a clever solution. Funny things can happen at startup and shutdown.

 

Thanks, Wayneh!

I will get an estimate of the total amperage drawn by the different 5V components, and reply to your message again.

Thanks, again,

Mike

PS: I meant to ask about fuses too. Do they go on the positive or negative side of the battery and do you put them near the component or nearer the battery? Do I need to use slow-blow or fact-blow fusues?

 

It's good you're thinking about fuses. It doesn't matter where in a current loop the fuse is; once it blows, no current flows thru that loop. As for placement, you want to think about what you're protecting, and try to protect as much as possible with the fuse. But sometimes one big fuse protecting everything will leave a lower current section of a circuit unprotected, since the big fuse will never blow even if that one small section smokes. So you may want more than one fuse. One on the 12v and another on the 5v would make sense. A PSU has current protection built-in.