Power isolation question

[mtodman (Matt)]

I need to power a number of devices from a single 12V 85AH Gel Cel battery. These devices include a notebook PC and a Gemini controlled Losmandy mount but also include dew heaters and PWM control circuit to control my light box.
Currently, there is no electrical isolation between any of these devices.

Is there a relatively simple way I can isolate the individual circuits from the likes of current surges and voltage fluctuations? I would hate for a short circuit on one device to destroy my Gemini computer.

Thanks

Matt

[kustard (Simon)]

To prevent a surge going into the system from a faulty device you could put a power diode in series (Anode to the +12, Cathode to the output) with the power line. With a diode you will have a voltage drop of around 0.8V which shouldn't really be an issue. This along with a fuse on each line is the simplest protection method.

Other ways are by using zener diodes to clamp surges and MOV (varistors) to shunt excessive voltages but these can be a bit more involved to get working simply.

HTH
Simon

[mtodman (Matt)]

Thanks Simon.
Would you say a 1N5408 wo0uld be ok?
http://jaycar.com.au/productView.asp...T&SUBCATID=388

I have a few of these spare.

Thanks

Matt

[DavidTrap (David)]

I think Matt may be referring to "RF" interference that can come with using one battery to power Dew Heaters and Mounts / Cameras. The variable output nature of the dew heaters (I think) uses a rapidly oscillating on & off circuit. This can cause interference to other devices. At present, I'm trying to run my dew heaters off on battery and Scope of another. If there is a way to "reduce" this RF I'd be keen to hear it too.

I was wondering if "ferrite suppression sleeves" would work??

DT

[mtodman (Matt)]

Hi David,

I found this circuit which I built to regulate my light box. I haven't tried it out on a common battery source yet. I'm hoping the choke & filter will be enough to do the job.

http://www.ctshooter.com/dimmer.html

[wasyoungonce (Brendan)]

A battery set-up is much like a car battery system A surge in one cct will not damage other ccts without major damaging to the battery cells.

This is very unlikely to happen.

A dropping or reverse diode dropping .8~.7VDC is of little use. Better to use a Shottky diode as they have lower fwd voltage drop (typical ~c .3V DC). Any FET will have a lower fwd V drop. But even these will not isolate your supply just give you a false sense of security with regards to " cct isolation" as they are mainly used in reverse polarity protection.

But, If you are worried about noise being fed back from the PWM dew heater ccts then it would be best to get a separate battery for that system as a standalone.

Any practical filtering devices would be prohibitive in costs & design as there are a whole range of frequencies, harmonics and surge types that need to be protected against.

[kustard (Simon)]

Well if the issue is RF interference then indeed the best method of prevent contamination of the power supply is to have separate power supplies.

I guess you could drive separate regulated circuitry but you'd lose more voltage across these circuits.

As wasyoungonce pointed out, a Shottky diode would be better than a standard power diode (my thinking cap wasn't on).

It's always concerned me about dewzapper PWM systems that generate high RF interference. IMHO you'd only need the systems to run at 100Hz at the most. I'm still yet to build my system up so I'll be playing around with the best frequency.

[mill (Martin)]

Just use an RF choke and be done with all the problems
It is just a couple of capacitors with an coil in the power line, the coil in series and the capacitors on either end of the coil, this will kill all of the transients.

[Hagar (Doug)]

Matt. I really hate to disappoint you here but the one problem you are likely to have is with your Gemini controller. As a general rule Gemini does not handle working well off a battery supply. It is very voltage critical and often does not handle very small voltage drops well at all.
You may be better to power it off a DC to DC converter which can step the battery supply up to 15 or so voltsfor the Gemini only.

Good luck with it all. You will need a pretty heafty battery to power all the gear you listed above.

[mtodman (Matt)]

Thanks to all who have responded here. It's given me a number of things to consider.
Hagar, your comments re the Gemini have me concerned. Do you something like this would do the job?
http://www.jaycar.com.au/productView...T&SUBCATID=665

Thanks

Matt

[Starkler (Geoff)]

Dew heaters run at a very low frequency with cycle time of seconds, and certainly not rf frequencies.